Molecular Membrane Biology, 2000, 17, 143± 156
The location and function of vitamin E in membranes (Review)
Xiaoyuan Wang and Peter J. Quinn*
Division of Life Sciences, King’s College London,
150 Stamford Street, London SE 1 8WA, UK
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
Summary
Vitamin E is a fat-soluble vitamin that consists of a group of
tocols and tocotrienols with hydrophobic character, but possessing a hydroxyl substituent that confers an amphipathic
character on them. The isomers of biological importance are
the tocopherols , of which a-tocopherol is the most potent
vitamin. Vitamin E partitions into lipoproteins and cell membranes, where it represents a minor constituent of most
membranes . It has a major function in its action as a lipid
antioxidant to protect the polyunsaturated membrane lipids
against free radical attack. Other functions are believed to be to
act as membrane stabilizers by forming complexes with the
products of membrane lipid hydrolysis, such as lysophospholipids and free fatty acids. The main experimental approach to
explain the functions of vitamin E in membranes has been to
study its effects on the structure and stability of model
phospholipid membranes . This review describes the function
of vitamin E in membranes and reviews the current state of
knowledge of the effect of vitamin E on the structure and phase
behaviour of phospholipid model membranes .
Keywords: Vitamin E , a-tocopherol, model membranes, phospholipid bilayers , free radical oxidation.
Abbreviations: DSC, differential scanning calorimetry, NMR,
nuclear magnetic resonance, ESR, electronic spin resonance,
SAXS, small-angle X-ray scattering; WAXS, wide-angle X-ray
scattering; a-T, a-tocopherol, RRR-a-tocopherol; c-T, c-tocopherol,
RRR-c-tocopherol; La , lamellar liquid-crystalline phase; Lb , lamellar
gel phase; Lc, lamellar crysta l phase; HII, inverted hexagonal phase;
PC, phosphatidylcholine; PE, phosphatidylethanolamine; DMPE ,
1,2-dimyristoyl-sn-glycerol-3-phosphoethanolamine; DPPE, 1,2-dipalmitoyl-sn-glycerol-3-phosphoethanolamine; DSPE, 1,2-distearoyl-sn-glycerol-3-phosphoethanolamine; DLPE, 1,2-dilauroyl-snglycerol-3-phosphoethanolamine; DOPE, 1,2-dioleoyl-sn-glycerol-3phosphoethanolamine; DLPC, 1,2-dilauroyl-sn-glycerol-3-phosphocholine;
DMPC,
1,2-dimyristoyl-sn-glycerol-3-phosphocholine;
DPPC, 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine; DSPC, 1,2distearoyl-sn-glycerol-3-phosphocholine; DOPC, 1,2-dioleoyl-sn-glycerol-3-phosphocholine; LDL, low density lipoprotein, VLDL, very low
density lipoprotein; HDL, high density lipoprotein; TBP, tocopherol
binding protein.
Introduction
Vitamin E is an ubiquitous, albeit a minor, component of
biological membranes . It is classified as a fat-soluble vitamin,
but, becaus e it possesses a hydroxyl group attached to the
ring structure, it is weakly amphipathic. In some respects, the
vitamin res embles cholesterol in its amphipathic character.
*To whom correspondence should be addressed.
e-mail: p.quinn@ kcl.ac.uk
Despite its relatively low concentration compared to other
membrane lipids , it is believed to play an important part in
pres erving the integrity of membranes . Foremost amongs t its
functions is its ability to protect polyunsaturate d lipids of the
lipid bilayer matrix of membranes agains t oxidation. Another
important function in membranes is the formation of
complexes between vitamin E and products of membrane
lipid hydrolysis such as lysophospholipids and free fatty
acids. The complexes thus formed tend to stabilize membranes and prevent the detergent-like actions of lipid
hydrolytic products on the membrane.
This review summariz es recent evidenc e for the role of
vitamin E in protecting membranes from free radical attack
and the cons equences of lipid oxidation in membranes. One
of the key factors in understanding how vitamin E fulfils its
functions in membranes is how it is localiz ed in the lipid
bilayer matrix and what effect does its pres ence have on the
structure and stability of membranes . Studies with model
membrane systems containing vitamin E which address
thes e questions are described.
Biochemistry of vitamin E
Vitamin E is us ed as the generic description for all tocol and
tocotrienol derivatives qualitatively exhibiting the biological
activity of a -tocopherol (IUPAC 1974). Tocopherols should
be regarded as derivatives of tocol. The tocopherols poss ess
a 4¢ , 8¢ , 12-trimethyltr idecyl phytol side chain and the
tocotrienols differ by the pres ence of double bonds at the
3¢ , 7¢ and 11¢ positions of the side chain. The a-, b-, c-, and disomers of tocopherol and tocotrienols differ in the number
and position of the methyl substituents attached to the
chromanyl ring. The structures and names of thes e isomers
are shown in figure 1. The structure of tocol indicates that
there are three centres of asymmetry; thes e are at C2 , C4¢
and C8¢ . The tocotrienol poss ess only one centre of
asymmetry in C2 , in addition to sites of geometrical
isomeris m at C3¢ and C7¢ . Thus , a number of stereo isomers
of the tocol and tocotrienol can exist.
In nature, the most abundant form of vitamin E was shown
conclusively to have the 2R, 4¢ R, 8¢ R configuration (Mayer et
al. 1963, Sheppard et al. 1993), and was called RRR-atocopherol. It has the highes t biological activity and accounts
for ~ 90% of the vitamin E activity found in tissues (Machlin
et al. 1982, Cohn 1997). Its melting point is 2.5± 3.58 C, so
that, at room temperature, it is a viscous oil. It is fluorescent
with an emission maximum of ~ 325 nm in hydrophobic
environments . Its infrared spectra show OH (2.8± 3.0 l m)
and CH (3.4± 3.5 l m) stretching and a characteristic band at
8.6 l m. The optica l rotations of thes e tocopherols are very
small and depend on the nature of the solvent.
Becaus e vitamin E is a fat-soluble vitamin, it tends to
partition into tissue lipids , locate in hydrophobic domains like
lipoproteins, or partition into the hydrophobic core of the
various cell membranes . The distribution of vitamin E in the
Molecular Membrane Biology
ISSN 0968-7688 print/ISSN 1464-5203 online Ó 2000 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/09687680010000311
X. Wang and P. J. Quinn
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
144
Figure 1.
Chemical structures of tocol (a) and tocotrienol (b), and a space-filling model (c) of RRR-a-tocopherol.
body changes from one tissue to another (Podda et al., 1996,
Hidiroglou et al. 1997, Viana et al. 1999). Adipos e tissue,
liver and muscle repres ent the major sites of vitamin E in the
body, with ~ 90% of the vitamin being contained in the
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
Vitamin E in membranes
adipos e tissue (Traber and Kayden 1987). Of all the
subcellula r membrane fractions, the greatest concentrations
of a-tocopherol were found in the Golgi membranes and
lysosomes (Buttriss and Diplock 1988, Zhang et al. 1996).
The molar ratio of a-tocopherol : phospholipid in these
membranes was in the order of 1 : 65 phospholipid molecules, which is about an order of magnitude greater than
found in the other subcellula r membranes . In all tissues , atocopherol was found to be the major form of vitamin E.
Tocopherol binding protein (TBP) is believed to be
involved in the transport and metabolis m of a-tocopherol in
tissues (Traber and Arai 1999). Three a-tocopherol binding
proteins have been isolated from the cytosol of mammalian
tissues and characteriz ed (Dutta-Roy 1999). The first one is
a hepatic protein of 30 kDa (Kuhlenkamp et al. 1993, Sato et
al. 1993, Wolf 1994) and is responsible for discrimination
between the homologues of vitamin E, by incorporating atocopherol from lysosomes to the endoplasmic reticulum for
packaging in VLDL (Traber et al., 1994). This could explain
why c-tocopherol only accounts for 10± 15% of plasma
tocopherol although it is efficiently absorbed. The s econd
a-tocopherol binding protein, which has a molecula r weight of
~ 15 kDa has been identified in the cytosol of various
tissues , including liver. It may be involved in the intracellula r
distribution and metabolis m of a-tocopherol in tissues (DuttaRoy et al. 1993a, b, 1994, Gordon et al. 1995, Dutta-Roy
1997). Unlike the 30 kDa TBP that only resides in hepatocytes, the 15 kDa TBP is pres ent in all major tissues . The
15 kDa TBP specifically binds a-tocopherol, in preference to
d- and c-tocopherol, and may exclusively transport atocopherol to intracellar sites. The third one is a plasma
membrane a-tocopherol binding protein (TBPpm ). This is
characteriz ed in human erythrocytes and liver, and may
regulate a-tocopherol levels in thes e cells (Dutta-Roy et al.
1994, Bellizzi et al. 1997a, b).
145
species and propagate lipid peroxidation (Burton and Ingold
1986, Liebler 1993).
The reactions involved in the scavenging function of
vitamin E are illustrated in figure 2. The oxidation of lipids is
s een to proceed by a free radical mediated chain process,
whereby a lipid peroxyl radical serves as a chain carrier.
Chain propagation occurs by abstraction of a hydrogen atom
from the target lipid by the peroxyl radical to produc e a lipid
hydroperoxid e and a carbon-centred lipid radical. The
carbon-centred lipid radical can react with molecula r oxygen
to generate another lipid peroxyl radical to produc e further
chain carrying radicals.
The principle role of a-tocopherol is to scavenge the lipid
peroxyl radical before it is able to attack the target lipid
substrate producing a-tocopheroxyloxyl radicals . Stereo
electronic features of a-tocopherol contribut e to the stabilization of tocopheroxyl radical and account for the high firstorder rate constant for hydrogen transfer from a-tocopherol
to peroxyl radicals (Burton and Ingold 1981, Burton et al.
1985). a-Tocopherol reacts with a peroxyl radical either by
concerted hydrogen transfer, or by sequentia l electron then
proton transfer, to form a lipid hydroperoxid e and the
tocopheroxyl radical. The stability of the tocopheroxyl radical
arises from delocalization of the unpaired electron about the
fully substituted chromanol ring system, rendering the radical
relatively unreactive (Kamal-E ldin and Appelqvis t 1996). In
vitro measurements of the relative rate of chain propagation
to chain inhibition by a-tocopherol have indicated that atocopherol scavenges the peroxyl radicals considerably
faster than the peroxyl radical reacts with lipid substrate.
The concentration of a-tocopherol in cell membrane is
relatively low. In most cell membranes, the amount of atocopherol is less than 2 mol/mol phospholipid. The question
arises as to how such small amounts of a-tocopherol protect
Functions of vitamin E in membranes
The vitamin E deficiency syndrom e and the benefit of vitamin
E to the body have been well known for many years, but the
mode of action of tocopherols at a molecula r level is not
clearly understood. There is considerable literature on the
subject, and some of the conclusions are conjectural. It is
believed that the main function of vitamin E is its action as an
antioxidant in membranes and lipoproteins (Fukuza wa et al.
1997, Surai et al. 1999), but more functions, such as
stabilizing membranes , inhibiting platelet aggregation and
maintaining the immune system; have been characteriz ed
recently.
Scavenging free radicals
a-Tocopherol and c-tocopherol constitut e essential compo-
nents of cellular defence mechanisms agains t endogenous
and exogenous oxidants . Unlike many other cellular
antioxidants that are constituent enzymes or enzymedependent systems, the antioxidant reaction of a-tocopherol
is non-enzymatic and fast. The principa l role of atocopherol, as an antioxidant is believed to be in scavenging lipid peroxyl radicals, which are the chain-carrying
Figure 2.
Scavenging reactions of a-tocopherol.
X. Wang and P. J. Quinn
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
146
membrane lipids agains t sustained free radical attack. Redox
cycles of a-tocopherol are believed to occur in membranes
(Liebler 1993). Regeneration of a-tocopherol in vitro from its
tocopheroxyl radical form mediated by vitamin A (Bohm et al.
1997, 1998), vitamin C (Kagan et al. 1992, Bowry and
Stocker 1993) and coenzym e Q (Ingold et al. 1993,
Stoyanovsky et al. 1995) have been demonstrated. There
is, however, some doubt about the significanc e of regeneration by vitamins A and C in vivo (Livrea and Tesorieve 1999).
An antioxidant role of coenzyme Q itself is likely to be
subsidiar y to its main function as a mobile redox proton
carrier in the energy-transducing membranes of mitochondria and chloroplasts (Navarro et al. 1998). Antioxidant
mechanisms of vitamin E and coenzym e Q can be envisaged
to operate individually or in tandem. In the first instance,
independent reactions with peroxyl radicals take place in
which each antioxidant acts as a phenolic scavenger of
radicals (Barclay et al. 1985). In the s econd cas e, there is
redox cycling in which vitamin E acts as the primary
scavenger of peroxyl radicals , and coenzyme Q recycles
vitamin E from its phenoxyl radical (Maguire et al. 1992,
Mukai et al. 1992, Kagan et al. 1994). Thes e reactions are
shown:
TO ‡QH2 ! TOH ‡ QH
…1†
TO ‡QH ! TOH ‡ Q
…2†
where QH2, QHÇ and Q repres ent ubiquinol, ubisemiquinone
and ubiquinone, respectively.
Electron carriers in rat liver microsomes , mitochondria,
and submitochondrial particles have been demonstrated to
regenerate vitamin E from its phenoxy radical by a
ubiquinone-dependent reduction (Maguir e et al. 1992,
Mukai et al. 1992). This suggests that vitamin E/coenzym e
Q interactions may be important in the antioxidant protection of electron transport membranes . In experiments with
mitochondria and submitochondrial particles devoid of
vitamin E, Ernster et al. (1992) found that reduced
coenzym e Q was able to effectively inhibit lipid peroxidation. This indicates that direct antioxidant action of
coenzym e Q in mitochondria is also possible. Neuzil and
Stocker (1994) showed that coenzyme Q can act as an
effective peroxyl and oxygen radical scavenger in LDL, and
that this reaction is essential for preventing chain-propagating pro-oxidant action of phenoxyl radicals of vitamin E in
LDL.
In contras t to a-tocopherol, c-tocopherol is a powerful
nucleophil e that traps electrophilic mutagens in lipophilic
compartments (Cooney et al. 1993, 1995, Christen et al.
1997). It, thus, complements glutathione, which similarly
scavenges electrophilic mutagens in the aqueous phas e
of the cell. An electrophilic mutagen prone to react with
c-tocopherol is peroxynitrite. Thus , c-tocopherol may
protect lipids , DNA, and proteins from peroxynitritedependent damage.
Membrane stabilization
There is some evidenc e to indicate that the formation of
complexes of a-tocopherol with certain membrane compo-
nents tends to stabiliz e bilayer structures, while other
evidenc e suggests that tocopherol destabiliz es membranes
and promotes fusion. An early obs ervation by Ahkong et al.
(1972) showed that when a-tocopherol was added to a
suspension of avian erythrocytes , the cells were induced to
fus e, presumably becaus e a-tocopherol destabiliz ed the
membrane lipid bilayer. Simila r studies with chromaffin
granules aggregated with synexin, however, showed that
the addition of a-tocopherol did not induc e the granules to
fus e (Creutz 1981). Another s eries of studies in which the
stability of model phospholipid bilayer membranes was
determined in the pres ence of different amounts of atocopherol also supported the notion that a-tocopherol
stabilizes lipid bilayers and prevents fusion. For example,
the aggregation rate of phosphatidy lcholin e vesicles containing free oleic acid is decreased (Ortiz and GoÂmez-Ferna ndez
1988) and Ca 2+-induced fusion of large unilamella r vesicles
of phosphatidy ls erine (Aranda et al. 1996) is inhibited by atocopherol. One explanation for the inhibitory effect of atocopherol on Ca 2+-induced fusion is that a-tocopherol
reduces the binding of Ca2+ to phosphatidy ls erine vesicles
(Sanchez-Migallon et al. 1996a).
a-Tocopherol is believed to form complexes with membrane lipid components that have a tendency to destabilize
the bilayer structure, thereby countering their effects and
rendering the membrane more stable. Kagan (1989) has
us ed paramagnetic resonance spectroscopy to demonstrate
the formation of complexes between free fatty acids and atocopherol in phospholipid bilayer membranes . Urano et al
(1988, 1993) have investigated the interaction of unsaturated
fatty acids with a-tocopherol, by using fluorescence and NMR
methods , which showed that a-tocopherol decreas ed the
fluidity of phospholipid liposomes that were perturbed by the
pres ence of free fatty acids with more than one double bond.
Evidenc e was also obtained for the formation of complexes
between lysophosphatidy lcholine and a-tocopherol, that was
said to explain the protectiv e effect of a-tocopherol agains t
the action of products of phospholipas e A2 hydrolysis on
biological membranes (Mukherjee et al. 1997). Porcine
pancreatic phospholipase A2 hydrolyses phosphatidylcholine
when in lamellar and micella r states. Grau and Ortiz (1989)
have found that a-tocopherol is able to inhibit the phospholipas e A2 activity only toward lamella r fluid membranes , thus
protecting phospholipids toward this lytic enzyme. This
compound decreas ed both the initia l rate and the extent of
hydrolysis . The inhibition is of the non-competitiv e type and
the evidenc e strongly suggests that it is due to an effect of atocopherol on the substrate, i.e. the membrane. Other
tocopherols , such as b-, c- and d-tocopherols , also displayed
the phospholipas e A2 inhibition, but consecutively to a lower
extent (Grau and Ortiz 1989). Cholesterol does not inhibit the
phospholipase A2 activity at concentrations even higher than
those of tocopherols , indicating that the effect of tocopherols
is not due to alteration of membrane fluidity (Grau and Ortiz
1989).
Molecula r model studies have been us ed to infer formation
of stable complexes with phospholipids compris ed of polyunsaturated fatty acyl substituents (Lucy 1972, Diplock and
Lucy 1973). It was concluded from such studies that the
pockets created by the stereoconfiguration of the cis double
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
Vitamin E in membranes
bonds of arachidonyl residues, for example, accommodate
the methyl groups at position C4¢ and C8¢ of the tocopherol
phytol side-chain, producing a strong association between
the two molecules . It was rationalized that such complexes
would be ideally suited to protect the vulnerable fatty acids of
membrane lipids from oxidation, reduce the passive permeability of membranes containing high proportions of polyunsaturated fatty acyl residues , and decreas e the
susceptibility of such phospholipids to hydrolysis by endogenous phospholipases.
The effect of the association of a-tocopherol with single
chain or very asymmetric phospholipids has been studied
using 31 P NMR, Fourier-transfer infrared spectroscopy and
light microscopy (Salgado et al. 1993a, b). It is suggested
that the complementary shapes of a-tocopherol and asymmetric phospholipids may be the reason for the stabilization
of the bilayer structure. The rationale for the stabilizing effect
of a-tocopherol is that, by formation of a molecula r complex
with molecules such as free fatty acids and lysophospholipids , the amphiphatic balance of the complex is restored to
that which approximates a bilayer-forming phospholipid. This
is illustrated in figure 3, that shows space-filling molecular
models of diacyl phosphatidy lcholin e and compares this with
complexes of a-tocopherol and lysophosphatidy lcholin e and
palmitic acid.
Modulation of eicosenoid metabolis m
Vitamin E up-regulates the activities of cytosolic phospholipas e A2 and cyclooxygena se (Chan et al. 1998). The
enhanced activity of these two rate-limiting enzymes in the
arachidonic acid cascade provides a mechanis m for the
obs ervation that vitamin E dos e-dependently enhances
releas e of prostacyclin, a potent vasodilator and inhibitor of
platelet aggregation (Pyke and Chan 1990, Tran and Chan
1990).
a-Tocopherol modulates the in vitro expression of some
significa nt proteins /enzymes in various cell types involved in
atherogenesis (Chan 1998). Vitamin E enrichment of
147
endothelia l cells down-regulates the expression of intracellular cell adhesion protein and vascula r cell adhesion
molecule-1, thereby reducing the oxidiz ed LDL-induced
adhesion of white cells to the endothelium (Cominacini et
al. 1997). Recent advances in the area of arachidonic acid
cascade have also demonstrated that a-tocopherol can
regulate thes e pathways, and its effect is not always shared
by other isomers of vitamin E (Chan 1998).
Regulation of protein kinas e C
The effect of a-tocopherol on protein kinas e C, has been
studied (Cachia et al. 1998, Azzi and Stocker 2000). aTocopherol inhibits smooth muscle cell proliferation (Boscoboinik et al. 1991a ), decreas es protein kinas e C activity
(Boscoboinik et al. 1991b), increas es phosphoprote in phosphatas e A2 activity (Ricciarelli et al. 1998), and inhibits
protein kinas e Ca phosphorylatio n-state and its activity (Azzi
and Stocker 2000). Thes e functions are not related to vitamin
E antioxidant action becaus e b-tocopherol, which has a
similar antioxidant activity, does not perform any of thes e
functions. a-Tocopherol effects on protein kinas e C inhibition
have also been reported in human platelets (Freedman et al.
1996), diabetic rat kidney (Koya et al. 1997, 1998), and
human monocytes (Devaraj et al. 1996). The mechanis m of
protein kinas e C inhibition by a-tocopherol may be attributable, in part, to its attenuation of the generation of
membrane-derived diacylglycerol, a lipid that activates
protein kinase C translocation and activity (Kunisaki et al.
1994, Tran et al. 1994). Cachia et al (1998) also suggested
that the inhibition of protein kinas e C activity is not due
directly to the antioxidant capacity of a-tocopherol, but
requires the integration of a-tocopherol into a membrane
structure. Addition of a-tocopherol to recombinant protein
kinas e C in the test tube does not result in inhibition of protein
kinas e C, suggesting that the inhibition of a-tocopherol is not
caus ed by a direct interaction with protein kinas e C (Azzi and
Stocker 2000).
Maintenanc e of the immune system
Although vitamin E is a relatively minor constituent of all
cellular membranes , it is found in high concentrations in
membranes of cells of the immune system. It is possible
that such membranes are at especially high risk of
oxidative damage (Coquette et al. 1986, Machlin 1992,
Beharka et al. 1997). The oxidant± antioxidant balance is
an important determinant of cell function in immune
systems, including maintaining the integrity and functionality of membrane lipids , cellula r proteins , nucleic acids,
and gene expression in cells of immune system. Vitamin
E, as an antioxidant, is said to play an important role in the
maintenanc e of the immune system (Meydani and Tengerdy 1991).
Effects of vitamin E on model membrane systems
Figure 3. Molecular models of dipalmitoylphosphatidylcholine, and
molecular complexes between a-tocopherol and lysophosphatidylcholine and palmitic acid.
In order to determine how a-tocopherol functions in cell
membranes , it is necessary to know how it is arranged in the
membrane and what influenc e it has on membrane structur e
148
X. Wang and P. J. Quinn
and stability. Codispersions of a-tocopherol with phospholipids in aqueous media have often been chosen as a model to
examine the effect of vitamin E on the lipid matrix of cell
membranes. A number of studies have been reported using
phospholipids with particula r acyl chain composition in
codispersion with vitamin E.
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
The effect of a-tocopherol on phase behaviour of model
membranes
Many studies have shown that a-tocopherol influences the
phas e behaviour of phospholipid model membranes. Examination of the thermal properties of mixed aqueous dispersion
of a-tocopherol with DSPC, DPPC or DMPC by DSC,
showed that increasing proportions of a-tocopherol caus e a
progressive broadening of the gel to a liquid-crystalline
phas e transition and a decreas e in enthalpy change of the
transition (DeKruijff et al. 1974, Mass ey et al. 1982,
McMurchie and McIntosh 1986, Villalõ Â an et al. 1986, Ortiz
et al. 1987, Quinn 1995). Similar results were obtained by
Fourier transform infrared and Raman spectroscopy res earch (Lefevre and Picquart 1996). Proportions of atocopherol greater than ~ 20 mol% resulted in almost
complete loss of transition enthalpy.
It has been consistently obs erved in the thermal studies
of mixtures of a-tocopherol with saturated phosphatidylcholines, that the pres ence of a-tocopherol apparently eliminates the pre-transition enthalpy. This does not mean that
the ripple structur e its elf is eliminated. Evidenc e from
synchrotron X-ray diffraction and freeze-fracture electron
microscopy have shown that different ripple structures are
induced in mixed aqueous dispersions of a-tocopherol and
PC (Wang et al. 2000). Figur e 4(a) shows the SAXS
patterns of codispersions of DSPC containing up to
10 mol% a-tocopherol at 258 C. It can be s een that the
number and intensity of the additiona l peaks increas e with
increasing proportions of a-tocopherol, suggesting that
these peaks originate from a-tocopherol enriched domains .
Electron micrographs of freeze-fracture replica s prepared
from thes e dispersions at 258 C showed areas with a ripple
structur e (figures 4(b) and (c)), suggesting that the atocopherol enriched domain forms a ripple phase. The
primary effect of a-tocopherol on phosphatidy lcholine might
be to form disordered ripples of large periodicity of ~ 50±
150 nm (figure 4(b)). When the proportion of a-tocopherol
is increased, the ripples of large periodicity were replaced
by the ordered ripples with a periodicity of 16 nm (figure
4(c)), which produce at least six reflections in the SAXS
region (figure 4(a)). A further increase in the proportion of
a-tocopherol induces planar bilayers with a worm-like
surface texture (figure 4(c), asterisk). A simila r ripple
phas e has also been observed in codispersions of atocopherol and unsaturated phosphatidylc holine (Wang
2000).
The effect of a-tocopherol on phas e behaviour of heteroacid PCs is complex, with evidence of domain formation.
Sanchez-Migallon et al. (1996b) examined the thermal
behaviour of a-tocopherol with 1,2-di18-PCs having 18 : 0
acylated in the sn-1 position and 18 : 0, 18 : 1, 18 : 3 or 20 : 4
acylated at the sn-2 position of the glycerol. They con-
Figure 4. (a) Static SAXS intensity patterns of DSPC containing up
to 10 mol% a-tocopherol recorded at 258 C. (b) and (c) Electron
micrographs of freeze-fractur e replica s prepared from codispersions
of DSPC containing 5 and 10 mol% a-tocopherol thermally
quenched from 258 C after heating from low temperature, respectively. The scale bar corresponds to 100 nm.
structed partial phas e diagrams of the mixed acyl dispersions , and concluded that there was fluid phase immiscibility
between the phospholipids and a-tocopherol. The magnitude
of the effect of a-tocopherol on transition enthalpy was found
to be dependent on the extent of unsaturation of the
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
Vitamin E in membranes
hydrocarbon chain located at the sn-2 position of the glycerol
backbone of the phospholipid. It was suggested that atocopherol s erved to reduce the differences between gel and
fluid states of the phospholipids that, in turn, are related to
the molecular shape of the unsaturated phosphatidylcholines.
Unlik e the bilayer-forming phosphalipids, the effect of atocopherol on the phas e behaviour of non-bilayer forming
lipids appears to be different. Ortiz et al (1987) examined
the effect of different proportions of a-tocopherol in DPPE
and DLPE and found that a-tocopherol induces the
formation of multiple peaks in differentia l scanning calorimetric curves which were interpreted as evidenc e of phase
separation within the bilayer. Similar results were obs erved
in the mixture of DMPE and a-tocopherol (Micol et al.
1990). a-Tocopherol has also been found to decreas e the
temperature of the lamellar to HII phase transition in
dielaidoyl PE, as well as to induc e phase s eparations of
isotropic and HII phas es in the gel phase phospholipid as
deduced from 31P-NMR spectroscopy (Micol et al. 1990).
X-ray diffraction experiments have given strong evidenc e
about the phas e s eparation in the codispersion of atocopherol and PEs (Wang and Quinn 1999a, b, Wang et
al. 1999). There are three interesting aspects for the atocopherol rich domain formed in the mixture of atocopherol and phosphatidylethanolamine. First, they form
a lamellar crystal phas e during low temperature equilibrium. Secondly, they form an HII phase at temperatures
much lower than that of La to HII transition of the pure PE,
even below the main transition of the pure PE. Finally,
they could form a Pn3m cubic phase at higher temperatures. Increasing the hydrocarbon chain length favours the
formation of the lamellar crystal phase, but not the
formation of the Pn3m cubic phas e. Moreover, the cubic
phas e was only observed in mixtures of PE containing atocopherol between 5± 20 mol%. In the mixture containing
20 mol% a-tocopherol, however, only the HII phas e was
obs erved, which is independent of the hydrocarbon chain
length of the phospholipid. Figure 5 shows static diffraction
patterns of mixtures of DLPE containing 7.5, 10 and
15 mol% a-tocopherol recorded at designated temperatures. It can be s een that cubic phas es begin to dominate
the diffraction patterns at 908 C in the mixture containing
7.5 mol% a-tocopherol (figure 5(a)), and represent the only
phas e at 998 C in the mixture containing 10 mol% atocopherol (figure 5(b)). It is noteworthy that in the mixture
containing 15 mol% a-tocopherol, the Pn3m phas e already
dominates the diffraction pattern at 818 C and completely
replaces the La phas e at 928 C; there was no evidenc e of
any transformation of this cubic space group, even after
heating to 1018 C (figur e 5(c)).
Static X-ray diffraction experiments were performed to
investigat e the relationship between the different phas es
obs erved in the codispersion of DPPE with 20 mol% atocopherol, and the results are shown in figure 6. Two
lamella r crystal phases (Lc1 and Lc2 ) are clearly distinguished in this experiment. The Lc1 is characteriz ed by a
relatively sharp diffraction at 6.4 nm in the SAXS, with a
group of peaks in the WAXS region (46.48 C). The Lc2
phas e is characteriz ed by a broader lamellar diffraction
149
Figure 5. Static small-angle X-ray scattering intensity profiles
versus reciprocal spacing of fully hydrated codispersions of (a) 7.5,
(b) 10, and (c) 15 mol% a-tocopherol in DLPE at designated
temperatures. Each diffraction pattern represents scattering accumulated in 5 min.
centred at 5.0 nm in the SAXS region, with another s et of
peaks in the WAXS region (528 C). Below 528 C, only one
phas e exists, suggesting that the stoichiometr y of phospholipid : a-tocopherol is about 4 : 1. It is also noteworthy
that the appearanc e of the HII phas e at 56.98 C coexists
with the Lc2 phas e, suggesting that the HII phas e
originates directly from the Lc2 phase and a phas e
transition s equence of the a-tocopherol-rich domain in the
initia l heating scan of Lc1 ® Lc2 ® HII can be inferred. There
is a change in lamella r d-spacing from 6.1 nm in the Lc1
phas e to 5.0 nm in the Lc2 phas e. Assuming that there is
no change in hydration between the two crystal phas es,
the d-spacing could be accommodated by a change in tilt
of the hydrocarbon chains from the vertical in Lc1 to 358
with respect to the bilayer normal in the Lc2 phas e. The
150
X. Wang and P. J. Quinn
tentative structura l arrangements of the a-tocopherol
enriched domain are illustrated in figure 6(c).
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
Location of a-tocopherol model membranes
Figure 6. Static X-ray scattering intensity patterns in SAXS (a) and
WAXS (b) from aqueous codispersions of DPPE containing 20 mol%
a-tocopherol recorded at different temperatures. (c) Schematic
representation of the different structures of a-tocopherol rich domain,
which have a stoichiometry of 4 : 1 for DPPE: a-tocopherol. Symbols
and ¨ j Ð Ð
indicate DPPE and a-tocopherol molecules,
s Ð Ð Ð
respectively.
The location of a-tocopherol in model membranes has been
the subject of considerable interest (GoÂmez-Ferna ndez et al.
1989, Quinn 1998). ESR probe experiments have shown that
the phytol chain of a-tocopherol anchors the molecule firmly
within the phospholipid bilayer (Niki et al. 1985). This is
further supported by the results from 13C and 2 H-NMR (Perly
et al. 1985, Urano et al. 1987, Ekiel et al. 1988, Salgado et al.
1993a , Suzuki et al. 1993), 19 F-NMR (Urano et al. 1993) and
fluorescenc e studies (Aranda et al. 1989). It was concluded
that the molecule rotates about an axis perpendicula r to the
plane of the phospholipid bilayer, and the motion of the rigid
chromanol ring is more constrained than the phytol chain in
which the frequency of gauche 6 rotamers increas es with
distanc e towards the centre of the bilayer. 13 C-NMR
(Srivastava et al. 1983) and 2H-NMR (Wassall et al. 1986)
relaxation measurements of a-tocopherol in phospholipids in
the fluid phase both indicate a restriction in the motiona l
freedom of both components of the mixture. Fourier-transform infrared studies of mixed aqueous dispersions of atocopherol and DPPC have shown that a-tocopherol does
affect the acyl chain conformation. Earlier studies by Villalain
et al. (1986) showed that the number of gauche 6 rotamers in
the acyl chains of the phospholipid in the gel phas e was
reduced by the pres ence of a-tocopherol. This , however, was
not confirmed by subs equent studies that showed that the
number of gauche 6 rotamers increas ed (Severcan and
Cannistraro 1990). The latter is consistent with the observations from ESR (Wassall et al. 1991, Suzuki et al. 1993,
Severcan 1997) and NMR (Wass ell et al. 1986, Suzuki et al.
1993).
Different biophysica l methods have been us ed to study the
orientation of a-tocopherol in phospholipid bilayer membranes. Some suggested that a-tocopherol locates in the
region of the aqueous interface of the structur e (Srivastava et
al. 1983, Perly et al. 1985). Some suggested it is ~ 1 nm
from the aqueous interface (Fragata and Bellemare 1980).
Becaus e the excitation and fluorescenc e emission wavelengths of a-tocopherol are appropriat e for fluorescence
energy transfer with certain probes , several experiments
using fluorescence quenching of probes have been performed to determine the location of the chromanol ring of atocopherol (Ohyashiki et al. 1986, Kagan and Quinn 1988,
Bisby and Ahmed 1989). Thes e experiments led to the
conclusion that the chromanol moiety of a-tocopherol is
oriented towards the lipid± water interface of the phospholipid
bilayer, but does not extend into the lipid± water interface.
A number of studies on the hydrogen bonding between the
a-tocopherol and phospholipid have been done. Several
Fourier-transfe r infrared spectroscopy measurements
showed evidenc e of H-bonding of the phenoxyl hydroxyl
group to either the carbonyl or phosphate oxygen of the
phospholipid molecules (GoÂmez-Ferna ndez et al. 1991,
Salgado et al. 1993b). Bilayer permeability experiments
reported by Urano et al. (1990) concluded that the phenoxy
group of a-tocopherol is hydrogen bonded to the carbonyl
Vitamin E in membranes
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
group of the ester carbonyl bond of the phospholipids
arranged in a bilayer configuration. For further confirmation
of this conclusion, time-dependent changes in the fluidity of
liposomes caus ed by the addition of Pr3+ were measured at
the surface and inner region of liposomes using a fluorescence polarization technique (Urano et al. 1993). The fluidity
increased in the surface region and decreas ed in the inner
region of liposomes containing a-tocopherol. These results
show that when Pr 3+ pass es through the liposom e membrane, the hydrogen bonds between a-tocopherol and the
ester carbonyl of PC may be broken, leading to the formation
of an a-tocopherol/Pr3+ complex, thus causing the fluidity to
increas e.
151
containing both PE and PC, regardles s of the saturation and
the length of hydrocarbon chains of the two phospholipids.
The effect of a-tocopherol on equimola r mixtures of
saturated PE and PC has been investigated using DSC
(Ortiz et al. 1987) and X-ray diffraction (Wang and Quinn
2000b). The DSC results were interpreted that a-tocopherol
was preferentially partitioned in the most fluid phas e,
irrespectiv e of whether this was PE or PC. However, the
conclusions drawn from the X-ray data appear to be different
(Wang and Quinn 2000b). The change in the WAXS intensity
of the sharp peak at 0.43 nm to temperature plotted in figure
7(a) was of the equimola r mixtures of DMPC and DPPE
containing up to 20 mol% a-tocopherol. The curves show
maxima corresponding to the midpoint of the phas e transition
Preferential interactions of vitamin E with particular
phospholipids
The effect of a-tocopherol on the phas e behaviour of mixed
diacylphospha tidylcholines to determine whether any preferential interactions take place has been reported. Ortiz et al.
(1987) have studied the effect of a-tocopherol on equimola r
mixtures of DPPC/DSPC and DMPC/DSPC. The equimola r
DPPC/DPSC mixture only showed a single endotherm which
was modified by the pres ence of a-tocopherol in a similar
manner to the individual PCs. The equimola r DMPC/DSPC
mixture, however, showed monotectic behaviour. With
increasing proportions of a-tocopherol, the highes t temperature endotherm was broadened and shifted towards lower
temperatures, while the lowes t temperature endotherm was
weakened and disappeared in the mixture containing
20 mol% a-tocopherol. This was interpreted such that atocopherol preferentially affects the lower melting component
which corresponds to DMPC. Increasing proportions of atocopherol in an equimola r mixture of 18 : 0/18 : 1- and 18 : 0/
22 : 6-PC also results in a two component transition enthalpy
(Stillwell et al. 1996). The lower temperature transition is
progressively broaded as the a-tocopherol content of the
mixture increas es, but the higher temperature component is
virtually unaffected by the pres ence of up to 10 mol% atocopherol. This was interpreted as a phas e separation of atocopherol within the mixed phospholipid dispersion, with a
preference for an association with the more unsaturated
molecular species of phospholipid.
The inverted hexagonal phas e induced by a-tocopherol in
PEs can be us ed as a marker for the PE+a-tocopherol
domain in PC/PE mixtures (Wang and Quinn 2000a , b). An
X-ray diffraction study of mixtures of a-tocopherol with DOPC
and DOPE showed that the effect of a-tocopherol on
unsaturated phospholipids follows a simila r pattern to that
obs erved for their unsaturated counterparts . However, in the
X-ray diffraction study of mixed aqeuous dispersions of atocopherol with DOPE/DOPC (1 : 1) and DOPE/DMPC (1 : 1),
it was found that lamellar gel and liquid-crystallin e phas es
dominated the phase structure. This is consistent with the
fact that a-tocopherol does not preferentially interact with PE.
The changes in the SAXS patterns are similar to thos e of
mixtures of a-tocopherol and phosphatidylcholines or of
equimola r mixtures of PE/PC (Wang and Quinn 2000a). This
again suggests that a-tocopherol preferentially interacts with
the PC in the mixture or distributes randomly in domains
Figure 7. Rate of normalized X-ray scattering intensity I of the
WAXS peak dI/dt recorded from heating scans at 28 /min of
codispersions of DMPC/DPPE (a) and DLPE/DSPC (b) containing
indicated mol% a-tocopherol plotted as a function of temperature.
Inset shows a plot of the relative heights, Peak 2 : Peak 1, plotted as
a function of a-tocopherol in the mixture.
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
152
X. Wang and P. J. Quinn
of the DMPC (Peak 1) and DPPE (Peak 2) components of
the mixture. The ratio in height, Peak 2 : Peak 1, is found to
increas e with increasing a-tocopherol in the mixture. This is
consistent with a preferentia l partition of a-tocopherol into
DMPC, so as to reduce the contribution of change in the
intensity of the wide-angle reflection due to DMPC relative to
DPPE. A simila r analysis of the WAXS intensity data of
mixtures of DLPE and DSPC containing different proportions
of a-tocopherol are pres ented in figure 7(b). The ins et to the
figure shows the relationship between the relative heights of
Peaks 2 and 1, and the mol% a-tocopherol in the mixture.
This shows that as the proportion of a-tocopherol in the
mixture increas es the contribution to the change in scattering
intensity of the WAXS peak from DSPC decreas es relative to
that from DLPE. This can be interpreted as a preferentia l
partitioning of a-tocopherol into the high melting point
phospholipid component of the mixture, which is again
phosphatidylcholine, DSPC.
from measurements of bandwidth and frequencies of the
CH2 antisymmetric and symmetric stretch vibrations of the
phospholipid acyl chains that the pres ence of a-tocopherol
causes a decreas e in the average number of gauche 6 chain
conformers at temperatures above the main phas e transition.
Interestingly, the bandwidth and frequency of maximum
absorbance does not appear to change in concert with
changes in temperature that were interpreted as evidenc e for
the coexistenc e of two phases. Evidenc e for complex
formation between a-tocopherol and phospholipids has also
been obtained from synchrotron X-ray diffraction studies
(Wang and Quinn 1999a , b, Wang et al. 1999). Codispersions of a-tocopherol and a group of heteroacid PCs have
been detected by the DSC method (Sanchez-Migallon et al.
1996a ), and lateral phas e separations containing different
amounts of a-tocopherol were suggested in the fluid phas e.
Phas e s eparation was also found in the mixture of atocopherol and PEs (Ortiz et al. 1987).
Vitamin E enriched domains in membranes
Conclusions
According to the fluid mosaic model of membrane structure
(Singer and Nicolson 1972), phospholipids, together with
some other lipids , such as cholesterol, form a fluid bilayer
where the incorporated proteins and lipids themselves are
free to diffus e laterally. The idea of a free lateral diffusion
implies that generally both lipids and proteins would be more
or less randomly distributed. However, a considerable
number of experimenta l and theoretica l data have accumulated recently, indicating that membranes are not laterally
homogeneous , but domains with distinct lipid and protein
composition exist (Brown and London 1997, Simons and
Ikonen 1997, Brown 1998, Rietveld and Simons 1998).
Therefore, biological membranes are likely to be compris ed
of a mosaic of lipid domains (Vaz and Almeids 1993). In fact,
s everal types of cholesterol-enriched domains have been
propos ed in plasma membranes (Bretscher and Munro 1993,
Murata et al. 1995, Harder et al. 1998, Keller and Simons
1998, Rietveld and Simons 1998).
In a model of a mosaic of lipid domains , where does
vitamin E locate? What is the evidenc e that vitamin E forms
enriched domains in membranes ? As has been noted
already, preferential interactions between a-tocopherol and
phospholipids, and indirect evidenc e for the formation of
specific complexes , have been obtained from a number of
different experimental approaches . The pres ence of two
signals from spin label studies of dispersions of DPPC
containing relatively high proportions of a-tocopherol (20 and
30 mol%) was reported at temperatures over the range 30±
508 C (Severcan and Cannistraro 1988). A relatively immobile
component was attributed to an a-tocopherol enriched
domain coexisting with bilayers of pure phospholipid.
Measurements of intrinsic fluorescenc e of 7 mol% a-tocopherol in bilayers of DPPC also led to the conclusion that atocopherol was clos ely associated in particular domains of
the phospholipid bilayer (Kagan and Quinn 1988, Aranda et
al. 1989). Information about the forces that may operate
between phospholipids and a-tocopherol have come from
Fourier transform infrared spectroscopy of mixed dispersions
of DPPC and a-tocopherol (Villalain et al. 1986). It was found
The function of vitamin E divides into those associated with
its role as an antioxidant and thos e which rely on its pres ence
as a constituent of biological membranes (Brigelius-F lohe
and Traber 1999, Wang and Quinn 1999c). Because redox
reactions require collisions between the reactants for
electron transfer to occur, the efficiency of action depends ,
in part, on the location of vitamin E in the proximity of
membrane constituents that are susceptible to oxidation. The
evidenc e for location of a-tocopherol in proximity to membrane phospholipids with polyunsaturate d fatty acyl substituents is bas ed on model predictions , and no direct
experimental evidence for such complexes has yet been
reported. Formation of complexes between a-tocopherol and
long-chain fatty acids and lysophospholipids , the hydrolytic
products of phospholipas e A action, have been characteriz ed. Stabilizing membranes by restoring an appropriate
amphipatic balance on formation of these complexes is
believed to be the mechanism underlying this action of atocopherol.
Membrane phospholipids with polyunsaturate d acyl substituents would tend to create domains of relatively high
fluidity in membranes . Evidence mainly from calorimetric
studies of mixed aqueous bilayer dispersions of saturated
molecular species of phospholipids has suggested that atocopherol tends to partition into fluid phas es in mixtures
exhibiting gel-fluid phas e immiscibility, irrespective of the
phospholipid class segregating into the respective phas e.
This result has been challenged by evidenc e from direct
structura l examination of mixed aqueous dispersions of
phospholipids . This X-ray data was consistent with the
formation of complexes between a-tocopherol and phosphatidylcholines which phase s eparate from pure phosphatidylethanolamines , irrespectiv e of whether the phosphatidylcholine is in a fluid- or gel-phas e configura tion. Further
work will be required to reconcile these opposing views.
Thus, the evidenc e that a-tocopherol is not randomly
distributed throughout the lipid bilayer matrix of biological
membranes, but instead forms complexes with specific
membrane constituents , is emerging. The evidence for
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
Vitamin E in membranes
location of a-tocopherol at membrane sites most susceptible
to oxidation is not direct. The formation of complexes with
membrane destabilizing agents is more convincing, sugges ting that, although present as a relatively minor membrane
component, it may exert a significant structural role via a
localiz ed effect. A preferential association with phosphatidylcholine, as opposed to phosphatidyleth anolamines , may
result from a more stable bilayer structure created by
phosphatidy lcholine± a-tocopherol complexes. Studies of atocopherol-phosphatidyle thanolamine mixtures indicates that
a-tocopherol is largely excluded from bilayer phas es of
phosphatidyle thanolamine, but phas e s eparates into domains of inverted hexagonal phas e of the phospholipid which
would clearly have a destabilizing effect on the membrane
lipid bilayer matrix. Failure to form stable complexes with
phosphatidyle thanolamine may be explained on the same
basis as that us ed to explain why a-tocopherol readily forms
complexes with free fatty acids and lysophospholipids,
namely, the amphipatic balance within the complex is
conduciv e to bilayer formation. The predominantly hydrophobic character of phosphatidyleth anolamin e is matched by
that of a-tocopherol and a combination of the two can only
exist in non-bilayer phas es.
There is clearly compelling evidence that, although
pres ent in relatively minor proportions in biological membranes, a-tocopherol segregates in membranes and forms
complexes with specific lipid constituents . Thes e reactions
have the effect of stabilizing the lipid bilayer matrix. The
wider significanc e of thes e preferentia l interactions , especially in regard to the putative antioxidant function of atocopherol, remains to be established.
Acknowledgements
Financial support and beamtime for some of the experiments
described in this review was obtained from the Daresbury Laboratory.
X.W. was supported by a KC Wong Education Foundation Studentship.
References
Ahkong, Q. F., Fisher, D. Tampion, W. and Lucy, J. A., 1972, The
fusion of erythrocytes by fatty acids, esters, retinol and atocopherol. Biochemical Journal, 136, 147± 155.
Aranda, F. J., Coutinho, A., Berberan-Santos, M. N., Prieto, M. J. E.
and GoÂmez-FernaÂndez, J. C., 1989, Fluorescence study of the
location and dynamics of a-tocopherol in phospholipid vesicles .
Biochimica et Biophysica Acta, 985, 26± 32.
Aranda, F. J., Sanchez-Migallon, M. P. and GoÂmez-Ferna ndez, J.
C., 1996, Influence of a-tocopherol incorporation on Ca2+-induced
fusion of phosphotidylserine vesicles . Archives of Biochemistry
and Biophysics, 333, 394± 400.
Azzi, A. and Stocker, A., 2000, Vitamin E: non-antioxidant roles.
Progress in Lipid Research, 39, 231± 255.
Barclay, L. R. C., Locke, S. J. and Macneil, J. M., 1985, Autoxidation
in micellesÐ synergism of vitamin C with lipid-soluble vitamin E
and water-soluble trolox. Canadian Journal of Chemistry, 63, 366±
374.
Beharka, A., Redican, S., Leka, L. and Meydani, S. N., 1997, Vitamin
E status and immune function. Methods in Enzymology, 282, 247±
263.
Bellizzi, M., Dutta-Roy, A. K., Duthie, G. G. and James, W. P. T.,
1997a, Vitamin E binding activity of red blood cells in smokers.
Free Radical Research, 27, 105± 112.
153
Bellizzi, M., Dutta-Roy, A. K. and James, W. P. T., 1997b, High Dglucose does not affect binding of a-tocopherol to human
erythrocytes. Molecular and Cellular Biochemistry, 170, 187± 193.
Bisby, R. H. and Ahmed, S., 1989, Transverse distribution of atocopherol in bilayer membranes studied by fluorescence quenching. Free Radical Biology Medicine, 6, 231± 239.
Bohm, F., Edge, R., Land, E . J., McGarrey, D. J. and Truscott, T. G.,
1997, Carotenoids enhance vitamin E antioxidant efficiency.
Journal of American Chemistry Society, 119, 621± 622.
Bohm, F., Edge, R., McGarrey, D. J. and Truscott, T. G., 1998, Betacarotene with vitamins E and C offers synergistic cell protection
against Nox. FEBS Letter, 436, 387± 389.
Boscoboinik, D., Szewczyk, A. and Azzi, A., 1991a, a-Tocopherol
(vitamin E) regulates vascular smooth muscle cell proliferation and
protein kinase C activity. Archives of Biochemistry and Biophysics,
286, 264± 269.
Boscoboinik, D., Szewczyk, A., Hensey, C. and Azzi, A., 1991b,
Inhibition of cell proliferation by a-tocopherol. Role of protein
kinas e C. Journal of Biological Chemistry, 266, 6188± 6194.
Bowry, V. W. and Stocker, R., 1993, Tocopherol-mediated peroxidationÐ the prooxidant effect of vitamin E on the radical-initiated
oxidation of human low-density-lipoprotein. Journal of the American Chemical Society, 115, 6029± 6044.
Bretscher, M. S. and Munro, S., 1993, Cholesterol and the golgiapparatus. Science, 261, 1280± 1281.
Brigelius-Flohe, R. and Traber, M. G., 1999, Vitamin E: function and
metabolism. FASEB Journal, 13, 1145± 1155.
Brown, D. A. and London, E ., 1997, Structure of detergent-resistant
membrane domains: does phase separation occur in biological
membranes? Biochemical Biophysics Research Communication,
240, 1± 7.
Brown, R. E ., 1998, Sphingolipid organization in biomembranes:
what physical studies of model membranes reveal. Journal of Cell
Science, 111, 1± 9.
Burton, G. W. and Ingold, K. H., 1981, Autoxidation of biological
molecules. 1. The antioxidant activity of vitamin E and related
chain-breaking phenolic antioxidants in vitro. Journal of the
American Chemistry Society, 103, 6472± 6477.
Burton, G. W. and Ingold, K. U., 1986, Vitamin E: applications of the
principles of physical organic chemistry to the exploration of its
structureandfunction.AccountsofChemicalResearch,19,194± 201.
Burton, G. W., Doba, T., Gabe, E. J., Hughes, L., Lee, F. L., Prasad,
L. and Ingold, K. H., 1985, Autoxidation of biological molecules. 4.
Maximizing the antioxidant activity of phenols. Journal of the
American Chemistry Society, 107, 7053± 7065.
Buttriss , J. L. and Diplock, A. T., 1988, The a-tocopherol and
phospholipid fatty acid content of rat liver subcellular membranes
in vitamin E and selenium deficiency. Biochimica et Biophysica
Acta, 963, 61± 69.
Cachia, O., Benna, J. E., Pedruzzi, E., Descomps, B., GougerotPocidalo, M. A. and Leger, C. L., 1998, a-Tocopherol inhibits the
respiratory burst in human monocytes. Attenuation of p47(phox)
membrane translocation and phosphorylation. Journal of Biological Chemistry, 273, 32801± 32805.
Chan, A. C., 1998, Vitamin E and atherosclerosis . Journal of
Nutrition, 128, 1593± 1596.
Chan, A. C., Wagner, M., Kennedy, C., Chen, E., Lanuville, O., Mezl,
V. A., Tran, K. and Choy, P. C., 1998, Vitamin E up-regulates
arachidonic acid releas e and phospholipase A2 in megakaryocytes. Molecular Cell Biochemistry, 189, 153± 159.
Christen, S., Woodall, A. A., Shigenaga, M. K., Southwell-Keely, P.
T., Duncan, M. W. and Ames, B. N., 1997, c-Tocopherol traps
mutagenic electrophiles such as NOx and complements atocopherol: physiological implications . Proceedings of the National
Academy of Science (USA), 94, 3217± 3222.
Cohn, W., 1997, Bioavailability of vitamin E. European Journal of
Clinica l Nutrition, 51, 80± 85.
Cominacini, L., Garbin, U., Pasini, A. F., Davoli, A., Campagnola, M.,
Contessi, G. B., Pastorino, A. M. and Lo Cascio, V., 1997,
Antioxidants inhibit the expression of intercellula r cell adhesion
molecule-1 and vascular adhesion molecule-1 induced by oxidized
LDL on human umbilical vein endothelial cells . Free Radical
Biology and Medicine, 22, 117± 127.
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
154
X. Wang and P. J. Quinn
Cooney, R. W., France, A. A., Harwood, P. J., Hatch-Pigott, V.,
Custer, L. J. and Mordan, L. J., 1993, c-Tocopherol detoxification
of nitrogen dioxide: superiority to a-tocopherol.Proceedings of the
National Academy of Science (USA), 90, 1771± 1775.
Cooney, R. V., Harwood, P. J., Franke, A. A., Narala, K., Sundstrom,
A. K., Berggren, P. O. and Mordan, L. J., 1995, Products of gammatocopherol reaction with NO2 and their formation in rat insulinoma
(RINm5F) cells . Free Radical Biology and Medicine, 19, 259± 269.
Coquette, A., Vray, B. and Vanderpas, J., 1986, Role of vitamin E in
the protection of the resident macrophage membrane against
oxidative damage. Archives of International Physiology and
Biochemistry, 94 529± 533.
Creutz, C. C., 1981, Cis-unsaturated fatty acids induce the fusion of
chromaffin granules aggregated by synexin. Journal of Cell
Biology, 91, 247± 256.
DeKruijff, B., Van Dijck, P. W. M., Demel, R. A., Schuijff, A., Brants,
F. and Van Deenen, L. L. M., 1974, Non-random distribution of
cholesterol in phosphatidylcholine bilayers . Biochimica et Biophysica Acta, 356, 1± 7.
Devaraj, S., Li, D. and Jialal, I., 1996, The effects of a-tocopherol
supplementation on monocyte function. Decreased lipid oxidation,
interleukin 1 b secretion, and monocyte adhesion to endothelium.
Journal of Clinical Investigation, 98, 756± 763.
Diplock, A. T. and Lucy, J. A., 1973, The biochemical modes of
action of vitamin E and selenium: an hypothesis. FEBS Letters,
29, 205± 210.
Dutta-Roy, A. K., 1997, a-Tocopherol-binding proteins: purification
and characterization. Methods in Enzymology, 282, 278± 297.
Dutta-Roy, A. K., 1999, Molecular mechanism of cellular uptake and
intracellula r translocation of a-tocopherol: role of tocopherolbinding proteins. Food and Chemical Toxicology, 37, 967± 971.
Dutta-Roy, A. K., Gordon, M. J., Campbell, F. M., Duthie, G. G. and
James, W. P. T., 1994, Vitamin E requirements, transport and
metaboilsmÐ role of a-tocopherol-binding proteins. Journal of
Nutrition Biochemistry, 5 562± 570.
Dutta-Roy, A. K., Gordon, M. J., Leishman, D. J., Paterson, B. J.,
Duthie, G. G. and James, W. P. T., 1993b, Purification and partial
characterization of an a-tocopherol-binding protein from rabbit
heart cytosol. Molecular Cell Biochemistry, 123, 139± 144.
Dutta-Roy, A. K., Leishman, D. J., Gordon, M. J., Campbell, F. M.
and Duthie, G. G., 1993a, Identifica tion of a low molecular mass
(14.2 kDa) a-tocopherol-binding protein in the cytosol of rat liver
and heart. Biochemistry and Biophysics Research Communications, 196, 1108± 1112.
Ekiel, I. H., Hughes, L., Burton, G. W., Joval, P. A., Ingold, K. U. and
Smith, I. C. P., 1988, Structure and dynamics of a-tocopherol in
model membranes and in solution. A broadline and high-resolution
NMR study. Biochemistry, 27, 1432± 1440.
Ernster, L., Forsmark, P. and Nordenbrand, K. 1992, The mode of
action of lipid-soluble antioxidants in biological membranes:
relationship between the effects of ubiquinol and vitamin E as
inhibitors of lipid peroxidation in submitochondrial particles.
Journal of Nutritional Science and Vitaminology, 548, 41± 46.
Fragata, M. and Bellemere, F., 1980, Model for singlet oxygen
scavenging by a-tocopherol in biomembranes. Chemistry and
Physics of Lipids, 27, 93± 99.
Freedman, J. E., Farhat, J. H., Loscalzo, J. and Keaney, J. F., 1996,
Alpha-tocopherol inhibits aggregation of human platelets by a
protein kinase C-dependent mechanism. Circulation, 15, 2434±
2440.
Fukuzawa, K., Matsuura, K., Tokumura, A., Suzuki, A. and Terao, J.,
1997, Kinetics and dynamics of singlet oxygen scavenging by atocopherol in phospholipid model membranes. Free Radical
Biology and Medicine, 22, 923± 930.
GoÂmez-Ferna ndez, J. C., Aranda, F. C. and Villalõ  an, J., 1991,
Location and dynamics of a-tocopherol in membranes. In Progress
in Membrane Biotechnology, J. C. GoÂmez-Ferna ndez, D. Chapman and L. Packer, eds (Basel: Birkhauser Verlag), pp. 98± 115.
GoÂmez-Ferna ndez, J. C., Villalõ  an, J., Aranda, F. J., Ortiz, A., Micol,
V., Coutinho, A., Berberan-Santos, M. N. and Prieto, M. J. E .,
1989, Localization of a-tocopherol in membranes, Annals of the
New York Academy of Science, 570, 109± 120.
Gordon, M. J., Campbell, F. M., Duthie, G. G. and Dutta-Roy, A. K.,
1995, Characterisation of a novel a-tocopherol-binding protein
from bovine heart cytosol. Archives of Biochemistry and Biophysics, 318, 140± 146.
Grau, A. and Ortiz, A., 1989, Dissimilar protection of tocopherol
isomers against membrane hydrolysis by phospholipase A2.
Chemistry and Physics of Lipids, 91, 109± 118.
Harder, T., Scheiffele, P., Verkade, P. and Simons, K., 1998, Lipid
domain structure of the plasma membrane revealed by patching of
membrane components. Journal of Cell Biology, 141, 929± 942.
Hidiroglou, N., Hayward, S., Behrens, W. and Madere, R., 1997,
Vitamin E levels in superficial and intra-abdominal location of
white adipose tissue in the rat. Journal of Nutrition Biochemistry,
8, 392± 396.
Ingold, K. U., Bowry, V. W., Stocker, R. and Walling, C., 1993,
Autoxidation of lipids and antioxidation by a-tocopherol and
ubiquinol in homogeneous solution and in aqueous dispersions
of lipidsÐ unrecognized consequences of lipid particle-size as
exemplified by oxidation of human low-density-lipoprotein.
Proceedings of the National Academy of Science (USA), 90,
45± 49.
IUPAC, 1974, Recommended reference materials for the realization
of physicochemical properties . Pure and Applied Chemistry, 40,
393± 472.
Kagan, V. E., 1989, Tocopherol stabilizes membrane against
phospholipase A, free fatty acids, and lysophospholipids. Annals
of the New York Academy of Science, 570, 121± 135.
Kagan, V. E. and Quinn, P. J., 1988, The interaction of a-tocopherol
and homologues with shorter hydrocarbon chains with phospholipid dispersions, a fluorescence probe study. European Journal of
Biochemistry, 171, 661± 667.
Kagan, V. E., Serbinora, E., Fork, T., Scita, G. and Packer, L., 1992,
Recycling of vitamin E in human low-density lipoproteins. Journal
of Lipid Research, 33, 385± 397.
Kagan, V. E., Stoyanovsky, D. O. and Quinn, P. J., 1994, Integrated
functions of coenzyme Q and vitamin E in antioxidant action. In
Free Radicals in the Environment, Medicine and Toxicology, H.
Nohl, H. Esterbauer and C. Rice-Evans, eds (London: Richelieu
Press), pp. 221± 248.
Kamal-Eldin, A., and Appelqvist, L. A., 1996, The chemistry and
antioxidant properties of tocopherol and tocotrienols . Lipids, 31,
671± 701.
Keller, P. and Simons, K., 1998, Cholesterol is required for surface
transport of influenza virus hemagglutinin. Journal of Cell Biology,
140, 1357± 1367.
Koya, D., Haneda, M., Kikkawa, R. and King, G. L., 1998, d-aTocopherol treatment prevents glomerular dysfunctions in diabetic
rats through inhibition of protein kinase C-diacylglycerol pathway.
Biofactors, 7, 69± 76.
Koya, D., Lee, I. K., Ishii, H., Kanoh, H. and King, G. L., 1997,
Prevention of glomerular dysfunction in diabetic rats by treatment
with d-a-tocopherol. Journal of the American Society of Nephrology, 8, 426± 435.
Kuhlenkamp, J., Ronk, M., Yusin, M., Stolz, A. and Kaplowitz, N.,
1993, Identifica tion and purification of a human liver cytosolic
tocopherol binding protein. Protein E xpression and Purification, 4,
382± 389.
Kunisaki, M., Bursell, S. E ., Umeda, F., Nawata, H. and King, G. L.,
1994, Normalization of diacylglycerol-protein kinase C activation
by vitamin E in aorta of diabetic rats and cultured rat smooth
muscle cells exposed to elevated glucose levels . Diabetes, 43,
1372± 1377.
Lefevre, T. and Picquart, M., 1996, Vitamin E Ð phospholipid
interaction in model multiplayer membranes: a spectroscopic
study. Biospectroscopy, 2, 391± 403.
Liebler, D. C., 1993, The role of metabolism in the antioxidant
function of vitamin E. Critical Reviews in Toxicology, 23, 147± 169.
Livrea, M. A. and Tesorieve, L., 1999, Interactions between vitamin A
and vitamin E in liposomes and in biological contexts. Methods in
Enzymology, 299, 421± 430.
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
Vitamin E in membranes
Lucy, J. A., 1972, Functional and structural aspects of biological
membranes: a suggested structural role for vitamin E in the control
of membrane permeability and stability. Annals of the New York
Academy of Science, 203, 4± 11.
Machlin, L. J., 1992, Beyond deficiency. New views on the function
and health effects of vitamins. Annals of the New York Academy of
Sciences, 669, 1± 6.
Machlin, L. J., Gabriel, E. and Brin, M., 1982, Biopotency of atocopherol as determined by curative myopathy bioassay in the
rat. Journal of Nutrition, 112, 1437± 1440.
Maguire, J. J., Kagan, V. E., Serbinova, E. A., Ackrell, B. A. and
Packer, L., 1992, Succinate ubiquinone reductase linked recycling
of a-tocopherol in reconstituted systems and mitochondriaÐ
requirement for reduced ubiquinone. Archives of Biochemistry
and Biophysics, 292, 47± 53.
Massey, J. B., She, H. S. and Pownall, H. J., 1982, Interaction of
vitamin E with saturated phospholipid bilayers . Biochemistry and
Biophysics Research Communications, 106, 842± 847.
Mayer, H., Schudel, P., Ruegg, R. and Isler, O., 1963, Helvitica
Chimica Acta, 46, 963± 699.
McMurchie, E. J. and McIntosh, G. H., 1986, Thermotropic
interaction of vitamin E with dimyristoyl and dipalmitoylphosphatidyl choline liposomes. Journal of Nutritional Science and
Vitaminology, 32, 551± 558.
Meydani, S. N. and Tengerdy, R. P., 1991, Vitamin E and the
immune system. In Vitamin E: Biochemical and clinica l application, L. Packer and J. Fuchs, eds (New York: Marcel Dekker),
pp. 549± 554.
Micol, V., Aranda, F. J., Villalõ  an, J. and GoÂmez Ferna ndez, J. C.,
1990, Influence of vitamin E on phosphatidylethanolamine lipid
polymorphism. Biochimica et Biophysica Acta, 1022, 194± 202.
Mukai, K., Itoh, S. and Morimoto, H., 1992, Stopped-flow kineticstudy of vitamin E regeneration reaction with biological hydroquinones (reduced forms of ubiquinone, vitamin K, and tocopherolquinone) in solution. Journal of Biological Chemistry, 267,
22277± 22281.
Mukherjee, A. K., Ghosal, S. K. and Maity, C. R., 1997, Lysosomal
membrane stabilization by a-tocopherol against the damaging
action of Vipera russelli venom phospholipase A2 . Cellular and
Molecular Life Sciences, 53, 152± 155.
Murata, M., Peranen, J., Schreiner, R., Wieland, F., Kurzchalia, T. V.
and Simons, K., 1995, VIP21/caveolin is a cholesterol-binding
protein. Proceedings of the National Academy of Sciences (USA),
92, 10339± 10343.
Navarro, F., Navas, P., Burgess, J. R., Bello, R. I., De Cabo, R.,
Arroyo, A. and Villalba, J. M., 1998, Vitamin E and selenium
deficiency induces expression of theubiquinone-dependent antioxidant system as the plasma membrane. FASEB Journal, 12,
1665± 1673.
Neuzil, J. and Stocker, R., 1994, Free and albumin-bound bilirubin
are efficient co-antioxidants for a-tocopherol, inhibiting plasma and
low-density-lipoprotein lipid-peroxidation. Journal of Biological
Chemistry, 269, 16712± 16719.
Niki, E., Kawakami, A., Saito, M., Yamamoto, Y., Tsuchiya, J. and
Kamiya, Y., 1985, Effect of phytol side chain of vitamin E on its
antioxidant activity. Journal of Biological Chemistry, 260, 2191±
2196.
Ohyashiki, T., Ushiro, H. and Mohri, T., 1986, Effects of a-tocopherol
on the lipid peroxidation and fluidity of porcine intestina l brush
border membranes. Biochimica et Biophysica Acta, 858, 294± 300.
Ortiz, A. and GoÂmez-Ferna ndez, J. C., 1988, Calcium-induced
aggregation of phosphatidylcholine vesicles containing free oleic
acid. Chemistry and Physics of Lipids, 46, 259± 266.
Ortiz, A., Aranda, F. J., and GoÂmez-Ferna ndez, J. C., 1987, A
differentia l scanning calorimetric study of the interaction of atocopherol with mixtures of phospholipids. Biochimica et Biophysica Acta, 898, 214± 222.
Perly, B., Smith, I. C. P., Hughes, L., Burton, G. W. and Ingold, K. U.,
1985, Estimation of the location of natural a-tocopherol in lipid
bilayers by 13 C-NMR spectroscopy. Biochimica et Biophysica
Acta, 819, 131± 135.
155
Podda, M., Weber, C., Traber, M. G. and Packer, L., 1996,
Simultaneous determination of tissue tocopherols, tocotrienols,
ubiquinols andubiquinones. JournalofLipidResearch, 37, 893± 901.
Pyke, D. D. and Chan, A. C. 1990, Effects of vitamin E on
prostacyclin releas e and lipid composition of the ischemic rat
heart. Archives of Biochemistry Biophysics, 277, 429± 433.
Quinn, P. J., 1995, Characterisation of clusters of a-tocopherol in gel
and fluid phases of dipalmitoylphosphatidylcholine. European
Journal of Biochemistry, 233, 916± 925.
Quinn, P. J., 1998, Localization of vitamin E in membranes. In FatSoluble Vitamins, Subcellular Biochemistry, P. J. Quinn and V. E .
Kagan, eds (New York and London: Plenum Press), Vol. 30, 319±
343.
Ricciarelli, R., Tasinato, A., Clement, S., Ozer, N. K., Boscobo-inik,
D. and Azzi, A., 1998, a-Tocopherol specifically inactivates cellular
protein kinase C alpha by changing its phosphorylation state.
Biochemical Journal, 334, 243± 249.
Rietveld, A. and Simons, K., 1998, The differentia l miscibility of lipids
as the basis for the formation of functional membrane rafts .
Biochimica et Biophysica Acta, 1376, 467± 479.
Salgado, J., Villalõ  an, J. and GoÂmez-Ferna ndez, J. C., 1993a, Magicangle spinning C-13 NMR spin-lattice relaxation study of the
location and the effects of a-tocopherol, ubiquinone-10 and
ubiquinol-10 in unsonicated model membranes. European Biophysical Journal, 22, 151± 155.
Salgado, J., Villalõ  an, J. and GoÂmez-Ferna ndez, J. C., 1993b, aTocopherol interacts with natural micelle-forming single-chain
phospholipids stabilizing the bilayer phase. Archives of Biochemistry and Biophysics, 306, 368± 376.
Sanchez-Migallon, M. P., Aranda, F. J., GoÂmez-Ferna ndez, J. C.,
1996a, Interaction between a-tocopherol and heteroacid phosphatidylcholines with different amounts of unsaturation. Biochimica et Biophysica Acta, 1279, 251± 258.
Sanchez-Migallon, M. P., Aranda, F. J. and GoÂmez-Ferna ndez, J.
C., 1996b, The interaction of a-tocopherol with phosphatidylserine
vesicles and calcium. Biochimica et Biophysica Acta, 1281, 23±
30.
Sato, Y., Arai, H., Myata, A., Tokita, S., Yamamoto, K., Tanabe, T.
and Inoue, K., 1993, Primary structure of a-tocopherol transfer
protein from rat liver. Homology with retinaldehyde-binding
protein. Journal of Biological Chemistry, 268, 17705± 17710.
Severcan, F., 1997, Vitamin E decreases the order of the
phospholipid model membranes in the gel phase: an FTIR study.
Bioscience Report, 17, 231± 235.
Severcan, F. and Cannistraro, S., 1988, Direct eletron spin
resonance evidence for a-tocopherol-induced phase separation
in model membranes. Chemistry and Physics of Lipids, 47, 129±
133.
Severcan, F. and Cannistraro, S., 1990, A spin-label ESR and
saturation transfer ESR study of a-tocopherol containing model
membranes. Chemistry and Physics of Lipids, 53, 17± 26.
Sheppard, A. J., Pennington, J. A. T. and Weihrauch, J. L., 1993,
Analysis and distribution of vitamin E in vegetable oils and foods.
In Vitamin E in Health and Disease, L. Packe and J. Fuchs, eds
(New York: Marcel Dekker), pp. 9± 31.
Simons, K. and Ikonen, E., 1997, Functional rafts in cell membranes.
Nature, 387, 569± 572.
Singer, S. J. and Nicolson, G. L., 1972, The fluid mosaic model of the
structure of cell membranes. Science, 175, 720± 730.
Srivastava, S., Phadke, R. S., Govil, G. and Rao, C. N. R., 1983,
Fluidity, permeability and antioxidant behaviour of model membranes incorporated with a-tocopherol and vitamin E acetate.
Biochimica et Biophysica Acta, 734, 353± 362.
Stillwell, W., Dallman, T., Dumaual, A. C., Crump, F. T. and Jenski,
L. J., 1996, Cholesterol versus a-tocopherolÐ effects on properties
of bilayers made from heteroacid phosphatidylcholines. Biochemistry, 35, 13353± 13362.
Stoyanovsky, D. A., Osipov, A. N., Quinn, P. J. and Kagan, V. E .
1995, Ubiquinone-dependent recycling of vitamin-e radicals by
superoxide. Archives of Biochemistry and Biophysics, 323, 343±
351.
Mol Membr Biol Downloaded from informahealthcare.com by Kings College London on 11/22/12
For personal use only.
156
X. Wang and P. J. Quinn
Surai, P. F., Noble, R. C. and Speake, B. K., 1999, Relationship
between vitamin E content and susceptibility to lipid peroxidation
in tissues of the newly hatched chick. Britis h Poultry Science, 40,
406± 410.
Suzuki, Y., Tsuchiya, M., Wassall, S. R., Govil, G., Kagan, V. E. and
Packer , L., 1993, Structural and dynamic membrane properties of
a-tocopherol and a-tocotrienol. Implication to the molecular
mechanism of their antioxidant potency. Biochemistry, 32,
10692± 10699.
Traber, M. G. and Arai, H., 1999, Vitamin E in humans: demand and
delivery. Annual Review of Nutrition, 16, 321± 347.
Traber, M. G. and Kayden, H. J., 1987, Tocopherol distribution and
intracellula r localization in human adipose tissue. American
Journal of Clinical Nutrition, 46, 488± 495.
Traber, M.G., Ramakrishnan, R and Kayden, J.H., 1994, Human
plasma vitamin E kinetics demonstrate rapid recycling of plasma
RRR-a-tocopherol. Proceedings of the National Academy of
Sciences, U.S.A., 91, 10005± 10008.
Tran, K. and Chan, A., 1990, R, R, R-a-tocopherol potentiates
prostacyclin release in human endothelial cells. Evidence for
structural specificity of the tocopherol molecule. Biochimica et
Biophysica Acta, 1043, 189± 197.
Tran, K. T., Proulx, P. and Chan, A. C., 1994, Vitamin E suppresses
diacylglycerol (DAG) level in thrombin-stimulated endothelial cells
through an increase of DAG kinase activity. Biochimica et
Biophysica Acta, 1212, 193± 202.
Urano, S., Iida, M., Otani, I. and Matsuo, M., 1987, Membrane
stabilization of vitamin E; interactions of a-tocopherol with
phospholipids in bilayer liposomes. Biochemical Biophysics
Research Communication, 146, 1413± 1418.
Urano, S., Kitahara, M., Kato, Y., Hasegawa, Y. and Matsuo, M.,
1990, Membrane stabilizing effect of vitamin E. Existance of a
hydrogen bond between a-tocopherol and phospholipids in bilayer
liposomes. Journal of Nutrition Science Vitamins, 36, 513± 519.
Urano, S., Matsuo, M., Sakanaka, T., Uemura, I., Koyama, M.,
Kumadaki, I. and Fukuzawa, K., 1993, Mobility and molecular
orientation of vitamin E in liposomal membranes as determined by
19
F-NMR and fluorescence. Archives of Biochemistry and
Biophysics, 303, 10± 14.
Urano, S., Yano, K. and Matsuo, M., 1988, Membrane stabilizing
effect of a-tocopherol and its model compounds on fluidity of
lecithin liposomes. Biochemical and Biophysics Research Communication, 150, 469± 475.
Vaz, W. L. C. and Almeids, P. F. F., 1993, Phase topology and
percolation in multi-phase lipid bilayers: is the biological membrane a domain mosaic? Current Opinion in Structural Biology, 3,
482± 488.
Viana, B., Barbas, C., Castro, M., Herrera, E . and Bonet, B., 1999, aTocopherol concentration in fetal and maternal tissues of pregnant
rats supplemented with a-tocopherol. Annals of Nutrition and
Metabolism, 43, 107± 112.
Villalõ  an, J., Aranda, F. J. and GoÂmez-Ferna ndez, J. C., 1986,
Calorimetric and infrared spectroscopic studies of the interaction
of a-tocopherol and a-tocopherol acetate with phospholipid
vesicles. European Journal of Biochemistry, 158, 141± 147.
Wang, X., 2000. The effect of vitamin E on the structure and phase
behaviour of phospholipid model membranes. PhD Thesis,
University of London.
Wang, X. and Quinn, P. J., 1999a, Vitamin E and its function in
membranes. Progress in Lipid Research, 38, 309± 336.
Wang, X. and Quinn, P. J., 1999b, Inverted hexagonal and cubic
phases induced by a-tocopherol in fully hydrated dispersions of
dilauroylphosphatidylethanolamine. Biophysical Chemistry, 80,
93± 101.
Wang, X. and Quinn, P. J., 1999c, The effect of a-tocopherol on the
thermotropic phase behaviour of dipalymitoylphosphatidylethanolamineÐ A synchrotron X-ray diffraction study. European Journal
of Biochemistry, 264, 1± 8.
Wang, X. and Quinn, P. J., 2000a, The distribution of a-tocopherol in
mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine. Biochemica et Biophysica Acta, in press.
Wang, X. and Quinn, P. J., 2000b, Preferentia l interaction of atocopherol with phosphatidylcholines in mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine.
European Journal of BIochemistry, 267, 1± 8.
Wang, X., Semmler, K., Richter, W. and Quinn, P. J., 2000, Ripple
phases induced by a-tocopherol in saturated diacylphosphatidylcholines. Archives of Biochemistry and Biophysics, 377, 304± 314.
Wang, X., Takahashi, H., Hatta, I. and Quinn, P. J., 1999, An X-ray
diffraction study of the effect of a-tocopherol on bilayers of
dimyristoylphosphatidylethanolamine. Biochimica et Biophysica
Acta, 1418, 335± 343.
Wassall, S. R., Thewalt, J. L., Wong, L., Gorrisen, H. and Cushley,
R. J., 1986, Deuterium NMR-study of the interaction of atocopherol with a phospholipid model membrane. Biochemistry,
25, 319± 326.
Wassall S. R., Wang, L., McCabe, R. C., Ehringer, W. D. and
Stillwell, W., 1991, Electron spin resonance study of the
interaction of a-tocopherol with phospholipid model membranes.
Chemistry and Physics of Lipids, 60, 29± 37.
Wolf, G., 1994, Structure and possible function of an a-tocopherolbinding protein. Nutrition Review, 52, 97± 98.
Zhang, Y., Turunen, M. and Appelkvist, E. L., 1996, Restricted
uptake of dietary coenzyme Q is in contrast to the unrestricted
uptake of a-tocopherol into rat organs and cells . Journal of
Nutrition, 126, 2089± 2097.