BASIC SCIENCE
Review Article
Fatty Acids
Biochemistry to Clinical Significance
MICHAEL LAPOSATA, MD, P H D
Fatty acids are a major source of lipids in the diet. Dietary fatty acids
are able to significantly influence the concentration of serum cholesterol
and thereby influence the risk of atherosclerosis. This brief review on
fatty acids will present information on the structure and nomenclature
The Western diet generally includes at least 30% to 40%
of calories as fat, mostly from triglycerides in oils, margarines, dairy products, and meats.1 Because >90% of
the mass of a triglyceride molecule is accounted for by
its 3 fatty acids, fatty acids are the major dietary lipid.
Cholesterol, contained in animal-derived products, is the
other major dietary fat. Dietary fatty acids significantly
influence the level of serum cholesterol and, thereby, influence the risk of atherosclerosis. This brief review of
fatty acids from biochemistry to clinical significance is
intended for physicians and scientists outside thefieldof
lipid biochemistry. I will first present the nomenclature
of fatty acids and fatty acid containing lipids to acquaint
the reader with the language in thefield.This will be followed by a discussion of the different metabolic pathways for fatty acids in cells. Finally, current evidence for
the atherogenicity of individual dietary fatty acids will
be presented, along with information on the fatty acid
content of commonly used dietary oils.
of fatty acids, the metabolic pathways for fatty acids in cells, and the
influence of dietary fatty acids on serum cholesterol levels. (Key words:
Fatty acids; Lipids; Atherosclerosis; Dietary fat) Am J Clin Pathol
1995;104:172-179.
acids exist, there are five common animal fatty acids
(Fig. 1). These fatty acids have both a familiar name and
a numerical identifier. In the numerical designation, the
number to the left of the colon indicates the number of
carbons in the fatty acid. The number immediately to
the right of the colon indicates the number of double
bonds in the fatty acid. The number after the delta sign
indicates the location of the double bonds counting from
the carbon at the carboxyl end of the fatty acid, which is
the number 1 carbon. Thus, 18:2 A 9, 12 is a fatty acid of
18 carbons with 2 double bonds that are between the
ninth and tenth and between the 11th and 12th carbons
in the fatty acid molecule. Its familiar name is linoleate
or linoleic acid. The carboxyl terminus of the fatty acid
is reactive and is the end of the molecule that binds the
fatty acid to other molecules.
Fatty acids can be grouped in families according to the
number of double bonds in the fatty acid. Fatty acids
with no double bonds are known as saturated fatty acids;
those with one double bond are called monounsaturated;
STRUCTURES A N D NOMENCLATURE
and those with two or more double bonds are called polyThe citations for the information in this section of the
unsaturated fatty acids. In oils and margarines, certain
report are several well-written review articles and monofatty acids are in higher concentration than others, and
graphs on fatty acid metabolism.2"5
based on the predominant fatty acids in these products,
oils are known primarily as polyunsaturated, monounFatty Acids
saturated, or saturated (Table 1). For monounsaturated
and polyunsaturated fatty acids, another classification,
Fatty acids are long chain molecules that have a carknown as the omega classification, is frequently used.
boxyl and a methyl end. Although many different fatty
The omega group is determined by the number of carbons between the methyl end of the molecule and the
From the Division of Clinical Laboratories, Department of Pathol- nearest double bond. For example, if the double bond
ogy, Massachusetts General Hospital/Harvard Medical School, Bosnearest the methyl end of the molecule is 3 carbons from
ton, Massachusetts.
the methyl end, the fatty acid is known as an omega-3 or
Manuscript received June 7, 1995; accepted June 7, 1995.
n-3 fatty acid. These fatty acids are found in large quanAddress reprint requests to Dr. Laposata: Clinical Laboratories, Mastities in fish. For omega-6 fatty acids, the final double
sachusetts General Hospital, Boston, MA 02115.
172
LAPOSATA
Fatty Acids
173
CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -C -OH
3 2 2 2 2 2 2 2 2 2 2 2 2 2 2
PALMITIC ACID 16:0
SATURATED FATTY ACID
0
II
CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -CH -C -OH
3 2
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
STEARIC ACID 18:0
SATURATED FATTY ACID
FIG. 1. Structure and nomenclature of common animal fatty acids.
CH
. C H . C H _CH _CH . C H _CH _CH _ CH
3 2 2 2 2 2 2 2
=CH
0
_CH . C H . C H _c H . C H . C H . C H . c . 0 H
2 2 2 2 2 2 2
OLEIC ACID 18:1 A 9 n-9 OR OMEGA- 9 FAMILY
MONOUNSATURATED FATTYACID
Q
II
CH -CH -CH -CH -CH -CH = CH -CH -CH = CH -CH -CH -CH -CH -CH -CH -CH -C -OH
3 2 2 2 2
2
2 2 2 2 2 2 2
UNOLEICACID
18:2 A 9,12
n-6 OR OMEGA-6 FAMILY
POLYUNSATURATED FATTY ACID
Q
II
CH -CH -CH-CH -CH -CH = CH -CH -CH = CH -CH -CH = CH -CH -CH = CH -CH -CH -CH -C-OH
3 2 2 2 2
2
2
2
2 2 2
ARACH1DONICACID 20:46 5, 8, 11, 14 n-6 OR OMEGA-6 FAMILY
POLYUNSATURATED FATTY ACID
bond is 6 carbons from the methyl end. Four major families of unsaturated fatty acids exist in animals—omega3, omega-6, omega-9, and omega-7 fatty acids. These are
also known as n-3, n-6, n-9, and n-7 fatty acids. The
omega-6 and omega-3 fatty acids are called essential fatty
acids because they cannot be generated from precursor
molecules in the human metabolic system, and therefore
must be obtained in the diet. There is an additional classification for unsaturated fatty acids that relates to the
stereochemistry of the hydrogens around double bonds.
The vast majority of fatty acids in nature have the hydrogen atoms across double bonds arranged in a cis configuration. However, the configuration can also occur in
a trans form. Trans fatty acids are present in the triglycerides of margarine in large amounts as a result of the
manufacturing progress. Trans fatty acids have been
shown to elevate the level of serum cholesterol.6 Cis double bonds introduce a bend in the fatty acid molecule,
but trans double bonds do not. Therefore, trans fatty
acids have the same straight chain three dimensional
structure as saturated fatty acids. It is thought that this
similarity in three dimensional structure may be in part
responsible for the atherogenic properties of trans fatty
acids.
Lipids Containing Fatty Acids
Triglycerides in cells are present primarily in large droplets because they are extremely hydrophobic. In the
plasma, triglycerides are transported within the core of li-
TABLE 1. FATTY ACID COMPOSITION OF COMMON COOKING OILS AND FATS'
Familiar Name
Fatty Acid
Butter
Butyric
Caproic
Caprylic
Capric
Laurie
Myristic
Palmitic
Stearic
Palmitoleic
Oleic
Gadoleic
Erucic
Linoleic
Linolenic
4:0
6:0
8:0
10:0
12:0
14:0
16:0
18:0
16:1
18:1
20:1
22:1
18:2
18:3
2.6
1.6
0.9
2.0
2.3
8.2
21.3
9.8
1.8
20.4
1.8
1.2
Coconut Oil
0.6
7.5
6.0
44.6
16.8
8.2
2.8
5.8
1.8
Palm Oil
Olive Oil
Canola Oil
Corn Oil
0.1
1.0
43.5
4.3
0.3
36.6
0.1
11.0
2.2
0.8
72.5
0.3
10.9
1.8
9.1
0.2
7.9
0.6
4.8
1.5
0.5
53.2
1.0
0.2
22.2
11.0
* Each amount is per 100 g of edible portion.
Vol. 104-No. 2
24.2
58.0
0.7
Soybean Oil
0.1
10.3
3.8
0.2
22.8
0.2
51.0
6.8
174
BASIC SCIENCE
Review Article
poproteins, which protects the hydrophobic triglyceride
from interacting with the aqueous environment of the
blood. A second major fatty acid containing lipid is phospholipid. Phospholipids contain two fatty acids, a phosphate group, and a head group. The head groups for the
four most common phospholipids are choline, ethanolamine, inositol, and serine. These account for the names
of the four major phospholipids-phosphatidylcholine
(PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidylserine (PS). Phospholipids in
cells are primarily components of membranes. Many
functional properties of the membrane are influenced by
the fatty acids within the phospholipids. Saturated fatty
acids decrease the fluidity of the membrane, and polyunsaturated fatty acids produce more a fluid membrane
structure. In the plasma, phospholipids are found in the
unilamellar outer shell of the lipoproteins because they
have polar head groups that can interact with the aqueous
environment. Although there are dietary sources of phospholipids, such as meats, most of the phospholipid in the
body is synthesized endogenously by esterification of two
fatty acids onto a glycerol-3-phosphate backbone with the
subsequent addition of a head group. The third fatty acid
containing lipid found in vivo is cholesteryl ester. Cholesteryl esters are molecules of cholesterol esterified with a
fatty acid. Sixty percent to 80% of the cholesterol in blood
is esterified, and there are enzymes in blood and in cells
that catalyze the esterification of fatty acid to cholesterol
to form cholesteryl esters. Cholesteryl esters are present in
the core of plasma lipoproteins because the solubility of
cholesteryl esters in aqueous medium is extremely low. In
cells, cholesteryl esters, like triglycerides, form lipid droplets. A small amount of fatty acid in cells is also present as
unesterified or nonesterified or free fatty acids. Nonesterified fatty acids are present in the blood bound to albumin.
METABOLIC PATHWAYS OF FATTY ACIDS
Figure 2 is a summary of the metabolic pathways described in this section. The topics discussed in this section are thoroughly reviewed in references 2, 3,4, 5, and
7. Additional citations are included for those points not
considered in the reviews.
Fatty Acid Uptake
The uptake of fatty acids into cells (Pathway 1, Fig. 2)
is relatively nonspecific, thereby permitting incorporation into cells of a wide variety of fatty acids present in
the plasma. Fatty acids are made available to cells by delivery from albumin and by hydrolysis of triglycerides
within the core of lipoproteins. Lipoprotein lipase on the
blood vessel wall degrades the lipoprotein associated tri-
glycerides to free fatty acids for uptake by cells. For the
most part, the fatty acid uptake process is not energy dependent. However, there appears to be involvement of
certain proteins in fatty acid uptake that require ATP for
fatty acid transport across the membrane. The capacity
of cells to incorporate free fatty acids is very high and
excess fatty acid within the cell is stored as triglyceride
that accumulates in cytoplasmic lipid droplets.
Fatty Acid Synthesis
The de novo synthesis of fatty acids (Pathway 2, Fig. 2)
is relatively low in most cells, because cells tend to rely
primarily on fatty acids incorporated from the plasma.
In the biosynthesis of fatty acids, a long chain saturated
fatty acid, primarily palmitate (16:0), is the major end
product. Fatty acids are synthesized by the successive
condensations of 2-carbon units. Acetyl-CoA is carboxylated to a 3-carbon fragment, malonyl-CoA, by the action of acetyl-CoA carboxylase. This is the rate limiting
enzyme in fatty acid synthesis. Malonyl-CoA is the immediate substrate for addition to the growing fatty acid
backbone. Of the 3 carbons in malonyl-CoA, with each
condensation cycle, 2 carbons are added to the fatty acid
being synthesized and the third carbon is lost as C0 2 .
The fatty acid synthetase of animals, once thought to be
a large aggregate of individual enzymes in multienzyme
complexes, is now known to be two polypeptides with
many different catalytic functions.
Synthesis of Fatty Acyl-CoA
Most metabolic reactions of fatty acids in cells involve
fatty acids in their activated or fatty acyl-CoA forms
(Pathway 3, Fig. 2). All cells contain enzymes known as
fatty acyl-CoA synthetases or fatty acid-CoA ligases to
convert fatty acids to their corresponding fatty acylCoAs. One enzyme, known as nonspecific long chain
acyl-CoA synthetase, is capable of activating a range of
fatty acids from 14 carbons to 22 carbons in length. A
separate acyl-CoA synthetase, specific for arachidonate
and other fatty acids which can be oxygenated to eicosanoids (discussed subsequently), has been shown to be distinct from the nonspecific long chain acyl-CoA synthetase.8 Evidence also exists for a fatty acyl-CoA synthetase
specific for long chain fatty acids greater than 22 carbons
in length.9 The acyl-CoA synthetases are membrane
bound enzymes in the microsomes of cells.
Esterification of Fatty Acids Into Phospholipids,
Triglycerides, and Cholesteryl Esters
Fatty acid which enter cells are preferably esterified
into phospholipids (Pathway 4, Fig. 2). When provided
A.J.C.P.-August 1995
LAPOSATA
175
Fatty Acids
ALBUMIN-FATTY ACIDS
TRIGLYCERIDES
FATTY ACID
LIPOPROTEIN
LIPASE
) :
EXTRACELLULAR
INTRACELLULAR
NONESTERDTED
FATTY ACIDS
FATTY ACID
ACYLATED
PROTEINS
FATTY ACTO
POLYUNSATURATED
FATTY ACIDS
i
FATTY ACID
ETHYL ESTERS
BSTERJFlAATION
FIG. 2. Intracellular fatty acid metabolism. The
numbers in thefigurecorrespond to numbers
in the text describing the metabolic pathway.
9
MITOCHONDRIAL
BETA-OXIDATION:
FA &22 CARBONS .
if
11
UNSATURATED
FATTY ACYL-CoA( i 1 DOUBLE BONDS) DESATURATION
1
' T
ELONGATION T
I DESATURATION
I FATTY ACYL-CoA
PEROXISOMAL
,
" ^ l l BETA-OXIDATION:
I
I | FA>32CARBONS
6
OXYGENATED
METABOLITES
W
1
I
|
FATTYACW
SYNTHESIS
ACETYL-CoA
ACETYL-CoA
ACETYL-CoA
ESTBRIFICATION
ELONGATED
FATTY ACYL-CoA
ELONCA1 <ON
(
PHOSPHOLIPIDS
in excess, they are incorporated into newly synthesized
triglycerides and stored as lipid droplets in the cells. Very
little of the fatty acid in cells is used to esterify cholesterol
and form cholesteryl ester, the storage form of cholesterol in the cells. The immediate fatty acid substrate for
esterification into phospholipids, triglycerides, and cholesteryl esters is fatty acyl-CoA. Acyltransferase enzymes
are responsible for the incorporation of fatty acyl-CoA
into phospholipids and into triglycerides. A specific enzyme in the cell, acyl-CoA-cholesterol acyltransferase or
AC AT, is responsible for the formation of cholesteryl esters from fatty acyl-CoA and cholesterol in cells. An
analogous enzyme exists in the blood, lecithin-cholesterol acyltransferase or LCAT. LCAT catalyzes a reaction in which the fatty acid from a phosphatidylcholine
(lecithin) molecule in a lipoprotein is removed and provided to cholesterol to form a cholesteryl ester.
Beta-Oxidation
A major pathway for oxidation of fatty acids in mammalian cells is mitochondrial beta-oxidation (Pathway 5,
Fig. 2). In this process, 2 carbon moieties are cleaved
from fatty acyl-CoA in successive cycles to produce acetyl-CoA. This acetyl-CoA can then be used to generate
ATP by way of its metabolism in the citric acid cycle.
Certain organs, such as the heart, obtain most of their
energy from beta-oxidation of fatty acids rather than the
metabolism of glucose. The beta-oxidation cycle includes 4 sequential enzymatic steps that shorten the fatty
acyl-CoA by two carbons.
Fatty acyl-CoA from the cytoplasm must be first converted to fatty acylcarnitine for transport across the mi-
T
TRIGLYCERIDES
CHOLESTERYL
ESTERS
tochondrial membrane. Once inside the mitochondrial
membrane, the fatty acylcarnitine is converted back to
its fatty acyl-CoA form to allow beta-oxidation to begin.
There is evidence that defects in the beta-oxidation of
medium chain length fatty acyl-CoA molecules contributes to the sudden infant death syndrome.10
Desaturation and Elongation of Fatty Acids
The desaturation of fatty acids in mammalian cells involves the insertion of a double bond into the fatty acylCoA molecule (Pathway 6, Fig. 2). This transformation
is catalyzed by a series of microsomal enzymes that require molecular oxygen and NADH. There are several
distinct desaturase enzymes that insert double bonds at
specific positions in the fatty acid molecule. Virtually all
cells studied in culture exhibit delta-9 desaturase activity
(insertion of a double bond between the ninth and tenth
carbons). With this enzyme, the cells are capable of synthesizing monounsaturated fatty acids, such as oleate,
from the saturated fatty acid stearate. In the desaturation
of fatty acids in mammalian cells, fatty acyl-CoA substrates that already contain double bonds are unable to
have a new double bond inserted between the last double
bond and the methyl end of the molecule. This indicates
that unsaturated fatty acids cannot change omega families, as the family designation is determined by the number of carbons between the methyl end of the molecule
and the nearest double bond (Fig. 1).
Fatty acyl-CoA molecules can also be elongated by incorporating two carbon units. There appear to be at least
two different microsomal elongation enzymes, one for
saturated and another for unsaturated fatty acids. All
Vol. 104-No. 2
176
BASIC SCIENCE
Review Article
ENZYME
FATTY ACID
18:2
DELTA - 6
DESATURASE
18 : 3
A 9,12
LINOLEATE
A 6, 9, 12
GAMMA - LINOLENATE
A 8,11,14
DIHOMOGAMMA LINOLENATE
A 5,8,11,14
ARACHIDONATE
I
ELONGASE
20 : 3
DELTA • S
DESATURASE
20 : 4
FIG. 3. The conversion of linoleate to arachidonate through sequential
desaturation and elongation reactions.
normal cells are apparently able to elongate palmitate to
stearate and elongate 18 carbon fatty acids to corresponding 20 carbon fatty acids. The desaturases and
elongases are organized in a metabolic sequence to permit synthesis of highly unsaturated and elongated fatty
acyl-CoAs. An important desaturation/elongation/desaturation cascade for which the omega-6 fatty acid linoleate is the primary substrate is shown in Figure 3. Linoleate (18:2 A 9, 12) can be desaturated by the insertion of
a double bond between the sixth and seventh carbon
atom to produce gamma-linolenate (18:3 A 6, 9, 12).
This is converted by the action of elongase, with the addition of two carbons, to a fatty acid with 20 carbons and
3 double bonds known as dihomogamma linolenate
(20:3 A 8, 11, 14). Finally, 20:3 can be transformed by
the addition of a double bond between thefifthand sixth
carbon to arachidonate (20:4 A 5, 8, 11, 14). Arachidonate is particularly important because it is the primary
fatty acid substrate for the cyclooxygenase and lipoxygenase enzymes that produce eicosanoids, a diverse family of biologically potent compounds. There is very little
arachidonate in the diet, and because most of the dietary
fatty acid of the omega-6 family is linoleate, an intact
pathway between linoleate and arachidonate is essential
for the production of eicosanoids. We have demonstrated in a number of studies that the arachidonate used
for eicosanoid synthesis must be in specific phospholipids residing in specific membranes of the cells."12 Arachidonate esterified in phospholipids within nuclear
membranes is most readily available for eicosanoid production, but because this pool of arachidonate is small,
the bulk of arachidonate for eicosanoid production derives from arachidonate esterified in the phospholipids
of the endoplasmic reticulum.12
Conversion of Arachidonate and Related Fatty Acids to
Oxygenated Metabolites
The substrates for eicosanoid production are the polyunsaturated free fatty acids liberated from phospholipids at
the time of cell activation, with arachidonate as the principal eicosanoid precursor fatty acid (Pathway 7, Fig. 2). Arachidonate and selected other polyunsaturated fatty acids
can be oxygenated. Different cells are programmed to produce their own specific array of oxygenated metabolites
from polyunsaturated fatty acids. Oxygenated metabolites
with 20 carbons, which include prostaglandins, thromboxanes, leukotrienes, and HETEs, are known as eicosanoids.
Although more than 100 oxygenated metabolites of polyunsaturated fatty acids have been identified, the biologic
functions of most of them have not been determined. Certain eicosanoids, however, have potent and clearly identified biologic activities. For example, thromboxane A2 generated from arachidonate in platelets is an extremely
procoagulant molecule that induces the aggregation of
platelets and thereby limits blood loss. In neutrophils, leukotriene B4, produced from arachidonate on cell activation, is a potent chemotactic agent. Stimulation of the release of arachidonate from cellular phospholipids through
the action of phospholipase enzymes is thefirststep in the
agonist-mediated synthesis of oxygenated metabolites. The
agonist stimulated release of fatty acids from phospholipids
is highly specific for arachidonate and other closely related
fatty acids such as eicosapentaenoate (EPA) (20:5 A 5, 8,
11,14,17). Because it has only one double bond more than
arachidonate, EPA is recognized by the enzymes that oxygenate arachidonate into eicosanoids, with the resultant
generation of oxygenated metabolites having one double
bond more than those formed from arachidonate. The biologic activities of the EPA-derived metabolites are often
significantly different from the activities of the arachidonate-derived eicosanoids because of this one extra double
bond. For example, thromboxane A3 derived from EPA is
much less potent than thromboxane A2 derived from arachidonate, and leukotriene B5 from EPA is much less potent than leukotriene B4 from arachidonate. The ingestion
of EPA in fish oil has been shown to have major effects
on a variety of biologic functions. There is evidence that
ingestion offish oil obtained either by eatingfishor ingestingfishoil supplements leads to a decrease in the mortality
from coronary heart disease. Greenland Eskimos who consumed more than 400 g offishper day were found to have
a very low death rate from coronary heart disease.13 Japanese on the island of Okinawa who consumed twice the
amount offish as Japanese on the mainland were shown to
have a lower death rate from coronary heart disease than
their mainland counterparts.14 In a study performed in the
Netherlands, Dutch men consuming two or more fish
meals per week were found to have a 50% lower mortality
from coronary heart disease than Dutch men who did not
eat fish.15
It should also be noted that 20:3 A 8, 11, 14 (dihommogamma linolenate or DHLA) with one double bond less
A.J.C.P. • August 1995
LAPOSATA
177
Fatty
than arachidonate can also be oxygenated into a variety of
different eicosanoids with biologic activities different from
those of corresponding arachidonate derived eicosanoids.
For example, prostaglandin E2 derived from arachidonate
can act as proinflammatory eicosanoid. In contrast, prostaglandin Ei derived from DGLA can be anti-inflammatory.
We have found that the ingestion of DGLA or fatty acid
precursors of DGLA may be therapeutic in patients with
inflammatory disorders such as autoimmune disease.1617 It
has been shown in a number of animal and human studies
that ingestion of fatty acids that elevate cellular DGLA concentrations in vivo decreases the inflammatory response. 17 ' 8 It appears that at least part of the anti-inflammatory effect of DGLA is mediated by increased levels of
prostaglandin Ei.
Catabolism of Very Long Chain Fatty Acids (VLCFA)
Peroxisomes are organelles bounded by a single membrane that have a number of critical functions in cells
(reviewed in reference 19). One of these is the degradation by beta-oxidation of fatty acids longer than 22 carbons (Pathway 8, Fig. 2). Patients who have peroxisomal
disorders, which may involve the complete absence of
peroxisomes or deficiencies of specific enzymes within
peroxisomes, often suffer from serious neurologic diseases because of accumulation of very long chain fatty
acids (VLCFA) in the cerebral white matter and the adrenal cortex. X-linked adrenoleukodystrophy is a peroxisomal disease associated with brain demyelination, rapid
deterioration, and death, usually within 3 years. Less
common is adrenomyeloneuropathy, which involves the
spinal cord and peripheral nerves. It is characterized by
severe spastic paraparesis, sensory loss in the legs, and
sphincter disturbances that develop over a period of 5
to 14 years. The accumulated VLCFA are derived from
endogenous synthesis by the microsomal fatty acyl-CoA
elongation system and from the diet. The simple restriction of dietary VLCFA, however, typically leads to no
biologic or clinical improvement. 20 New trials of a variety of inhibitors of the fatty acid elongation system are
ongoing as a result of the recent popular movie, Lorenzo 's Oil, which featured a boy with adrenoleukodystrophy.
The Modification of Proteins by Fatty Acids
It has long been known that fatty acids influence the
function of various proteins by noncovalent interactions. However, in the last 15 years, it has been learned
that fatty acids can modify the ability_pf proteins to bind
to membranes (Pathway 9, Fig. 2) by covalently binding
to the proteins (reviewed in reference 21). A fatty acid
may bind to a protein via an amide linkage cotranslationally, primarily at an N-terminal glycine moiety of the
protein. The fatty acid in this linkage is almost exclusively 14:0 (myristate), and it does not turnover independent of the protein. Fatty acid acylation of proteins via
thioester linkages involves the binding of fatty acids primarily to internal cysteines. This linkage occurs posttranslationally, and the fatty acid moiety attached to the
protein turns over independent of the protein. The fatty
acid specificity for thioester-linked fatty acids is moderately relaxed. We have shown that although palmitate
is the predominant fatty acid in this group, myristate,
arachidonate, and eicosapentaenoate can all become
bound via thioester linkages.22,23 A third mechanism by
which fatty acids can become covalently bound to proteins is one in which the fatty acids do not directly bind
to the protein. Glycosyl phosphatidylinositol (GPI) anchored proteins are proteins bound to membrane phosphatidylinositol through the head group of the phospholipid. The fatty acid moieties of the phospholipid are not
directly attached to the protein but are embedded in the
bilayer of the membrane. The synthesis of the GPI anchored proteins occurs in the microsomes with independent synthesis of the protein and phosphatidylinositol,
followed by combination of the two to form the GPI anchored protein. We have demonstrated the fatty acid
acylation of a number of important platelet proteins,
such as the von Willebrand factor receptor on the platelet
surface,24 the alpha subunits of various G-proteins, 25 and
P-selectin.26 Proteins that contain covalently bound myristate do not become as effectively bound to membranes
as proteins modified by palmitate, presumably because
myristate is two carbons shorter. There are many examples of how defects in the fatty acid acylation of proteins
have functional consequences. A notable example is the
p21 ras protein. 27 When this protein is mutated in a specific fashion, it is able to transform cells into a cancerous
phenotype. This mutant protein is normally acylated
with fatty acids, but if the fatty acid acylation of the mutant protein is inhibited, it is no longer able to transform
cells.
Formation of Fatty Acid Ethyl Esters
Fatty acid ethyl esters are esterification products of
fatty acids and ethanol (Pathway 10, Fig. 2). Following a
report that acutely intoxicated individuals were found to
have FAEE predominantly in the organs damaged by
ethanol abuse, it was suggested that FAEE are mediators
of ethanol-induced organ damage. 28 The presence of ethanol in cells, particularly in hepatocytes and pancreatic
acinar cells, leads to the formation of substantial quantities of fatty acid ethyl esters.29 We have recently demon-
Vol. 11 •No. 2
178
BASIC SCIENCE
ReviewArticle
strated that fatty acid ethyl esters are present in the blood
of humans following ethanol ingestion,30 and that fatty
acid ethyl esters or their metabolites are toxic for intact
human cells.31
ASSOCIATION BETWEEN SERUM
CHOLESTEROL AND DIETARY FATTY ACIDS
It has been well established that the level of serum cholesterol is a strong indicator of the risk of atherosclerotic
coronary heart disease.32"34 Although serum cholesterol
levels are affected by the amount of cholesterol in the
diet, it is clear that the amount of saturated fatty acid
ingested also has an impact on the serum cholesterol.
The major saturated fatty acid in the diet is palmitate
(16:0).35 In the American diet, palmitate constitutes
about approximately 60% of total saturated fatty acid.35
It is the predominant saturated fatty acid in most meats
and dairy fat. Therefore, a major reduction in palmitate
intake could be achieved by reducing the intake of animal fat. It is quite clear that palmitate, relative to unsaturated fatty acids, causes an elevation in the total serum
cholesterol.36"39 The increase in total cholesterol concentration in the plasma induced by palmitate occurs primarily in the low density lipoprotein (LDL)-cholesterol
fraction. The mechanism whereby palmitate raises the
LDL-cholesterol concentration is not fully understood,
but it appears to act by suppressing the expression of
LDL receptors.40"41 Myristic acid (14:0), another saturated fatty acid found in the diet, is at least as hypercholesterolemic as palmitate.36 An intermediate length saturated fatty acid, lauric acid (12:0), which appears in a few
oils such as coconut oil, is also clearly associated with
elevations of the serum cholesterol concentration.36 In
contrast with other saturated fatty acids, stearic acid does
not appear to elevate the serum cholesterol.42 Most of the
available data indicate that stearate is neutral in its action
on LDL-cholesterol concentration. It is not known why
stearic acid does not produce a hypercholesterolemic
effect similar to other long chain saturated fatty acids.
There are two predominant monounsaturated fatty acids
in the diet. These are oleic acid (cis 18:1) and elaidic acid
(trans 18:1). As noted above, trans fatty acids are in high
concentrations in margarine. Oleic acid accounts for
about 45% of the total fatty acid in the American diet.35
Available evidence suggest that oleic acid neither raises
nor lowers the serum cholesterol concentration.36-3739
On the other hand, data indicate that trans monounsaturated fatty acids are not neutral and raise LDL-cholesterol concentrations, when compared with oleic acid,6 by
an unknown mechanism. Polyunsaturated fatty acids,
particularly linoleic acid, the predominant polyunsaturated fatty acid in the diet, were considered for many
A.J.C.P.-
years to produce a serum cholesterol reduction. However, there has been a growing reservation about the wisdom of recommending Hnoleate as dietary fatty acid.
Polyunsaturated fatty acids are more prone to oxidation
and in that role could promote the development of atherosclerosis.43 In addition, evidence suggests that dietary
Hnoleate has little if any cholesterol lowering effect.37
Current recommendations now suggest that the intake
of Hnoleate should not exceed 7% of total calories.44
There are many different dietary oils, all with different
fatty acid compositions, available in food stores. Table 1
shows the fatty acid composition of butter and a number
of different oils. Several of these are composed largely of
saturated fatty acids such as butter, coconut oil, and
palm oil. Olive oil is primarily a monounsaturated fatty
acid containing oil, as is cannola oil. There are many predominantly polyunsaturated fatty acid containing oils.
These include soybean oil, peanut oil, and safflower oil.
Thus, the selection of oils and margarines has a definite
impact on the cholesterol level in the plasma.
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