Ammo acid transport in the lens
V. Everett Kinsey
Most arnino acids usually found in other tissues are present in the lens. In the rabbit all of
these compounds are in concentrations higher than those in the aqueous humors and much
higher than concentrations in vitreous humor. Accumulation takes place through active transport by processes which require glucose, the anaerobic utilization of which can provide all
the energy needed to maintain normal concentration gradients. The mechanisms responsible
for transport of amino acids are probably confined to the epithelium, are highly temperature
dependent, can be inhibited by various metabolic poisons and selectively, in varying degrees,
by each other. At least three systems, one each for neutral, acidic, and basic amino acids,
appear to be involved in their active transport. The steady state concentration of amino
acids in the lens is determined by the balance between the rate of entrance through active
transport and exit by diffusion.
-L ifty years ago Van Slyke and Meyer1
traced ammo acids from the intestine into
the blood, then into the tissues where they
were found to be concentrated five- to tenfold. Only in recent years has the concentration of amino acids in the lens been
shown to exceed that in the ocular fluids
which bathe it,2 and have investigations of
factors concerned with their accumulation
led to some clarification of the kinetics and
site of action of the systems responsible for
active transport.3"5
Accumulation and transport, when applied to exchange of substances between
the lens and its environment, are not the
same things. Accumulation is the difference
between the quantity of a substance that
enters and leaves it by any means, and is
the variable usually determined experimentally. Transport refers to actual movement of a substance; it can be either active
or passive. ChristensenG described transport as "the process by which a solute is
transferred from one phase to another."
When movement is active, energy must be
provided by cellular metabolism; when
passive, it is dependent only on molecular
mobility, although sometimes it is "facilitated" by mediation of cell structure or
chemical groups.7
Transport is considered to be active
when it takes place against an electrochemical gradient. The electric gradient
need not be produced by transfer of a
charge on the substance in question, an
amino acid, for example. It is essential,
however, that the chemical gradient exists
between the same species in free solution
on either side of the boundary which separates two phases, in most instances a
cell membrane.
In the lens, Brindleys has shown the existence of a potential difference across the
capsule, the inside being negative with
respect to the outside. Neutral and acidic
From the Kresge Eye Institute, Detroit, Mich.
This study was supported in part by Research
Grants B-1100 and B-2885 from the National
Institute of Neurological Diseases and Blindness
of the National Institutes of Health, United
States Public Health Service; the United States
Atomic Energy Commission, Contract No. COO152-45, and Research to Prevent Blindness, Inc.
691
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Investigative Ophthalmology
August 1965
692 Kinsey
amino acids, which cany net negative
charges, therefore must move across the
capsule against an electric potential. Basic
amino acids, which carry positive charges,
move in the direction of the potential gradient and, therefore, cannot be considered
to be actively transported solely on the
basis of differences in chemical concentration between lens and environmental
fluids.
Amino acid composition
The concentration of free amino acids
in lenses of rabbits2 and rats9 has been determined by ion exchange chromatography on sulfonated polystyrene resins (Table I). The lens contains all of the amino.
acids usually found in other tissues, with
the exception of tryptophan, cysteine, and
possibly hydroxyproline. The absence of
cysteine seems unusual, because it is present in aqueous humor in small concentrations, and it is also a constituent of glutathione, a polypeptide known to be synthesized in relatively large quantities in
situ. Possibly cysteine is bound to structural elements of the lens.
Additional information concerning concentration of amino acids in lenses has
been provided by Dardenne and Kirsten,10
who analyzed the non-protein-bound ninhydrin-positive substances present in lenses
of rats and cattle. The free amino acid
composition of the lenses is not accurately
reflected by their data because proteinfree extracts were analyzed only after being hydrolyzed for 24 hours in 6N HC1
and so include unknown quantities of
amino acids from polypeptides, such as
glutathione and ophthalmic acid.
Distribution relative to ocular fluids
The concentration of each amino acid
is higher in the lens than in the anterior
or posterior aqueous, and much higher
than in the vitreous humor (Fig. I). 2
The concentration ratio of the two acidic
amino acids, aspartic and glutamic, is highest; the ratios for the basic amino acids,
arginine, lysine, and histidine, are among
Table I. Concentration of free amino acids
in lenses of rabbits2 and rats''
Rabbits
(mmoles/
Kg. lens
icater)
Alanine
/S-Alanine
Y-Amino-n-butyric acid
Arginine
Asparagine and
glutaminef
Aspartic acid
Cystathionine
Cysteine-cystine
Glutamic acid
Glycine
Histidine
Hydroxyproline
Lsoleucine
Leucine
Lysine
Methionine
Ornithine
Phenylalanine
Proline
Serin e
Taurine
Threonine
Tryptophan
Tyrosine
Valine
Rais
(mmoles/
Kg. lens
water)
2.54
.53
Trace
.45*
1.97
2.27
.66
.19
.49
.08
0.0
3.93
.35
0.0
5.83
1.79
.35
0.0
.22
.34
.54
.16
.30
.41
1.05
1.42
6.85
.51
0.0
.88
.02
.12
1.64
.29
0.0
.45
.78
2.29
.30
.10
1.32
.65
19.3
1.84
Trace
2.16
.66
.86
°In original text,2 value was incorrectly given as 0.33.
fAsparagine color equivalent used for analytical value.
the lowest; and values for the neutral
amino acids, except leucine and isoleucine,
are intermediate between acidic and basic
compounds. The ratio of the total quantity of amino acids and peptides in lenses
of fresh calf and cattle eyes is likewise
higher than that of aqueous humor, the
values being 6.7 and 5.3, respectively.'1
Rate of accumulation
Evidence that the lens concentrates
amino acids from the fluids which bathe
it has been obtained from studies performed both in vivo and in vitro.'1-5 The
concentration of 14C-labeled a-aminoisobutyric acid (a-AIB) in lenses of live rabbits
rises within 24 hours after its parenteral
administration to 2J/2 and 25 times that
in aqueous and vitreous humor, respectively.11 The rate of accumulation of a-AIB
in rabbit lenses, and hydroxyproline in calf
lenses, cultured in media which contained
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Amino acid transport in lens 693
Volume 4
Number 4
CONCENTRATION OF FREE
(m mol«i ptr Kg. wotar)
IN
IN
IN
IN
AMINO
ACIDS
VITREOUS
POST. AO.
LENS
ANT. AO.
PHENYLALANINE
OCUTAMIC ACID
O.YCINE
Fig. 1. Relative concentration of free amino acids in lens, aqueous and vitreous humors
of the rabbit eye. (From Reddy and Kinsey: INVEST. OPHTH. 1: 635, 1962.)
no other amino acids, is shown in Fig. 2.
Because of differences in weights of lenses
and volume of culture fluid (5 ml. for rabbits and 10 ml. for calves), the data are
not entirely comparable; however, it is apparent that the amino acids accumulate
rapidly in lenses of both species.
Factors affecting accumulation
The effects of temperature, omission of
glucose or calcium, and addition of various
poisons to culture media, on accumula-
tion of a-AIB in rabbit lenses, are shown
in Table II. Uptake of this amino acid is
highly dependent upon temperature, the
apparent Q10 being approximately "4. Absence of glucose from the medium caused a
reduction in accumulation of a-AIB to less
than 20 per cent of normal. A similar observation was made by Kern4 on the uptake of hydroxyproline in calf lenses.
The metabolic poisons, iodoacetate, ouabain, and cyanide, and omission of calcium also depress transport in varying de-
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In vestigative Ophthalmology
August 1965
694 Kinsey
TIME IN HOURS
Fig. 2. Accumulation of 1<lC-labeled a-AIB in rabbit lenses and hydroxyproline in calf lenses cultured in media containing no other amino acids.
(Data for calf lenses from Kern/1 recalculated.)
grees. The inhibitory effect of ouabain on
a-AIB uptake occurred only after a delay
of approximately one hour, an effect noted
too with calf lenses.4
Thoft and Kinoshita12 found calcium
was also required for normal accumulation
of a-AIB in rat lenses. These investigators
reported that the leakage rate of a-AIB
increased only slightly when calcium was
not present, and concluded that part of
the reduction in uptake must be caused
by inhibition of active transport.
Kinsey and Reddy5 reported accumulation of a-AIB in rabbit lenses to be reduced approximately 30 per cent in the
absence of oxygen, as it was also in the
presence of 3 x 10~3M dinitrophenol, suggesting that anaerobic processes are incapable of providing all of the energy required for its transport. These results differed from those of Kern, who showed
that in calf lenses hydroxyproline concentrated in nitrogen as effectively as in air;
and dinitrophenol, in concentrations as
high as 0.67 mmole per liter, did not affect
accumulation. They seemed inconsistent
too, with the work of Kinoshita and coworkers,13 who observed that, when glucose was included in the medium, oxygen
was not required for maintaining the cations sodium and potassium at normal levels. Thus, in calf lenses sufficient energy
can be obtained from anaerobic glycolysis
to transport not only amino acids but cations as well. As a consequence, the question
of whether uptake of a-AIB in rabbit
lenses can also be supported solely by
energy derived anaerobically was reinvestigated.9
Lenses were cultured in the usual manner under aerobic conditions, and uptake
of tracer a-AIB was measured and compared with that obtained when the gas
phase was changed to 95 per cent nitrogen
and 5 per cent carbon dioxide. Traces of
oxygen were removed from the gas mixture in a manner similar to that employed
by Kinoshita and co-workers,13 in which
the gas was passed serially through three
washes of a solution containing vanadous
sulfate. To minimize possible differences in
temperature, the aerobic and anaerobic experiments were performed simultaneously
in the same water bath. The effect of dinitrophenol was also reinvestigated. The
dosage (10~4M) was less than that employed in the earlier study. If it is assumed
that the response of the lens to this poison
Table II. Influence of various factors on
accumulation of "C-labeled a-AIB in
rabbit lenses cultured for 24 hours in a
medium containing no other amino acids5
Condition
or poison
Control
37° C.
Reduced
temperature
23° C.
Glucose
absent
Anaerobio-
Concentration
Concentration of
a-AIB,
Conlens water
trol
0
medium, initially (%)
13.0 ±3.66 (67)
100
2.2 ±0.47 (12)
17
1.7 ±0.57 ( 7)
13
8.8 ±2.42 (12)
68
3 x l(HM
7.4 ±1.07 ( 6)
57
3 x i(HM
0.74 ± 0.19 ( 5)
3.4 ±0.88 ( 9)
1.55 ±0.40 (13)
6
26
12
5.1 ±1.2 ( 8)
39
sis
Dinitrophenol
Iodoacetate
Ouabain
Sodium
cyanide
Calcium 0
absent
3 x 10-2M
3
5 x 10~ M
•Number of lenses in parentheses.
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Volume 4
Number 4
Amino acid transport in lens 695
Table III. Influence of anaerobiosis and
dinitrophenol (10~4M) on accumulation of
"C-labeled a-AIB in rabbit lenses cultured
for 24 hours in a medium containing no
other amino acids
Concentration of a-AIB,
Anaerobiosis
Control, aerobic
Dinitrophenol
Control, no DNP
lens water
media, initially
13.1 ±2.7 (19)
13.2 ±2.0 (19)
10.8 + 1.7 (15)
12.0 ±2.9 (15)
is like that of other tissues, this concentration should be enough to uncouple oxidative phosphorylation but not to affect
other metabolic processes. Contralateral
lenses were used as controls in all instances.
Table III shows that, contrary to results
obtained previously, there was no difference in uptake of a-AIB, either when oxygen was excluded from the gas phase or
when dinitrophenol was present in the
media. An explanation for the earlier observations, particularly those obtained under anaerobic conditions, is not apparent.
Two sets of experiments had been performed more than a year apart, and accumulation consistently appeared to be decreased in the absence of oxygen. However, separate baths were used for the
aerobic and anaerobic experiments, and
possibly by chance the temperature of the
baths differed. A variation of only two degrees would be enough to account for the
observed difference in uptake. The inhibition in accumulation of a-AIB with the
higher concentration of dinitrophenol used
in the earlier study could have been due
to an effect on the lens, unrelated to energy
metabolism. Whatever the explanation, it
now seems that energy for amino acid
transport in rabbit lenses also can be provided solely from, anaerobic metabolism,
although, as KinoshitaV* studies suggest,
it is very likely that under physiologic conditions ATP derived from respiration also
contributes to the energy pool.
The possibility of a close relationship
between amino acid and potassium transport in the lens, as in tumor cells, was
considered by Kern.4 He noted that ouabain reduced the levels of both substances
proportionately. However, he also observed that net movements of potassium
and hydroxyproline were not always in
the same direction. Kinsey and Reddy0
measured the accumulation of tracer a-AIB
in rabbit lenses cultured in media containing no other amino acids, in which
the concentration of potassium varied from
0 to 15 mmoles per liter, and also measured
the rate of uptake of tracer potassium in
media containing 5 mmoles per liter of potassium in which the concentration of a-AIB
varied from 0 to 15 mmoles per liter (Table
IV). The accumulation of a-AIB is not
significantly affected by different concentrations of potassium, and uptake of the latter ion is just slightly greater (p < < .01)
in the absence of amino acids. Thus there
appears to be little, if any, interdependence
of transport of amino acids and potassium
in the lens.
Uptake of tracer a-AIB is affected by
the presence of nonlabeled a-AIB, glycine
or methionine5 in the culture media (Fig.
3). Accumulation of tracer a-AIB decreases asymptotically with increased concentration of total amino acid, suggesting
that the system responsible for transport
of a-AIB into the lens becomes saturated.5
The existence of mutual competitive effects
on transport was studied by determining
the relative effect of 4.5 mmoles per liter of
Table IV. Accumulation of ^'C-a-AIB as
affected by potassium, and accumulation
of 42K as affected by nonlabeled a-AIB, in
cultured rabbit lenses (24 hours)
ConConcentraRatio of
centration of Ratio of contion of concentration of a-AIB centration of
"'C-a-AlB,
**K,
Kin
in
lens water
lens water
media
media
(mM.) media, initially
(mM.) media, initially
0
12.0 ±3.2 (13)
0
12.8 ±0.82 ( 8 )
5
11.0 ±0.92 ( 7 )
5
10.4 ± 1.7 ( 8)
15
10.7±1.00 ( 7 )
15
11.5 + 2.4(12)
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Investigative Ophthalmology
August 1965
696 Kinsey
found that concentrations of 6 mmoles per
liter of proline and methionine caused a
reduction in uptake of hydroxyproline, the
values being 36 and 53 per cent of normal,
respectively; furthermore, the amino acids
in TC 199 caused a decrease in accumulation of tyrosine and tryptophan. Kinoshita
and colleagues13 observed transport of
a-AIB in rat lenses to be effectively inhibited by methionine.
2
4
6
8
10
12
14
16
18
CONCENTRATION AMINO ACID IN MEDIA (mmoles/L)
Fig. 3. Effect of nonlabeled a-AIB, glycine, and
methionine in the media on the accumulation of
ll
C-labeled a-AIB in lenses cultured for 24 hours.
twenty-one L - and three D- amino acids on
accumulation of tracer a-AIB. The results
(Fig. 4) are expressed on a scale of 100,
where 100 units correspond to uptake
when only tracer amino acid is present in
the media.
The gamma isomer of a-AIB and both
the basic and acidic amino acids have little
or no effect in saturating the carrier system
for a-AIB (upper left-hand chart, Fig. 4).
However, all neutral amino acids decrease
accumulation in varying amounts, methionine being an even more effective inhibitor
than a-AIB. All D- forms of the neutral
amino acids were less effective inhibitors
than the L - isomers.
This demonstration of the competitive
effect of amino acids on accumulation led
Kinsey and Reddy to conclude that at
least three separate systems, perhaps involving carriers or carrier sites specific for
neutral, acidic, and basic amino acids, are
responsible for transporting them into the
lens.
Competition among amino acids for
transport into the lens in species other than
the rabbit has been demonstrated by several investigators. Thus, Kern4 compared
the rate of accumulation of hydroxyproline
in calf lenses incubated in salt solution
with that in TC 199 and in a salt solution
containing 6 mmoles per liter of alanine, all
as a function of the final concentration of
hydroxyproline in the media (Fig. 5). He
Kinetics
From the data in Fig. 3, the quantity of
a-AIB accumulated by rabbit lenses during a 24 hour period of culture was calculated on the assumption that lenses do not
distinguish between labeled and nonlabeled amino acids.5 The data (Fig. 6)
show that accumulation increased with
concentration in the media and reached a
maximum at approximately 5.5 /^moles per
lens. These values were then employed to
calculate the apparent Michaelis-Menten
constant, Km, and the maximum velocity
of transport, Vmax. The reciprocal of the
amount of a-AIB in the lens was plotted
against that of the concentration in the
media using the method described by Lineweaver-Burk. This way of treating the data
does not take into account the quantity of
amino acid which has leaked out of the
lens or changes in the concentration of
labeled and nonlabeled amino acid in the
culture media during the 24 hour period of
culture. A detailed evaluation of the coefficients, employing methods similar to
those used for calculating the same constants for potassium,14 will be described
more fully in a later paper. This evaluation
gives a lower value for Km than previously
reported, viz., 1.3 compared with 2.5
mmoles per liter, but approximately the
same value for Vmax, viz., 0.23 yumole per
lens per hour. The turnover rate of a-AIB
in rabbit lenses, also calculated by the
newer method is 2.5 per cent per hour.
Site of active transport
Accumulation of amino acids is dependent upon the presence of the surface mem-
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Amino acid transport in lens 697
Volume 4
Number 4.
14
Fig. 4. Effect of various amino acids (4.5 mM.) on the transport of C-labeled a-AIB into
lenses cultured for 24 hours. (From Kinsey and Reddy: INVEST. OPHTH. 2: 229, 1963.)
HYDROXYPROLINE
IN LENS (/iM/ml )
SALT MEDIUM
8-
6-
4-
2-
0J
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
HYDROXYPROLINE IN MEDIUM
Fig. 5. Effect of competing amino acids on the partition of hydroxyproline between calf
lens and incubating medium. The period of incubation was 20 hours. (From Kern: INVEST.
OPHTH. 1: 368, 1962.)
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2.0
Investigative Ophthalmology
August 1965
698 Kinsey
TRACER ONLY
5mM 01 AIB
0.3 mM IODOACETATE
0.0
1000
2000
TIME IN MINUTES
Fig. 6. Accumulation of 14C-labeled «-AIB by
encapsulated lenses cultured in a medium containing no other amino acids. (From Kinsey and
Fig. 7. Effect of nonlabeled a-AIB on the total
amount of amino acid accumulated by lenses in
24 hours. (From Kinsey and Reddy: INVEST.
Reddy: INVEST. OPHTH. 2: 229, 1963.)
OPHTH. 2: 229, 1963.)
branes, the capsule, and epithelium. These
membranes are also the site of the metabolically active processes concerned with
active transport. Kern4 found that the ratio
of concentration of hydroxyproline in the
lens plus its separated capsule, to that in
the media, was 1.4 compared with 15 for
an intact lens after 20 hours of incubation.
Kinsey and Reddy5 cultivated lenses without capsule and epithelia for various periods in media containing tracer a-AIB
only, tracer and 5 mmoles per liter of nonlabeled a-AIB, and tracer with 0.3 mmole
per liter of iodoacetate. They found that
tracer a-AIB rapidly entered decapsulated
lenses and reached a maximum concentration within approximately one hour (Fig.
7). The presence of either nonlabeled
a-AIB or iodoacetate, both of which depress transport into intact lenses, had no
appreciable effect on accumulation in
lenses without capsule and epithelia. These
results indicate that the mechanisms responsible for accumulation are associated
with surface membranes, and, at least in
decapsulated lenses, underlying structures
are not involved to an appreciable extent.
The question of whether the capsule or
epithelium, or both, actively transport
a-AIB into the lens was investigated by
studying separately15 penetration across
the anterior and posterior surfaces. Ap-
proximately two and one-half times as
much 14C-labeled a-AIB was found in the
lens within one hour following exposure
of 60 per cent of the anterior surface to
the isotope as when penetration occurred
across the same area of the posterior surface (Table V). Removal of the epithelium
and capsule caused a significant reduction
in the rate of transport across the anterior
surface, whereas there was only a slight
increase in the quantity of a-AIB that
entered the lens through the posterior surface. Movement of a-AIB through the anterior surface was reduced by nonlabeled
a-AIB, and by iodoacetate and ouabain,
the latter to a lesser extent. Neither iodoacetate nor nonlabeled a-AIB had any appreciable effect on penetration through the
posterior surface. Thus all criteria suggestive of active transport of amino acids into
the lens were shown to be associated with
the anterior surface membranes. Since the
anterior and posterior capsules appear to
have similar physicochemical characteristics, the difference in behavior of the two
sides of the lens led Kinsey and Reddy13
to conclude that the single layer of epithelial cells, present only on the anterior
surface, is the site of the mechanism responsible for active transport of amino
acids in the lens.
In conclusion, the lens behaves like a
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Amino acid transport in lens 699
Volume 4
Number 4
Table V. Influx of "C-a-AIB in counts per
minute per lens when a single surface is
incubated for one hour in media
containing 105 counts per minute per
milliliter of radioactivity15
III. The absorption of amino acids from the
blood by the tissues, J. Biol. Chem. 16: 197,
1913-1914.
2. Reddy, D. V. N., and Kinsey, V. E.: Studies
on the crystalline lens. IX. Quantitative analysis of free amino acids and related compounds, INVEST. OPHTH. 1: 635, 1962.
Anterior
1,240 ± 370 (42)
Control
755 + 200(16)
Decapsulated
Iodoacetate
750 + 245 (14)
(3 x 10-3M)
Ouabain
985 + 255 ( 9)
(10- 5 M)
Nonlabeled
605 ± 170 (13)
a-AIB (HHM)
Posterior
540 ± 270 (37)
690 + 215(10)
680 ± 230 (11)
3. Kinoshita, J. H., Kern, H. L., and Merola,
L. O.: Factors affecting the cation transport
of calf lenses, Biochim. et biophys. acta 47:
458, 1961.
4. Kern, H. L.: Accumulation of amino acids
by calf lens, INVEST. OPHTH. 1: 368, 1962.
510 + 155 (20)
Number of lenses shown in parentheses.
pump-leak system whereby amino acids
are actively transported into it by processes which depend on the presence of
the epithelium, and leave by passive diffusion, probably chiefly across the posterior capsule. The balance between the
rate of entrance through the action of the
pump and exit by diffusion establishes
their steady state concentration, which for
all amino acids is higher in the lens than
in the aqueous and vitreous humors. The
amino acid pump requires energy which is
derived primarily from the utilization of
glucose; it can be saturated, and exhibits
selectivity, suggesting that three or more
systems are concerned in the active transport of amino acids.
5. Kinsey, V. E., and Reddy, D. V. N.: Studies
on the crystalline lens. X. Transport of amino
acids, INVEST. OPHTH. 2: 229, 1963.
6. Christensen, H. N.: Biological Transport, New
York, 1962, W. A. Benjamin, Inc., p. 43.
7. Danielli, J. F.: Morphological and molecular
aspects of active transport, Symp. Soc. Exper.
Biol. 8: 502, 1954.
8. Brindley, G. S.: Resting potential of the lens,
Brit. J. Ophth. 40: 385, 1956.
9. Kinsey, V. E., and Reddy, D. V. N.: Unpublished.
10. Dardenne, U., and Kirsten, C : Presence and
metabolism of amino acids in young and old
lenses, Exper. Eye Res. 1: 415, 1962.
11. Reddy, D. V. N., and Kinsey, V. E.: Transport of alpha aminoisobutyric acid into ocular
fluids and lens, INVEST. OPHTH. 1: 41, 1962.
12. Thoft, R. A., and Kinoshita, J. H.: The effect
of calcium on rat lens permeability, INVEST.
OPHTH. 4: 122, 1965.
13. Kinoshita, J. H., Merola, L. O., and Hayman,
S.: The effect of aldoses on accumulation of
a-aminoisobutyric acid (a-AIB) by the rabbit
lens, J. Biol. Chem. 240: 310, 1965.
14. Kinsey, V. E., and McLean, I.: Studies on the
crystalline lens. XIII. Kinetics of potassium
transport, INVEST. OPHTH. 3: 585, 1964.
REFERENCES
1. Van Slyke, D. D., and Meyer, G. M.: The
fate of protein digestion products in the body.
15. Kinsey, V. E., and Reddy, D. V. N.: Studies
on the crystalline lens. XI. The relative role
of the epithelium in
OPHTH. 4: 104, 1965.
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transport,
INVEST.