1 Physiologisches Institut der Universität Würzburg, D-97070 Würzburg, Germany; and 2 Department of Physiology, College of Medicine, University of Arizona, Tucson, Arizona 85724-5051
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ABSTRACT |
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D-Serine normally
contributes up to 3% to total plasma serine and up to 23% in chronic
renal failure. D-Serine is
metabolized by tubular D-amino
acid oxidase (D-AAO), and high
D-serine plasma levels are
nephrotoxic; both events are localized in the straight part of the
proximal tubule. We therefore investigated if and how
D-serine is reabsorbed there. We
microinfused
14C-labeled
D- or
-L-serine + [3H]inulin into early
proximal (EP), late proximal (LP), or early distal (ED) tubule sections
of superficial nephrons and into long loops of Henle (LLH) of rats in
vivo and in situ. The fractional reabsorption (FR) of the
14C label was determined from the
14C:3H
ratio in the final urine. At 0.36 mM, FR of
D-[14C]serine
was 86% (EP), 90% (LP), and 0 (ED, LLH). FR of
D-serine could be saturated and
inhibited by L-serine (and vice
versa). D-methionine, but not
D-glutamate or
D-arginine, blocked FR of D-serine (LP). We conlude that
filtered D-serine is able to
enter the pars recta cells, thereby getting
access to D-AAO. The uptake carrier has a very low stereospecificity
and is, therefore, different from that in the proximal convolution. The
colocalization of exclusive reabsorption and metabolism makes the
pars recta the tubule site for the recycling of
the carbon structure of D-amino
acids and, at the same time, the target of
D-serine nephrotoxicity.
kidney; D-amino acid transport; nephrotoxicity; Henle's loop
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INTRODUCTION |
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D-SERINE IS A NORMAL plasma constituent in mice (18) and humans (3, 17). It stems from spontaneous in vivo racemization of aged proteins of long-lived tissue (20), from brain cells releasing D-serine (12), and from uptake of cooked or otherwise processed dietary proteins (7, 14). A bacterial source of D-serine has been discounted as D-serine does not occur in bacterial walls and has been found in mice living under germ-free conditions (16). D-Serine contributes 0.5% to 3% to total plasma serine in normal volunteers and up to 23% in patients with highly elevated serum creatinine levels (3, 17). This increase of plasma D-serine concentrations in chronic renal failure suggests that the kidney keeps the plasma D-serine level low. Metabolism of D-serine by tubular D-amino acid oxidase (D-AAO) (18) seems to be the main mechanism by which the kidney fulfills this role.
High doses of intraperitoneally injected D-serine damage rat proximal straight tubules within several hours, causing acute tubular necrosis with proteinuria, glucosuria, and generalized hyperaminoaciduria (4, 5, 10, 15, 19, 31, 32). From these observations, two main questions arise: 1) how does D-serine enter the cells of the proximal straight tubule, and 2) why is it toxic for these cells? The first question is addressed in this paper, whereas parallel reports from our laboratory deal with the second one (24, 25).
We demonstrated earlier that, in contrast to its L-isomer, D-serine is not reabsorbed significantly if microperfused through proximal convoluted tubules in the rat kidney in vivo and in situ (27). Transport studies with membrane vesicles prepared from rabbit renal outer cortex and from outer stripe of outer medulla (13) showed that luminal cortical membrane vesicles accumulate L- and D-serine by a single common transport system, which has a much lower affinity (Km = 30 mM) for D- than for L-serine (Km = 3.7 mM). Luminal membrane vesicles from the outer medulla take up L-serine by two kinetically distinct processes (Km = 0.37 and 10 mM, respectively), whereas D-serine transport is not detected in these vesicles (13). Uptake of L-serine into basolateral vesicles has been demonstrated too. It can be inhibited by L-phenylalanine, but not by D-serine (13). Shimomura et al. (21) microperfused rabbit proximal nephron segments in vitro and found a high intracellular accumulation of the radiolabel of L-serine and, to a smaller extent, of D-serine if they were offered at the basolateral side. Accumulation was similar in the convoluted and straight portions of the proximal tubule (21). Thus, in rabbits, there is some evidence from in vitro experiments that D-serine can enter the cells of the proximal tubule not only by the same luminal carrier as the L-isomer, albeit at a lower affinity, but also via a basolateral carrier.
As D-serine nephrotoxicity is localized in the straight part of the proximal tubule (5), and as renal D-serine metabolism by D-AAO is localized there (6, 29), we wanted to investigate whether, in contrast to the proximal convolution (27), D-serine is reabsorbed in the straight part. For this purpose, we microinfused 14C-labeled D- or L-serine into early proximal (EP), late proximal (LP), or early distal (ED) tubule sections of superficial nephrons, as well as into the long loops of Henle (LLH) of juxtamedullary nephrons of the rat in vivo and in situ and determined the fractional recovery of the 14C label compared with co-microinfused 3H-inulin in the final urine.
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MATERIALS AND METHODS |
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Male Munich-Wistar rats were used for the following groups of
experiments: 1) EP, LP, and ED
experiments [shown in Figs. 2 to 5; 198-357 g body wt (269 g
mean body wt); Harlan Sprague Dawley, Indianapolis, IN];
2) LP experiments [shown in
Fig. 6; 152-382 g body wt (275 g mean body wt); Charles River,
Sulzfeld, Germany]; and 3) LLH
experiments [120-143 g body wt (136 g mean body wt); Charles
River]. Group 1 was
fed on Teklad 4% Mouse/Rat Diet 7001; groups
2 and 3 were fed on
Altromin Standard Diet 1320. All groups had free access to water. The
animals were anesthetized with Inactin (120 mg/kg body wt; Byk-Gulden,
Konstanz, Germany). A tracheostomy was performed and polyethylene
cannulae were placed in the right jugular vein for infusions. The
animals were infused with Ringer solution at a rate of 0.15 ml · min1 · kg
body wt
1. The
Ringer solution contained the following (in g/l): 9 NaCl, 0.4 KCl, 0.25 CaCl2, and 0.2 NaHCO3. We prepared the kidney for tubule micropuncture using standard techniques (1)
Microinfusion into superficial
nephrons.
After identification of the nephron segments by intravenous injection
of lissamine green SF (Chroma-Gesellschaft; Schmidt, Köngen,
Germany) at a dose of 0.02 ml of a 100 g/l solution titrated with NaOH
to pH 7.4, we micropunctured the tubule using glass capillaries. The
latter had ground tips (outer tip diameter: 9-11 µm) and were
mounted on a microperfusion pump (28). Microinfuson sites were
1) the first superficial loop of the
proximal tubule (EP), 2) the last
superficial loop of the proximal tubule (LP), and
3) the first superficial loop of the
distal tubule (ED) (see Fig. 1). In all
cases, the microinfusate (10 nl/min) added to the endogenous flow rate
of tubular fluid. The microinfusate (pH 6.7) contained (in g/l) 9 NaCl,
0.4 KCl, 0.25 CaCl2, and 2.0 MOPS, as well as 7.4 GBq/l
[3H]inulin (5.18 GBq = 140 mCi/g; American Radiolabeled Chemicals, St. Louis, MO), 0.36 mmol/l
1-14C-labeled
D-serine (2.04 GBq = 55 mCi/mmol; American Radiolabeled Chemicals), or
14C(U)-labeled
L-serine (6.2 GBq = 167 mCi/mmol; New England Nuclear, Boston, MA; or 2.04 GBq = 55 mCi/mmol;
American Radiolabeled Chemicals), in addition to unlabeled
D- or
L-serine,
D-methionine,
D-glutamate, or
D-arginine, as indicated in
RESULTS. Microinfusion lasted for 10 min. Starting shortly before microinfusion, the ipsilateral urine was
collected from the ureteral catheter in 15-min fractions for 1 h, and
the 14C and
3H counts (in dpm) of each
fraction were determined in a liquid scintillation spectrometer (LS
6000SE; Beckman, Anaheim, CA; or 1600 TR; Canberra-Packard, Frankfurt
am Main, Germany). To be sure that the
14C activity found in the urine
still represented serine,
14C-containing urine
samples, as well as the microinfusion fluid, were analyzed by HPLC
(25a), and 1-min fractions of the outflowing eluent were collected in
scintillation vials. The 14C
counts of each fraction were determined in a liquid scintillation spectrometer. As a result, the radiochemical purity of the
microinfusion fluid was >95%. In the urine samples, it was >90%.
As a further control, the urine of the contralateral kidney was
collected from a bladder catheter in 15-min fractions during the same
1-h period. The
[3H]inulin counts (if
any) and the 14C-labeled
D- or
L-serine counts in the
contralateral urine, never exceeding 10% of those in the ipsilateral
urine, were subtracted from the latter. After this correction, the
fractional recovery was calculated from the sum of the
14C and
3H counts (in dpm), respectively,
of the 1-h collecting period, as shown in Fig. 1.
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Microinfusion into LLH.
The experiments on LLH were performed as described previously (8, 9).
Briefly, the papilla of the left kidney was exposed and a single
ascending limb of the LLH was punctured near the hairpin bend with a
glass micropuncture pipette with an external tip diameter of 5-6
µm and mounted to a microperfusion pump (28). The tip of
this pipette was coated with platinum glaze to make it easily
visible (11). The loop was then infused with a TES (N-tris[hydroxymethyl]methyl-2-aminoethanesulfonic
acid)-buffered (10 mmol/l) solution containing
[3H]inulin and
D-[14C]serine
(7.28 mmol/l), as described above. The microinfusion solution also
contained lissamine green (20 g/l), so that the flow in the loop could
be seen and so that we could determine whether there was any
extravasation from the loop that would make the infusion technically
unacceptable. The microinfusion was generally maintained at 10 nl/min.
After the microinfusion was well established (usually 2-3 min), we
made collections of urine emerging from the ducts of Bellini with a
second micropuncture pipette (external tip diameter: 12-14 µm).
The urinary volume recovery at this collection site was much smaller
than that obtained from the ureteral catheter during microinfusion into
superficial nephrons. Therefore, the lowest
D-[14C]serine
concentration used had to be higher (7.28 mmol/l) than that used for
superficial nephrons (0.36 mmol/l). The radioactivity in the collected
fluid and the initial perfusion solution was measured in a liquid
scintillation counter (1600 TR; Canberra-Packard) to determine the
fractional recovery of the infused
D-[14C]serine
in the urine of the ducts of Bellini. Two to five collections of
70-100 nl were made in each infusion experiment (the number depending on the length of time the infusion could be maintained), and
the mean value for the fractional recovery for all collections was used
as the value for that microinfusion experiment.
Chemicals. The D- and L-serine, D-methionine, D-glutamate, and D-arginine were purchased from Sigma (Deisenhofen, Germany), TES and MOPS were obtained from Serva (Heidelberg, Germany), and all other chemicals were from Merck (Darmstadt, Germany).
Calculations and statistics.
The maximal reabsorption rate
(Jmax; pmol/s per
loop of Henle) and the apparent Michaelis constant (mmol/l) of
D-serine reabsorption from short
loops of Henle were estimated from the mean reabsorption rates
(J) at the concentrations (C),
whereby J = (1 recovery) × microinfusion rate × C (pmol/s per loop of Henle), by the
three linearized plots of the Michaelis-Menten equation
(1/J vs. 1/C, C/J vs. C, and
J vs.
J/C).
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RESULTS |
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High doses (7.5 mmol/kg body wt) of D-serine, leading to D-serine plasma concentrations of 12 to 27 mmol/l within 15 min in Wistar rats (unpublished obervations), are nephrotoxic and cause a generalized hyperaminoaciduria within several hours (see INTRODUCTION). As similarly high D-serine concentrations were used in the present microinfusion experiments, lasting for 10 min, the early time course of leucine (Leu) and ornithine (Orn) excretion was determined in pilot experiments. As a result, fractional excretion (FE) of these amino acids (FE control: Leu and Orn < 1%) did not significantly increase after 10 min and started to rise to severe values (>5%) only after 60 min. Thus significant toxic effects of microinfused D-serine can be excluded, to a great extent, in the present experiments.
Figures 2-6 show the fractional recovery (see
METHODS) of the
14C label of
D- and
L-serine (i.e., the original
data). As the reabsorptive process, rather than excretion, is the major
focus of this paper, the following results are presented as fractional
reabsorption [FR (%) = 100 fractional recovery
(%)], i.e., the distance on the ordinate between the height of the
columns and 100% (Figs. 2-6).
In the first series of experiments, we microinfused a solution (10 nl/min) containing 0.36 mmol/l 14C-labeled D-serine into EP, LP, and ED tubule sections appearing at the surface of the kidney, and a solution containing 7.28 mmol/l 14C-labeled D-serine into the ascending limb of an LLH near the hairpin bend. As can be seen from Fig. 2, FR of D-[14C]serine was ~86% during EP, ~90% during LP, but essentially zero during ED, as well as during LLH microinfusion. The ED and LLH values for fractional recovery were not significantly different from 100%. Thus D-serine is nearly completely reabsorbed if it is microinfused upstream from the short loops of Henle at an initial concentration of 0.36 mmol/l, but it is not reabsorbed at all downstream of the hairpin bend of the LLH (7.28 mmol/l). Therefore, the tubule section beyond the last accessible loop of the proximal tubule (i.e., the pars recta) is responsible for the high D-serine FR in the loop of Henle.
To evaluate, in further sets of experiments, whether the high
reabsorption observed during EP and LP microinfusion was carrier mediated, we repeated the experiments with the highly elevated D-serine concentrations of 20 and 80 mmol/l. As shown in Fig. 3, FR of
D-serine during EP microinfusion
decreased from 86% at 0.36 mmol/l to 70%
(n = 3) at 20 mmol/l and to 22%
(n = 5) at 80 mmol/l.
Similar to these EP values, FR of
D-serine during LP microinfusion
decreased from 90% at 0.36 mmol/l to 55%
(n = 6) at 20 mmol/l and to 21%
(n = 6) at 80 mmol/l. These
data demonstrate that at least the major part of
D-serine reabsorption from the pars recta reflects mediated transport.
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Apparent kinetic parameters roughly estimated from the LP values at 0.36, 20, and 80 mmol/l (Fig. 3) were as follows: Jmax = 3.80 ± 0.24 pmol/s per pars recta (SD); concentration at 1/2 Jmax (Km) = 24.1 ± 2.5 mmol/l (SD); and Hill coefficient = 0.99 (see DISCUSSION).
To test whether L-serine had any effect on D-serine reabsorption, we repeated the EP and LP microinfusion experiments with 0.36 mmol/l D-[14C]serine in the presence of 80 mmol/l L-serine in the microinfusate (Fig. 3). During EP microinfusion, the FR of 0.36 mmol/l D-[14C]serine in the presence of 80 mmol/l L-serine (31%) was not significantly different from that in the presence of 80 mmol/l D-serine (22%). During LP microinfusion, however, the FR of D-[14C]serine (0.36 mmol/l) was completetely inhibited by 80 mmol/l L-serine. These results demonstrate that 1) L-serine inhibits D-serine reabsorption in the pars recta, and 2) D-serine reabsorption is an entirely mediated process.
In a further set of experiments (Fig. 4),
we examined the FR of
L-[14C]serine
(0.36, 20, and 80 mmol/l) during EP and LP microinfusion. During EP, as
well as during LP microinfusion (compare Figs. 3 and 4),
L-serine was reabsorbed to about
the same high extent (92% for EP; 89% for LP) as the
D-isomer (86% for EP; 90% for LP) at 0.36 mmol/l. At 80 mmol/l, however, the results with
L-serine were quite different
from those with D-serine. During
EP microinfusion, ~55% of
L-[14C]serine
was reabsorbed (Fig. 4), compared with only ~22% of
D-[14C]serine
(Fig. 3). During LP microinfusion at 80 mmol/l, however, FR of
L-[14C]serine
dropped to only 4% (Fig. 4), compared with 21% in the case
of
D-[14C]serine
(Fig. 3).
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We also tested whether D-serine (80 mmol/l) had any effect on the reabsorption of the L-isomer (0.36 mmol/l). During EP microinfusion, L-[14C]serine FR was only slightly diminished by D-serine (83% vs. 92% in the control), whereas during LP microinfusion, FR of L-[14C]serine dropped from 89% (control) to 35% (Fig. 4). These data confirm our earlier conclusion that the serine-transporting carriers of the proximal convoluted tubule (27) highly prefer the L-isomer. This results in a higher capacity of the whole proximal tubule to reabsorb L-serine than to reabsorb D-serine. The present data show, in addition, that D- and L-serine inhibit each other during reabsorption in the pars recta.
In a further set of LP microinfusion experiments, we evaluated the
specificity of the reabsorptive mechanism for
D- and
L-serine in the pars
recta by adding
D-methionine to the LP
microinfusate containing 0.36 mmol/l
14C-labeled
D- or
-L-serine. In the presence of 80 mmol/l D-methionine, the FR of
D-[
14C]serine
decreased from 90% to 2%, but that of
L-[14C]serine
only decreased from 89% to 74% (Fig. 5).
During EP microinfusion, there was no influence of 80 mmol/l
D-methionine on the reabsorption of 0.36 mmol/l
L-[14C]serine
(90 ± 1% vs. 92 ± 2%, n = 4 and 5, respectively).
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In a final set of LP microinfusion experiments, we wanted to test the
extent to which
D-glutamate
or
D-arginine+
inhibits the reabsorption of
D-[14C]serine
(0.36 mmol/l) in the pars recta. As these
experiments had to be performed in Würzburg (instead of in
Tucson, as in those experiments detailed above), we purchased the
Munich-Wistar rats from a German dealer (see
METHODS). It turned out that the FR
of
D-[14C]serine
in the control experiments (LP; 0.36 mmol/l) was much smaller (55%;
see Fig. 6) than those found with
Munich-Wistar rats purchased in Tucson (90%; see Fig. 2). The FR of
L-[14C]serine
(LP; 0.36 mmol/l) differed as well, albeit to a smaller extent: 73%
vs. 89% (see Figs. 4 and 6). We then repeated the LP microinfusion
experiments with 20 and 80 mmol/l
D-serine (Fig. 6). The results
were again significantly different from those reported above (compare
LP values in Figs. 3 and 6). Roughly estimating the kinetics in this
case (see DISCUSSION), we obtained
the following values:
Jmax = 1.88 ± 0.38 pmol/s per pars recta (SD);
Km = 17.3 ± 6.8 mmol/l
(SD); Hill coefficient = 0.98. Thus
Jmax of the
pars recta of the Würzburg rats was
significantly smaller than that of the Tucson rats, whereas the
affinity constant was in the same range in both groups.
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In contrast to the results with D-methionine (Fig. 5), the FR of 0.36 mmol/l D-[14C]serine was not significantly inhibited when 80 mmol/l D-glutamate or D-arginine were added to the LP microinfusate (Fig. 6).
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DISCUSSION |
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The present data demonstrate that radiolabeled
D-serine is reabsorbed
by a high-capacity/low-affinity process
(Km 20 mmol/l) in
the pars recta at similar rates as
L-serine. This is in contrast to
our recent data, obtained from rat proximal convoluted tubules microperfused in vivo and in situ, in which only the
L-isomer was reabsorbed to a
significant extent (27). These earlier findings were confirmed by the
present results that
D-[14C]serine
reabsorption is inhibited by
L-serine to a greater extent during LP than during EP microinjections (Fig. 3), and vice versa (Fig.
4). Both results can be explained by a much higher stereospecificity of
serine reabsorption in the convoluted than in the straight part of the
proximal tubule. This is somewhat at variance with the results of
uptake studies with luminal membrane vesicles from outer cortical and
outer medullary (outer stripe) rabbit tubules (13). These authors
concluded from their results that outer cortical vesicles, stemming
mostly from the pars convoluta, take up
L-serine (Km = 3.7 mmol/l) and
D-serine (Km = 30 mmol/l) by the
same transport system, whereas the outer medullary vesicles (pars recta) do not take up
D-serine at all. However, these
data are hard to compare with the present findings, not only because of
possible species differences, but also because
1) kinetic data estimated from
vesicular uptake and from in vivo reabsorption usually differ by one or
more orders of magnitude, for unknown reasons (22);
2) the outer stripe of the outer
medulla does not contain only the pars recta
of the proximal tubule; 3)
preparations of luminal membranes usually also contain basolateral
membranes, to some extent; and 4)
D-AAO is absent in vesicles, but works in series with the luminal
uptake in vivo. The apparent kinetic parameters roughly estimated in
this study may, therefore, represent overall kinetics, including the
D-serine breakdown in the cells of the pars recta (see below).
The present results, especially the observation that
D-serine reabsorption is fully
inhibited by high concentrations of
L-serine (Fig. 3), demonstrate
that D-serine reabsorption from
the loop of Henle is an entirely mediated process. Whether this
represents a novel carrier, or
D- and
L-serine are accepted by one of
the known amino acid carriers to a similar extent, is not clear.
Recently, Utsunomiya-Tate et al. (30) have cloned a system ASC-like,
Na+-dependent neutral amino acid
transporter, ASCT2, from mouse testis cDNA. Expressed in oocytes, it
has a high affinity (Km 0.020 mmol/l) for L-alanine,
L-threonine,
L-cysteine, and
L-glutamine. L-Alanine transport by ASCT2 can
be inhibited by D-serine.
Although expression of ASCT2 mRNA has been demonstrated in mouse kidney (30), it does not seem to be very likely that ASCT2 is involved in
L- and D-serine
reabsorption in the loop of Henle because ASTC2 has a 100-fold lower
Km for L-serine than that found in
this study for D-serine. Moreover,
L-alanine transport by ASCT2 is
only slightly inhibited by
D-alanine (30), whereas
D-alanine is
reabsorbed in microperfused loops of Henle at the same high rate (23)
as D-serine was in the present
study, an observation supporting the possibility that
D-alanine and
D-serine share a common carrier in the pars recta, with similar transport rates.
We have recently reported that transport of taurine differs in short loops of Henle of Munich-Wistar rats coming from different sources (26). The present data, obtained with rats purchased from the same two dealers, show that this is also true for D-serine reabsorption. In both cases, the rats bought in Tucson reabsorbed the respective amino acid from short loops of Henle at a significantly higher rate than the rats purchased in Würzburg (see METHODS for sources). This time, we roughly estimated the overall kinetic constants of D-[14C]serine reabsorption and ended up with a significantly lower maximal transport rate (fmol/s per loop of Henle) in the Würzburg rats than in the Tucson rats, whereas the affinity constant was in the same range. As the microinfusate was diluted when entering the tubule lumen, the intratubular D-serine concentrations were lower than those in the microinfusion pipette and, therefore, these kinetic parameters are not very useful as absolute values. However, the difference in Jmax at unchanged Km values suggests that the density or activity of the carrier per membrane area is higher or that the tubule section involved in reabsorption is longer in Tucson rats than in Würzburg rats.
As mentioned above, reabsorption of
D-amino acids in the
pars recta delivers these substrates to
D-AAO localized in the peroxysomes of the same nephron section (6, 29).
This FAD-coupled enzyme converts neutral
-D-amino acids into their
respective
-keto acids, which in turn can be used to form their
respective, more useful,
-L-amino acids by
transamination. We therefore tested whether amino acids that are
accepted well (D-methionine),
slightly (dibasic D-amino
acids), or not at all
(D-glutamate) by the renal D-AAO
(2) affect D-serine
reabsorption. Indeed,
D-methionine is a strong
inhibitor of D-serine
reabsorption (Fig. 5), whereas D-arginine only has a small
effect, and D-glutamate has
no effect at all (Fig. 6). This could mean that
1) the carrier has a specificity similar to that of D-AAO and/or 2)
D-methionine inhibits D-AAO and,
thereby, competitively inhibits the metabolic sink for
D-serine. In the latter case,
the chemical gradient for
D-serine across the luminal
membrane and, therefore, its removal from the lumen would decrease. The
second process seems to play a partial role, at least, because
1) reabsorption of
D-alanine (another good
substrate of D-AAO) in the loops of Henle is partly inhibited in the
presence of the two D-AAO inhibitors, benzoate and indole carboxylic
acid (23), and 2) reabsorption of
LP-microinfused L-serine was
much less inhibited by
D-methionine than reabsorption
of D-serine (Fig. 5). Although,
to our knowledge, no information is available on D-serine transport across the
peroxisomal membrane, competition for a carrier could take place there too.
The colocalization of D-serine metabolism and the exclusive D-serine reabsorption in the pars recta can explain why nephrotoxicity of high D-serine doses is confined to the S3 segment (4). Our recent data (24, 25) suggest that the nephrotoxicity is caused by H2O2 (and/or oxygen radicals) formed during D-serine metabolism by peroxisomal D-AAO. Because D-serine filtered at the glomerulus is not reabsorbed in the proximal convoluted tubule and is, therefore, concentrated about three times until the tubular fluid reaches the pars recta, even moderately elevated D-serine plasma levels, as observed in chronic renal failure (see INTRODUCTION), might affect the tubule cells of the pars recta.
In conclusion, D-serine is able to enter tubule cells of the pars recta from the lumen, thereby getting access to D-AAO, the enzyme responsible for the metabolism of neutral D-amino acids. D-Serine reabsorption is mediated by a carrier that has a very low stereospecificity, if any. For this reason, the carrier in the straight part is different from that or those in the proximal convolution, and might be a novel one. D-serine, once reabsorbed, ends up in the metabolic sink of D-AAO, thereby keeping up the chemical D-serine gradient directed from lumen into the cell. The colocalization of exclusive reabsorption and metabolism makes the pars recta the tubule site for recycling the carbon structure of D-amino acids and, at the same time, the target of D-serine nephrotoxicity.
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ACKNOWLEDGEMENTS |
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We thank Olga Brokl and Kristen Evans for encouragement and support at the University of Arizona, and Dr. Michael Gekle for valuable discussions.
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FOOTNOTES |
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This study was supported, in part, by North Atlantic Treaty Organization Collaborative Grant 0104/88 and National Institute of Diabetes and Digestive and Kidney Diseases Research Grant DK-16294.
Parts of this study have been presented, in abstract form, at the 29th Annual Meeting of the American Society of Nephrology, New Orleans, November 1996 (J. Am. Soc. Nephrol. 7: 1305, 1996) and at the 76th Congress of the Deutsche Physiologische Gesellschaft in Rostock, March 1997 (Pflügers Arch. 433: R165, 1997).
Address for reprint requests and other correspondence: S. Silbernagl, Physiologisches Institut der Universität Würzburg, Röntgenring 9, D-97070 Würzburg, Germany.
Received 1 December 1997; accepted in final form 25 February 1999.
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