RENAL TUBULAR TRANSPORT OF PROLINE, HYDROXYPROLINE, AND GLYCINE IN HEALTH AND IN FAMILIAL HYPERPROLINEMIA

Renal imino acid and glycine transport system ontogeny and involvement in developmental iminoglycinuria

Renal maturation occurs post-natally in many species and reabsorption capacity at birth can vary substantially from the mature kidney. However, little is known regarding the maturation of amino acid transport mechanisms, despite the well-known physiological state of developmental iminoglycinuria. Commonly seen during early infancy, developmental iminoglycinuria is a transient version of the persistent inherited form of the disorder, referred to as iminoglycinuria, and manifests as a urinary hyperexcretion of proline, hydroxyproline and glycine. The transporters involved in developmental iminoglycinuria and their involvement in the improvement of renal reabsorption capacity remain unknown. qPCR (quantitative real-time PCR) and Western blot analysis in developing mouse kidney revealed that the expression of Slc6a18, Slc6a19, Slc6a20a and Slc36a2 was lower at birth (approx. 3.4-, 5.0-, 2.4- and 3.0-fold less than adult kidney by qPCR respectively) and increased during development. Furthermore, immunofluorescence confocal microscopy demonstrated the absence of apical expression of Slc6a18, Slc6a19, Slc6a20a and the auxiliary protein collectrin in kidneys of mice at birth. This correlated with the detection of iminoglycinuria during the first week of life. Iminoglycinuria subsided (proline reduction preceded glycine) in the second week of life, which correlated with an increase in the expression of Slc6a19 and Slc6a20a. Mice achieved an adult imino acid and glycine excretion profile by the fourth week, at which time the expression level of all transporters was comparable with adult mice. In conclusion, these results demonstrate the delayed expression and maturation of Slc6a18, Slc6a19, Slc6a20a and Slc36a2 in neonatal mice and thus the molecular mechanism of developmental iminoglycinuria.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Deciphering the mechanisms of intestinal imino (and amino) acid transport: The redemption of SLC36A1

The absorption of zwitterionic imino and amino acids, and related drugs, is an essential function of the small intestinal epithelium. This review focuses on the physiological roles of transporters recently identified at the molecular level, in particular SLC36A1, by identifying how they relate to the classical epithelial imino and amino acid transporters characterised in mammalian small intestine in the 1960s-1990s. SLC36A1 transports a number of D- and L-imino and amino acids, beta- and gamma-amino acids and orally-active neuromodulatory and antibacterial agents. SLC36A1 (or PAT1) functions as a proton-coupled imino and amino acid symporter in cooperation with the Na+/H+ exchanger NHE3 (SLC9A3) to produce the imino acid carrier identified in rat small intestine in the 1960s but subsequently ignored because of confusion with the IMINO transporter. However, it is the sodium/imino and amino acid cotransporter SLC6A20 which corresponds to the betaine carrier (identified in hamster, 1960s) and IMINO transporter (identified in rabbit and guinea pig, 1980s). This review summarises evidence for expression of SLC36A1 and SLC6A20 in human small intestine, highlights the differences in functional characteristics of the imino acid carrier and IMINO transporter, and explains the confusion surrounding these two distinct transport systems.

Isolation and function of the amino acid transporter PAT1 (slc36a1) from rabbit and discrimination between transport via PAT1 and system IMINO in renal brush-border membrane vesicles

Reabsorption of amino acids is an important function of the renal proximal tubule. pH-dependent amino acid transport has been measured previously using rabbit renal brush-border membrane vesicles (BBMV). The purpose of this investigation was to determine whether this pH-dependent uptake represents H(+)/amino acid cotransport via a PAT1-like transport system. The rabbit PAT1 cDNA was isolated (2296bp including both 5' and 3' untranslated regions and poly(A) tail) and the open reading frame codes for a protein of 475 amino acids (92% identity to human PAT1). Rabbit PAT1 mRNA was found in all tissues investigated including kidney. When expressed heterologously in a mammalian cell line, rabbit PAT1 mediates pH-dependent, Na(+)-independent uptake of proline, glycine, l-alanine and alpha-(methylamino)isobutyric acid. Proline uptake was maximal at pH 5.0 (K(m) 2.2+/-0.7 mM). A transport system with identical characteristics (ion dependency, substrate specificity) was detected in rabbit renal BBMV where an overshoot was observed in the absence of Na+ but in the presence of an inwardly directed H+ gradient. In the presence of Na+ and under conditions in which PAT1 transport function was suppressed, a second proline uptake system was detected that exhibited functional characteristics similar to those of the IMINO system. The functional characteristics of rabbit PAT1 in either mammalian cells or renal BBMV suggest that PAT1 is the low-affinity transporter of proline, glycine and hydroxyproline believed to be defective in patients with iminoglycinuria.

Molecular cloning of the mouse IMINO system: an Na+- and Cl--dependent proline transporter

Neurotransmitter transporters of the SLC6 family play an important role in the removal of neurotransmitters in brain tissue and in amino acid transport in epithelial cells. Here we demonstrate that the mouse homologue of slc6a20 has all properties of the long-sought IMINO system. The mouse has two homologues corresponding to the single human SLC6A20 gene: these have been named XT3 and XT3s1. Expression of mouse XT3s1, but not XT3, in Xenopus laevis oocytes induced an electrogenic Na+-and-Cl--dependent transporter for proline, hydroxyproline, betaine, N-methylaminoisobutyric acid and pipecolic acid. Expression of XT3s1 was found in brain, kidney, small intestine, thymus, spleen and lung, whereas XT3 prevailed in kidney and lung. Accordingly we suggest that the two homologues be termed 'XT3s1 IMINO(B)' and 'XT3 IMINO(K)' to indicate the tissue expression of the two genes.

The molecular basis of neutral aminoacidurias

Recent success in the molecular cloning and identification of apical neutral amino acid transporters has shed a new light on inherited neutral amino acidurias, such as Hartnup disorder and Iminoglycinuria. Hartnup disorder is caused by mutations in the neutral amino acid transporter B(0) AT1 (SLC6A19). The transporter is found in kidney and intestine, where it is involved in the resorption of all neutral amino acids. The molecular defect underlying Iminoglycinuria has not yet been identified. However, two transporters, the proton amino acid transporter PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) appear to play key roles in the resorption of glycine and proline. A model is presented, involving all three transporters that can explain the phenotypic variability of iminoglycinuria.

Direct monitoring of prolidase activity in cultured skin fibroblasts using capillary electrophoresis

Capillary electrophoresis (CE) was used as an alternative to current analysis schemes for detecting prolidase activity in erythrocytes and skin fibroblast cultures because of its unique selectivity and high resolving power. Kinetic measurement of peptide bond hydrolysis was performed using porcine kidney prolidase on different substrates (Gly-Pro, Leu-Pro and Ala-Pro) and by following the disappearance of the peptide-substrate's peak. The K(m) values obtained were in agreement with those previously reported. Interestingly, in the case of Phe-Pro as the substrate, simultaneous analysis of the product and parent peptide was possible, thus showing the superiority of the capillary electrophoresis (CE) assay with respect to the standard spectrophotometric method. The application of the CE technique to the characterization of prolidase activity in control and prolidase-deficient skin cultured fibroblasts was successful. Enzyme activity was easily calculated in all controls tested and the K(m) values determined were slightly lower than those obtained with the colorimetric reaction, thus confirming our assumption that the CE assay shows higher specificity than the ninhydrin technique. Our results demonstrate the feasibility of using CE as a simple and reliable technique for determining prolidase activity.

Type II hyperprolinaemia in a pedigree of Irish travellers (nomads)

We describe a study of 312 subjects in 71 families near related to a proband with type II hyperprolinaemia. The subjects were Irish travellers (nomads) among whom consanguineous marriage and high fertility are common. Thirteen additional cases of type II hyperprolinaemia were discovered; all were offspring of consanguineous unions. A further 50 subjects were found to have mild hyperprolinaemia. We found a strong association between type II hyperprolinaemia and seizures during childhood but no significant association with mental handicap. Most adults with type II hyperprolinaemia enjoyed normal health and there was no evidence that maternal hyperprolinaemia compromised fetal development. The documented association between type II hyperprolinaemia and seizures may be related to the neuromodulatory or reducing-oxidising effects of proline and pyrroline-5-carboxylate, respectively, that has been shown in vitro. Alternatively, another genetic defect closely linked to the type II hyperprolinaemia allele could be the explanation.

Uptake of proline by brushborder vesicles isolated from human kidney cortex

Proline transport into renal brushborder membrane vesicles isolated from human kidney is mediated by two uptake systems. The high-affinity system is stimulated by a Na gradient and appears to be shared with glycine while the low-affinity system is not. Uptake curves of low concentrations of proline exhibit a Na-gradient-dependent overshoot indicative of electrogenic transport. The proline transport systems observed in isolated human renal brushborder membrane vesicles appear to have characteristics similar to those in rat kidney membranes.

High proline levels in the brains of mice as related to specific learning deficits

Hyperprolinemic PRO/Re mice have been studied as potential models for hyperprolinemia in man. In addition to high proline levels, some heretofore unreported amino acid abnormalities in the brains of PRO/Re mice are described. The T-maze and shuttlebox learning abilities of PRO/Re mice were compared with those of CD-1 mice having normal proline levels. PRO/Re mice had a significant deficit for T-maze learning, but a significantly greater aptitude for shuttlebox learning when compared to CD-1 mice. By studying the F3 progency of the PRO/Re X CD-1 cross, these strain-specific differences in learning ability for different tasks were shown to be unrelated to the differences in brain proline levels. F3 mice could be subdivided into two distinct groups: those with high proline (HP+) and low proline (HP-) titers. Other amino acids in brain tissues were essentially identical in both groups. A comparison of learning abilities of these HP+ mice with their HP- littermates showed no meaningful differences. However, the slightly slower rate at which HP+ mice acquired shuttlebox learning was sufficiently consistent over the 8 day training period so that it became significant. These results do not support the hypothesis that high levels of proline in brain tissues and blood are necessarily accompanied by impaired learning and memory, but are in agreement with those studies of hyperprolinemia in man that suggest no consistent learning deficits in hyperprolinemic subjects. The results seem to validate the suitability of the PRO/Re mouse as a model for hyperprolinemia in man. The data suggest also that the altered amino acid pattern in brains of PRO/Re mice has multiple etiologies.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitive detection of amino acids in human serum and dried blood disc of 3 mm diameter for diagnosis of inborn errors of metabolism

Sensitive high-performance liquid chromatographic determination of amino and imino acids in human serum (5 microliters) and dried blood (2.6-2.8 microliters) on a paper disc (3 mm diameter) of normal and abnormal newborns with inborn errors of metabolism (phenylketonuria, maple syrup urine disease and tyrosinosis) is described. Amino and imino acids in the biological specimens were extracted with ethanol and derivatized with 4-fluoro-7-nitrobenzo-2-oxa-1,3-diazole at 60 degrees C and at pH 8.0 for 1 min. The fluorescent derivatives were separated on mu Bondapak C18 and detected fluorometrically (530 nm/470 nm). The method was about one order of magnitude more sensitive than the similar method using o-phthalaldehyde. The amino acid contents obtained by the proposed method were comparable to those obtained by the amino acid analyser with use of o-phthalaldehyde.

Glycine uptake by erythrocytes in iron deficiency anemia

Glycine uptake by erythrocytes from cases of iron deficiency anemia, megaloblastic anemia, and anemia of chronic renal failure and hypothyroidism has been studied. Concentrative uptake, characteristically observed only in iron deficiency, is dependent on a favorable Na+ gradient and is inhibited by p-chloromercuribenzoate. Transport appears to be mediated by a carrier whose possible relation to iron deficiency is discussed.

Delineation of sodium-stimulated amino acid transport pathways in rabbit kidney brush border vesicles

We have confirmed previous demonstrations of sodium gradient-stimulated transport of L-alanine, phenylalanine, proline, and beta-alanine, and in addition demonstrated transport of N-methylamino-isobutyric acid (MeAIB) and lysine in isolated rabbit kidney brush border vesicles. In order to probe the multiplicity of transport pathways available to each of these 14C-amino acids, we measured the ability of test amino acids to inhibit tracer uptake. To obtain a rough estimate of nonspecific effects, e.g., dissipation of the transmembrane sodium electrochemical potential gradient, we measured the ability of D-glucose to inhibit tracer uptake. L-alanine and phenylalanine were completely mutually inhibitory. Roughly 75% of the 14C-L-alanine uptake could be inhibited by proline and beta-alanine, while lysine and MeAIB were no more effective than D-glucose. Roughly 50% of the 14C-phenylalanine uptake could be inhibited by proline and beta-alanine; lysine was as effective as proline and beta-alanine, and the effects of pairs of these amino acids at 50 mM each were not cumulative. MeAIB was no more effective than D-glucose. We conclude that three pathways mediate the uptake of neutral L, alpha-amino acids. One system is inaccessible to lysine, proline, and beta-alanine. The second system carries a major fraction of the L-alanine flux; it is sensitive to proline and beta-alanine, but not to lysine. The third system carries half the 14C-phenylalanine flux, and it is sensitive to proline, lysine, and beta-alanine. Since the neutral, L, alpha-amino acid fluxes are insensitive to MeAIB, we conclude that they are not mediated by the classical A system, and since all of the L-alanine flux is inhibited by phenylalanine, we conclude that it is not mediated by the classical ASC system. L-alanine and phenylalanine completely inhibit uptake of lysine. MeAIB is no more effective than D-glucose in inhibiting lysine uptake, while proline and beta-alanine appear to inhibit a component of the lysine flux. We conclude that the 14C-lysine fluxes are mediated by two systems, one, shared with phenylalanine, which is inhibited by proline, beta-alanine, and L-alanine, and one which is inhibited by L-alanine and phenylalanine but inaccessible to proline, beta-alanine, and MeAIB. Fluxes of 14C-proline and 14C-MeAIB are completely inhibited by L-alanine, phenylalanine, proline, and MeAIB, but they are insensitive to lysine. Proline and MeAIB, as well as alanine and phenylalanine, but not lysine, inhibit 14C-beta-alanine uptake. However, beta-alanine inhibits only 38% of the 14C-proline uptake and 57% of the MeAIB uptake. We conclude that two systems mediate uptake of proline and MeAIB, and that one of these systems also transports beta-alanine.

Molecular specificity of tubular reabsorption of L-proline. A microperfusion study in rat kidney

In microperfusion experiments the reabsorption of 3H and 14C labelled L-proline by two recently defined transport systems (one with high capacity and low affinity, the other one having the opposite characteristics) was measured in vivo et situ on addition of several amino acids and some N-methylated derivatives. The high capacity system is apparently an unspecific system for neutral amino acids. The methylation of the amino group does not change the affinity to the system. The affinity decreases in the order phenylalanine > glutamine > alanine > proline, hydroxyproline > glycine. The low capacity system seems to be a specific reabsorption mechanism for imino acids like proline, hydroxyproline, sarcosine an N-methylalanine. Common neutral amino acids are not accepted. The different characteristics of both transport systems are also demonstrated by the finding that the affinity of phenylalanine for the high capacity system is about 5 times higher but its affinity for the low capacity system is about 50 times lower than the affinity for proline.

Kinetics of L-proline reabsorption in rat kidney studied by continuous microperfusion

Renal tubular reabsorption of 3H and 14C labelled L-proline was measured in vivo et situ by continuous microperfusion of single proximal tubules of the rat. The reabsorption is shown to be saturable. Passive diffusion plays a relatively small role in the reabsorption. A maximum possible permeability coefficient of 25 micrometers 2.s-1 for proline was calculated. Two transport systems were found, one with a small affinity and a high capacity, the other with a very high affinity and a small capacity. The following values were estimated. Jmax 1 = 2.6 +/- 0.28 (SEM) nmol.m-1.S-1 Km1 = 11.8 +/- 1.7 (SEM) mmol.1-1 Jmax 2 = 9.6 +/- 1.92 (SEM) pmol.m-1.s-1 Km2 = 29.3 +/- 7.8 (SEM) mumol.1-1. Whereas the first system reabsorbs the bulk of the filtered load, the activity of the second system explains the extremely small amount of proline found in the final urine. Diisopropylphosphorofluoridate--a specific inhibitor of dipeptidyl peptidase IV--decreases the reabsorption of L-proline and L-alanine but has no influence on the reabsorption of the basic amino acid L-arginine and the acidic amino acid L-glutamic acid. This result correlates with a recent speculation that dipeptidyl peptidase IV is involved in proline and alanine reabosrption.

Uptake of glycine by human kidney cortex

The transport of glycine was investigated in histologically normal adult human kidney cortical slices. Uptake occurs against a gradient and shows concentration dependence. Kinetic analysis reveals two systems for transport of glycine with apparent transport Km values of 0.511 and 34.2 mM. Glycine transport on the high-Km system is competitively inhibited by 50 mML-proline. Transport inhibition on the low-Km system could not be directly evaluated, but on theoretic grounds appears not to be inhibited by L-proline or hydroxyproline. Alpha-aminoisobutyric acid, valine, and thioproline are also shown to inhibit glycine uptake. Low medium sodium or anaerobic incubation depress the uptake of glycine. These observations are consistent with previous reports of glycine transport in rat kidney and support the proposals for the mechanism of familial iminoglycinuria based on in vivo investigations.

Late-onset nonketotic hyperglycinemia and spinocerebellar degeneration

Investigation of a 15-year old boy with progressive optic atrophy and spinocerebellar degeneration revealed elevated plasma, cerebrospinal fluid, and urine glycine concentrations. During an oral glycine loading test, the patient's plasma glycine concentration rose to a higher level than control values, although the initial rate of rise was slower; there was no concomitant rise in the plasma serine concentration. An oral serine loading test resulted in a prompt rise of both glycine and serine serum concentrations. The renal glycine clearance was elevated, and the renal tubular glycine reabsorption was diminished. These findings of decreased intestinal uptake and increased renal tubular glycine clearance suggest that a generalized derangement of glycine entry into cells may account for the phenotypic manifestations of the disorder.

The transport and metabolism of L-proline-14C in the rat in vivo

The physiologic disposition, metabolic fate, and renal clearance of intravenously injected 14C-L-proline was determined in the rat. The disappearance of radioactivity from plasma occurred with a biphasic curve, the initial high levels reaching a nadir about 30 min after injection with subsequent increasing amounts of radioactivity. Examination of the 14C components in plasma revealed that 14C-proline disappeared rapidly during the first 30 min. At this time, the labeling of circulating plasma proteins ensued and continued to increase during the following 45 min of observations. Plasma glucose became labeled 10 min after injection and, thereafter, increased its 14C content. The extensive labeling of plasma proteins and glucose accounted for the increasing 14C found in plasma 30 min after injection. The course of radioactive labeling of brain, kidney, diaphragm, and liver was assessed. The greatest number of cpm/mg of tissue was found in the kidney. Determination of the distribution ratio, the ratio of cpm/ml intracellular nonprotein 14C to that in plasma in kidney revealed a peak of 3.9 within 15 min, a value comparable to that found in vitro. Twelve percent of the administered radioactivity was excreted as 14CO2 within 180 min. The oxidation was inhibited by known transport and metabolic inhibitors, the greatest effect observed with hydroxyproline, followed in order by thioproline, 3,4-dihydroproline, and glycine. The fractional urinary excretion of proline, Cproline/Cinulin, was determined and found to be 1% or less. This was increased by inhibitors, the greatest effect due to hydroxyproline followed in order by dehydroproline and glycine, a result similar to the observed extent of inhibition of proline oxidation to 14C O2. The physiologic disposition of proline was not altered by ligation of the renal vasculature.

Histidinuria: a renal and intestinal histidine transport deficiency found in two mentally retarded children

Two siblings presenting slight mental retardation showed an abnormal elimination of histidine, their blood levels for the same amino acid being normal. The percentage of tubular resorption of histidine was calculated in both boys, and the values were 40.1 per cent (case 1) and 52.8 per cent (case 2). All other amino acids essayed were normal. After an oral overload test with histidine, a low intestinal absorption was found in the two boys, the values of this test in the parents being intermediate between those of the children and of the three normal controls and corresponding to heterozygosity. In view of the studies carried out on the two boys, it is possible to conclude that they are suffering from an impairment in their histidine membrane transport system which affects the kidney and intestines. Since they are siblings a genetically determined trait may be suspected.

Role of epithelial architecture and intracellular metabolism in proline uptake and transtubular reclamation in PRO/re mouse kidney

The homozygous PRO/Re mouse has less than 1 percent of the very high proline oxidase activity that characterizes normal kidney cortex. In PRO/Re mouse the endogenous proline concentration is eight times normal in plasma and four times normal in kidney cortex cell, but 50 times normal in urine. The integrity of the membrane transport systems for proline uptake at the antiluminal surface of absorbing epithelium is retained in PRO/Re kidney, as determined by the slice method. Clearance studies in vivo under steady-state conditions indicate that the integrity of the luminal uptake system shared by glycine and proline, and serving proline absorption, is also intact. The exaggerated renal clearance of proline in PRO/Re mice (50 times normal) is explained when its raised intracellular concentration, caused by impaired proline oxidation, is considered. Backflux into urine flowing down the nephron will occur under these conditions, thus impairing net reclamation of proline in PRO/Re kidney. The findings reveal that membrane transport and intracellular metabolism of a substrate are, indeed, independent functions, but that metabolism of a substance can influence its transcellular transport.

Amino acid reabsorption in the rat nephron. Free flow micropuncture study

The concentration of nine endogenous free L-alpha-amino acids (ALA, LEU, ILE, PHE, TYR, LYS, GLU, PRO, GLY) and of taurine were determined simultaneously along the nephron of the rat kidney using free-flow micropuncture techniques without altering plasma amino acid concentration or kidney function. The amount of each amino acid was determined after dansylation (14C-labelled dansyl-chloride) in the micropuncture sample followed by thinlayer chromatography. The main site of reabsorption is the proximal tubule. After 15-20% of the proximal tubule length the bulk of reabsorption has taken place (18.9 plus or minus 3.4% S.E. of the filtered load remaining). Net reabsorption continues to a small but significant extent along the distal nephron (disal tubule and collecting duct). Reabsorption of taurine is less rapid (% remaining of filtered load at the early proximal tubule 37.0 plus or minus 4.6%). The transtubular concentration ratio of all amino acids except taurine follows a homogeneous course. Under the experimental conditions of this study no distction with respect to different systems of reabsorption "neutral", "basic", "acidic", "imino-glycine") could be made.

Transport and metabolism of sarcosine in hypersarcosinemic and normal phenotypes

An adolescent male proband with hypersarcosinemia was discovered incidentally in a French-Canadian family; no specific disease was associated with the trait. The hypersarcosinemia is not diminished by dietary folic acid even in pharmacologic doses (30 mg/day). The normal absence of sarcosine dehydrogenase in cultured human skin fibroblasts and in leukocytes was confirmed, thus eliminating these tissues as useful sources for further investigation of mutant sarcosinemic phenotypes and genotypes. The response in plasma of sarcosine and glycine, after sarcosine loading, distinguished the normal subject from the subjects who were presumably homozygous and heterozygous for the hypersarcosinemia allele. Sarcosine clearance from plasma was delayed greatly (t(1/2), 6.1 hr) in the presumed homozygote and slightly (t(1/2), 2.2 hr) in the presumed heterozygote, while plasma glycine remained constant in the former and rose in the latter. Normal subjects clear sarcosine from plasma rapidly (t(1/2), 1.6 hr) while their plasma glycine trend is downward. The phenotypic responses suggest that hypersarcosinemia is an autosomal recessive trait in this pedigree. Renal tubular transport of sarcosine was normal in the proband even though he presumably lacked the sarcosine oxidation which should normally occur in kidney. Sarcosine catabolism is thus not important for its own renal uptake. Sarcosine interacts with proline and glycine during its absorption in vivo. Studies in vitro in rat kidney showed that sarcosine transport is mediated, saturable, and energy dependent. Sarcosine has no apparent transport system of its own; it uses the low K(m) transport systems for L-proline and glycine to a minor extent and a high K(m) system shared by these substances for the major uptake at concentrations encountered in hypersarcosinemia. Intracellular sarcosine at high concentration will exchange with glycine on one of these systems, which may explain a paradoxical improvement in renal transport of glycine after sarcosine loading in the hypersarcosinemic proband.