Effects of inhibition of cystathionase activity on glutathione and metallothionein levels in the adult rat

The effects of alterations in sulfur metabolism on hepatic and renal metallothionein and glutathione metabolism were studied in the adult rat using inhibition of two enzymes of these pathways, hepatic cystathionase and renal gamma-glutamyl transpeptidase. Rats were fed a diet containing both methionine (0.66%) and cystine (0.20%) for 1 week before receiving three consecutive daily intraperitoneal injections of propargylglycine, a selective cystathionase inhibitor, at various doses (2.5–375 μmol/kg). When hepatic cystathionase was inhibited greater than 90% (≥50 μmol propargylglycine/kg), renal and hepatic metallothionein and hepatic glutathione were unaltered except at the highest dose. On the other hand, renal glutathione was increased twofold with a concomitant decrease in renal gamma-glutamyl transpeptidase activity (50% of control). In another experiment, when renal gamma-glutamyl transpeptidase was inhibited greater than 90% with three consecutive daily injections of acivicin, a selective gamma-glutamyl transpeptidase inhibitor (10 mg/kg IP), renal glutathione content was unaltered while hepatic glutathione was decreased. Renal and hepatic metallothionein were not changed. Thus, the cysteine pools for metallothionein and glutathione appear unrelated under the present experimental conditions. In addition, following either propargylglycine or acivicin injections, renal and hepatic glutathione pools appear to be altered differently. These results suggest that renal glutathione may be preferentially maintained even when hepatic glutathione is decreased.     

Acivicin induce filamentous growth of Saccharomyces cerevisiae

The antibiotic acivicin is a known inhibitor of gamma-glutamyl transpeptidase (gammaGTP). We found that acivicin can induce filamentous growth in both diploid and haploid cells of Saccharomyces cerevisiae. This phenomenon is not related to the inhibition of gammaGTP or interference in glutathione metabolism. Interestingly, yeasts used in the brewing industry are more sensitive to acivicin, suggesting that this dimorphological differentiation may be related to some characteristics of these particular strains.     

Enhanced gamma-glutamyl transpeptidase expression and superoxide production in Mpv17-/- glomerulosclerosis mice.

Recently, gamma-glutamyl transpeptidase, which initiates cleavage of extracellular glutathione, has been shown to promote oxidative damage to cells. Here we examined a murine disease model of glomerulosclerosis, involving loss of the Mpv17 gene coding for a peroxisomal protein. In Mpv17-/- cells, enzyme activity and mRNA expression (examined by quantitative RT-PCR) of membrane-bound gamma-glutamyl transpeptidase were increased, while plasma glutathione peroxidase and superoxide dismutase levels were lowered. Superoxide anion production in these cells was increased as documented by electron spin resonance spectroscopy. In the presence of Mn(III)tetrakis(4-benzoic acid)porphyrin, the activities of gamma-glutamyl transpeptidase and plasma glutathione peroxidase were unchanged, suggesting a relationship between enzyme expression and the amount of reactive oxygen species. Inhibition of gamma-glutamyl transpeptidase by acivicin reverted the lowered plasma glutathione peroxidase and superoxide dismutase activities, indicating reciprocal control of gene expression for these enzymes.     

Uptake and metabolism of glutamine in cultured kidney cells

The metabolism of glutamine was investigated in cultured rat kidney cells. Glutamine utilization and product formation were followed as a function of time at either 10 microM or 1 mM initial glutamine concentration. At 1 mM glutamine, glutamate and gamma-glutamylglutamate were the major products formed at the end of a 5-min incubation period; glutamate accounted for 46% while gamma-glutamylglutamate accounted for 33% of the glutamine utilized. With time, glutamate continued to accumulate while gamma-glutamyl peptide formation leveled off. The role of gamma-glutamyl transpeptidase was assessed by using hippurate, a physiological activator of gamma-glutamyl transpeptidase and acivicin, L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid, an inhibitor of gamma-glutamyl transpeptidase. Hippurate, 4 mM, increased the utilization of glutamine and the formation of glutamate, gamma-glutamyl peptides and ammonia. Exposure of cells to acivicin resulted in 98% inhibition of gamma-glutamyl transpeptidase without effecting phosphate-dependent glutaminase activity. Acivicin inhibition resulted in a decreased utilization of glutamine and product formation as compared to control; 5-oxoproline appearance fell 70%. The fractional distribution of glutamine carbon and nitrogen into its metabolic products in control, hippurate and acivicin-treated cells showed no change at the end of 60 min. The data provide evidence that gamma-glutamyl transpeptidase utilizes glutamine and forms gamma-glutamyl peptides in cultured kidney cells.     

Stimulation of intralysosomal proteolysis by cysteinyl-glycine, a product of the action of gamma-glutamyl transpeptidase on glutathione

The mechanism of the stimulatory effect of glutathione on proteolysis in mouse kidney lysosomes and a lack of an effect in lysosomes from the liver was investigated. The stimulation in kidney lysosomes was inhibited by serine plus borate, a reversibly inhibitor of gamma-glutamyl transpeptidase. Treatment of mouse kidney lysosome suspensions with L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (acivicin), an irreversibly inhibitor of the transpeptidase, also inhibited the effect of glutathione, but this inhibition was completely relieved by washing and addition of freshly prepared kidney membranes or purified gamma-glutamyl transpeptidase to the incubation mixtures. Cysteinyl-glycine, a product of the action of gamma-glutamyl transpeptidase, stimulated proteolysis in acivicin-inhibited kidney lysosome preparations similarly to glutathione, and cysteine had no effect at equivalent concentrations. Glutathione also stimulated proteolysis in liver lysosomes in the presence of washed kidney membranes or gamma-glutamyl transpeptidase, but the effect was similar to that produced by equivalent concentrations of cysteine. These results suggest that the stimulatory effect of glutathione was mediated by the action of gamma-glutamyl transpeptidase present in contaminating cell membrane fragments in the lysosome preparations, and that glutathione does not take part in intralysosomal proteolysis. However, the possibility that cysteinyl-glycine is a physiological intralysosomal disulfide reductant in kidney lysosomes has not been excluded.     

Perturbation of hepatic glutathione level and glutathione-related enzyme activities by repeated administration of aminopyrine in rats

The influence of repeated administration of aminopyrine on the tissue glutathione level and related enzyme activities was investigated in rats. Reduced glutathione level in the liver was not changed after 5 days of treatment but a significant increase was seen after 15 days of aminopyrine treatment. Oxidized glutathione level was unaltered throughout the experiment. Repeated administration of aminopyrine for 5 days caused a marked increase in gamma-glutamyl transpeptidase activities in liver whole homogenates as well as in the nuclear fraction, but not in liver microsomes. These results suggest that gamma-glutamyl transpeptidase located in plasma membrane may be induced by repeated administration of aminopyrine for 5 days. The activities of cytosolic glutathione peroxidase, which modulates glutathione level, were also significantly increased by aminopyrine treatment. Under the same conditions, glutathione peroxidase activity with H2O2 as a substrate was unaltered, while a time-dependent increase in the activity was found when cumene hydroperoxide was used as a substrate, even after a single administration of aminopyrine. The intracellular cysteine level was increased accompanying the increased gamma-glutamyl transpeptidase activities. Therefore, induced gamma-glutamyl transpeptidase may play a role in the reclamation of extracellular oxidized glutathione.     

Effects of beta-carotene on oxidative stress in normal and diabetic rats

Increasing interest in the role of oxidative stress and beta-carotene in disease and prevention led us to examine the results of beta-carotene's administration in diabetic rats, a model for high-oxidative stress. In this experiment, amounts of lipid peroxidation, glutathione, and glutathione disulfide, and activity levels of catalase, glutathione peroxidase, glutathione reductase, superoxide dismutase, and gamma-glutamyl transpeptidase were measured in the liver, kidney, and heart of Sprague-Dawley rats with streptozotocin-induced diabetes, and after treatment with 10 mg/kg/day of beta-carotene for 14 days. Beta-carotene treatment resulted in the reversal of the diabetes-induced increase in hepatic and cardiac catalase activity, the decreased levels of glutathione disulfide in the heart, and the increased cardiac and renal levels of lipid peroxidation. Treatment with beta-carotene exacerbated the increased glutathione peroxidase activity in the heart and the decreased catalase activity in the kidneys. In contrast to reduced hepatic glutathione levels in untreated diabetic rats, beta-carotene treatment increased glutathione levels in diabetic rats. Increased hepatic gamma-glutamyl transpeptidase activity in diabetic rats was not reduced by treatment. Thus, beta-carotene therapy for 14 days prevented/reversed some, but not all, diabetes-induced changes in oxidative stress parameters.     

Interaction of gamma-glutamyl transpeptidase with glutathione involves specific arginine and lysine residues of the heavy subunit.

Gamma-glutamyl transpeptidase, an enzyme of importance in glutathione metabolism, consists of two subunits, one of which (the light subunit, Mr 22,000; residues 380-568; rat kidney) contains residue Thr-523, which selectively interacts with the substrate analog acivicin to form an adduct that is apparently analogous to the gamma-glutamyl enzyme intermediate formed in the normal reaction (Stole, E., Seddon, A. P., Wellner, D., and Meister, A. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 1706-1709). The present studies indicate that specific arginine and lysine residues of the heavy subunit (Mr 51,000; residues 31-379) participate in catalysis by binding the substrates. Selective labeling studies of the enzyme with [14C]phenylglyoxal showed that Lys-99 and Arg-111 were modified. This appears to be the first instance in which phenylglyoxal was found to react with an enzyme lysine residue. Incorporation of [14C]phenylglyoxal into Lys-99 was decreased in the presence of acceptor site selective compounds. Incorporation into both Lys-99 and Arg-111 was decreased in the presence of glutathione. The findings suggest that Lys-99 and Arg-111 interact, respectively, with the omega- and alpha-carboxyl groups of glutathione. That these putative electrostatic binding sites are on the heavy subunit indicates that both subunits contribute to the active center. Two additional heavy subunit arginine residues become accessible to modification by phenylglyoxal when acivicin is bound, suggesting that interaction with acivicin is associated with a conformational change.     

Pink-eyed dilution protein modulates arsenic sensitivity and intracellular glutathione metabolism

Mutations in the mouse p (pink-eyed dilution) and human P genes lead to melanosomal defects and ocular developmental abnormalities. Despite the critical role played by the p gene product in controlling tyrosinase processing and melanosome biogenesis, its precise biological function is still not defined. We have expressed p heterologously in the yeast Saccharomyces cerevisiae to study its function in greater detail. Immunofluorescence studies revealed that p reaches the yeast vacuolar membrane via the prevacuolar compartment. Yeast cells expressing p exhibited increased sensitivity to a number of toxic compounds, including arsenicals. Similarly, cultured murine melanocytes expressing a functional p gene were also found to be more sensitive to arsenical compounds compared with p-null cell lines. Intracellular glutathione, known to play a role in detoxification of arsenicals, was diminished by 50% in p-expressing yeast. By using the glutathione-conjugating dye monochlorobimane, in combination with acivicin, an inhibitor of vacuolar gamma-glutamyl cysteine transpeptidase, involved in the breakdown of glutathione, we found that p facilitates the vacuolar accumulation of glutathione. Our data demonstrate that the pink-eyed dilution protein increases cellular sensitivity to arsenicals and other metalloids and can modulate intracellular glutathione metabolism.     

Extracellular metabolism of glutathione accounts for its disappearance from the basolateral circulation of the kidney

Glutathione labeled in each of its amino acid residues, the corresponding free amino acids, and gamma-glutamyl-amino acids were used to evaluate their renal basolateral transport and metabolism at physiological levels of glutathione. Recovery of label in the venous outflow was compared to that of co-administered inulin after a single-pass in vivo infusion of rat kidney. Metabolites of glutathione and of its constituent amino acids were determined. No net basolateral transport of glutathione was detected; instead there was extensive breakdown of glutathione by the actions of basolateral gamma-glutamyl transpeptidase and dipeptidase. Glutamate and 5-oxoproline showed net basolateral uptake. Recoveries of 35S greater than those of inulin were found after perfusion of [35S]cysteine and [35S]glutathione suggesting rapid net tubular reabsorption of cyst(e)ine. Recovery of label from perfused [U-14C]glycine was equivalent to that of inulin consistent with little or no net flux. Co-administration of large amounts of unlabeled metabolites together with the labeled glutathiones led to label recoveries closer to those of inulin, consistent with competitive inhibition of labeled metabolite transport. Treatment of rats with an inhibitor of gamma-glutamyl transpeptidase decreased basolateral glutathione metabolism and thus indirectly decreased transport of labeled metabolites. No net basolateral transport of gamma-glutamyl-amino acids was detected. Significant amounts of label perfused as [Glu-U-14C]glutathione appeared in the gamma-glutamyl-amino acid fraction of the renal venous outflows, providing direct evidence that glutathione is used in vivo for the formation of gamma-glutamyl-amino acids.     

Inhibition of potassium bromate-induced renal oxidative stress and hyperproliferative response by Nymphaea alba in Wistar rats

KBrO3-mediated renal injury and hyperproliferative response in Wistar rats. In this communication, we report the efficacy of Nymphaea alba on KBrO3 (125 mg/kg body weight, intraperitoneally) caused reduction in renal glutathione content, renal antioxidant enzymes and phase-II metabolising enzymes with enhancement in xanthine oxidase, lipid peroxidation, gamma-glutamyl transpeptidase and hydrogen peroxide (H202). It also induced blood urea nitrogen, serum creatinine and tumor promotion markers, viz., ornithine decarboxylase (ODC) activity and DNA synthesis. Treatment of rats with Nymphaea alba (100 and 200 mg/kg body weight) one hour before KBrO3 (125 mg/kg body weight, i.p.) resulted in significant decreases in xanthine oxidase (P < 0.05), lipid peroxidation, gamma-glutamyl transpeptidase, H202 generation, blood urea nitrogen, serum creatinine, renal ODC activity and DNA synthesis (P < 0.001). Renal glutathione content, glutathione metabolizing enzymes and antioxidant enzymes were also recovered to significant levels (P < 0.001). These results show that Nymphaea alba acts as chemopreventive agent against KBrO3-mediated renal injury and hyperproliferative response.     

Metabolism of gamma-glutamyl amino acids and peptides in mouse liver and kidney in vivo.

The metabolism in vivo of gamma-glutamyl amino acids and peptides was studied in the mouse after administration of loading doses of L-gamma-glutamyl-2-aminobutyrate and several other gamma-glutamyl compounds, including glutathione. A great and rapid accumulation of glutamate, glutamine, aspartate and pyrrolidone carboxylate was observed in the kidney. Similarly, after administration of a tracer dose of L-gamma-[14C]glutamyl-L-2-aminobutyrate a rapid incorporation of label into kidney glutamate, glutamine and aspartate was found. These results suggest that both the hydrolytic and gamma-glutamyl transfer reactions catalyzed by gamma-glutamyl transpeptidase are active in the renal handling of gamma-glutamyl compounds. Indirect evidence was obtained that L-gamma-glutamyl-2-aminobutyrate is partially taken up by the kidney cell in an intact form. In contrast to the kidney, administration of several gamma-glutamyl derivatives did not cause an increase in liver glutamate, glutamine and pyrrolidone carboxylate. After administration of L-gamma-glutamyl-2-aminobutyrate only a slight increase in liver aspartate and pyrrolidone carboxylate was observed. Experiments with L-gamma-[14C]glutamyl-L-2-aminobutyrate suggest that this derivative is largely first degraded to its component amino acids (probably in the kidney) before entering into the metabolism of the liver cell. gamma-Glutamyl transpeptidase may function in the metabolism and transport of glutathione and other gamma-glutamyl compounds in a manner analogous to the function of dipeptidases and disaccharidases in the metabolism and transport of dipeptides and disaccharides respectively.     

Attenuation of potassium bromate-induced nephrotoxicity by coumarin (1,2-benzopyrone) in Wistar rats: chemoprevention against free radical-mediated renal oxidative stress and tumor promotion response

We report the modulatory effect of coumarin (1,2-benzopyrone) on potassium bromate (KBrO(3)) mediated nephrotoxicity in Wistar rats. KBrO(3) (125 mg/kg body weight, i.p.) enhances gamma-glutamyl transpeptidase, renal lipid peroxidation, xanthine oxidase and hydrogen peroxide (H(2)O(2)) generation with reduction in renal glutathione content and antioxidant enzymes. It also enhances blood urea nitrogen, serum creatinine, ornithine decarboxylase (ODC) activity and [(3)H]-thymidine incorporation into renal DNA. Treatment of rats orally with coumarin (10 mg/kg body weight and 20 mg/kg body weight) resulted in a significant decrease in gamma-glutamyl transpeptidase, lipid peroxidation, xanthine oxidase, H(2)O(2) generation, blood urea nitrogen, serum creatinine, renal ODC activity and DNA synthesis (P < 0.001). Renal glutathione content (P < 0.01) and antioxidant enzymes were also recovered to significant level (P < 0.001). These results show that coumarin may be used as an effective chemopreventive agent against KBrO(3)-mediated renal oxidative stress, toxicity and tumor promotion response in Wistar rats.     

Gamma-glutamyl-glutathione. Natural occurrence and enzymology

The natural occurrence of gamma-glutamyl-glutathione (gamma-glutamyl-gamma-glutamylcysteinylglycine) in bile was established by analytical and chromatographic studies on the isolated and chemically synthesized materials. Evidence that it is formed in kidney was obtained. The origin of gamma-glutamyl-glutathione was explored through studies on the interaction of glutathione with gamma-glutamyl transpeptidase. When purified gamma-glutamyl transpeptidase was incubated with various concentrations (4 microM-50 mM) of glutathione, the initial rates of formation of gamma-glutamyl-glutathione were substantial at all concentrations of glutathione studied and were greater than the rates of formation of glutamate at physiological levels of glutathione (1-10 mM). The findings indicate that gamma-glutamyl transpeptidase catalyzes transpeptidation in vivo. That gamma-glutamyl-glutathione is formed in vivo and that it is a significant product of the reaction between glutathione and gamma-glutamyl transpeptidase under physiological conditions suggest that this polyanionic tetrapeptide may have a physiological role. gamma-Glutamyl-glutathione is not a substrate of glutathione reductase or of glutathione S-transferase, but it is a substrate of gamma-glutamyl-cyclotransferase. That gamma-glutamyl-glutathione has an additional negative charge as compared to glutathione suggests that it may be more effective than glutathione in forming complexes with certain metal ions and other cations.     

Induction of renal oxidative stress and cell proliferation response by ferric nitrilotriacetate (Fe-NTA): diminution by soy isoflavones

Ferric nitrilotriacetate (Fe-NTA) is a known potent nephrotoxic agent. In this communication, we report the chemopreventive effect of soy isoflavones on renal oxidative stress, toxicity and cell proliferation response in Wistar rats. Fe-NTA (9 mg Fe/kg body weight, intraperitoneally) enhances gamma-glutamyl transpeptidase, renal lipid peroxidation, xanthine oxidase and hydrogen peroxide (H2O2) generation with reduction in renal glutathione content, antioxidant enzymes, viz., glutathione peroxidase, glutathione reductase, catalase, glucose-6-phosphate dehydrogenase and phase-II metabolising enzymes such as glutathione-S-transferase and quinone reductase. Fe-NTA treatment also induced tumor promotion markers, viz., ornithine decarboxylase (ODC) activity and thymidine [3H] incorporation into renal DNA. A sharp elevation in the levels of blood urea nitrogen and serum creatinine has also been observed. Treatment of rats orally with soy isoflavones (5 mg/kg body weight and 10 mg/kg body weight) resulted in significant decreases in gamma-glutamyl transpeptidase, lipid peroxidation, xanthine oxidase, H2O2 generation, blood urea nitrogen, serum creatinine, renal ODC activity and DNA synthesis (P < 0.001). Renal glutathione content (P < 0.01), glutathione metabolizing enzymes (P < 0.001) and antioxidant enzymes were also returned to normal levels (P < 0.001). Thus, our data suggest that soy isoflavones may be used as an effective chemopreventive agent against Fe-NTA-mediated renal oxidative stress, toxicity and cell proliferation response in Wistar rats.     

GAMMA-Glutamyl transpeptidase of sheep-kidney cortex. Isolation, catalytic properties and dissociation into two polypeptide chains.

Gamma-Glutamyl transpeptidase was isolated from sheep kidney cortex as an apparently homogeneous, highly active protein. At optimal pH and in the absence of acceptors, the enzyme catalyzes the release of about 510 mumol of p-nitroaniline per mg protein per min from the model substrate L-gamma-glutamyl-p-nitroanilide. Polyacrylamide gel electrophoresis in a sodium dodecylsulfate buffer system showed the presence of a large (Mr approximately 65000) and a small (Mr approximately 27000) polypeptide chain. Dissociation into two polypeptide chains was also achieved in 8 M urea. Amidination with dimethylsuberimidate produced a crosslinked protein of molecular weight approximately 90000. In the course of this work a convenient procedure was developed for the determination of gamma-glutamyl transpeptidase activity using L[glycine-2-3H]glutathione as the substrate. In this procedure the release of cysteinyl-[2-3H]glycine from glutathione is followed, after separation of the radioactive di-peptide from unreacted glutathione on a small Dowex-1 acetate column. The reactions with gamma-glutamyl-p-nitroanilide and glutathione are both strongly activated by several metal ions (Ca2+, Mg2+, Na+ and K+) and by a number of amino acids and peptide acceptors. The products of the reaction with glutathione were identified as cysteinylglycine, gamma-glutamylglutathione and glutamate. The formation of these products is consistent with the function of gamma-glutamyl transpeptidase in both the gamma-glutamyl transfer reaction and in the hydrolysis of the gamma-glutamyl bond. The activating effect of metal ions in the reaction with glutathione was shown to be dependent on the acceleration of the transfer reaction; the rate of hydrolysis of the gamma-glutamyl bond remaining unchanged.     

Chemomodulatory effect of Ficus racemosa extract against chemically induced renal carcinogenesis and oxidative damage response in Wistar rats

Ferric nitrilotriacetate (Fe-NTA) is a well-known renal carcinogen. In this communication, we show the chemopreventive effect of Ficus racemosa extract against Fe-NTA-induced renal oxidative stress, hyperproliferative response and renal carcinogenesis in rats. Fe-NTA (9 mg Fe/kg body weight, intraperitoneally) enhances renal lipid peroxidation, xanthine oxidase, gamma-glutamyl transpeptidase and hydrogen peroxide (H(2)O(2)) generation with reduction in renal glutathione content, antioxidant enzymes, viz., glutathione peroxidase, glutathione reductase, catalase, glucose-6-phosphate dehydrogenase and phase-II metabolising enzymes such as glutathione-S-transferase and quinone reductase. It also enhances blood urea nitrogen, serum creatinine, ornithine decarboxylase (ODC) activity and thymidine [(3)H] incorporation into renal DNA. It also enhances DEN (N-diethylnitrosamine) initiated renal carcinogenesis by increasing the percentage incidence of tumors. Treatment of rats orally with F. racemosa extract (200 and 400 mg/kg body weight) resulted in significant decrease in gamma-glutamyl transpeptidase, lipid peroxidation, xanthine oxidase, H(2)O(2) generation, blood urea nitrogen, serum creatinine, renal ODC activity, DNA synthesis (P<0.001) and incidence of tumors. Renal glutathione content (P<0.01), glutathione metabolizing enzymes (P<0.001) and antioxidant enzymes were also recovered to significant level (P<0.001). Thus, our data suggests that F. racemosa extract is a potent chemopreventive agent and suppresses Fe-NTA-induced renal carcinogenesis and oxidative damage response in Wistar rats.     

Absence of a role of gamma-glutamyl transpeptidase in the transport of amino acids by rat renal brushborder membrane vesicles

Summary The role of the enzyme, gamma-glutamyl transpeptidase on the uptake of amino acids by the brushborder membrane of the rat proximal tubule was examined by inhibiting it with AT-125 (l-[S, 5S]--amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid). AT-125 inhibited 98% of the activity of gamma-glutamyl transpeptidase when incubated for 20 min at 37°C with rat brushborder membrane vesicles. AT-125 given to ratsin vivo inhibited 90% of the activity of gamma-glutamyl transpeptidase in subsequently isolated brushborder membrane vesicles from these animals. AT-125 inhibition of gamma-glutamyl transpeptidase bothin vivo andin vitro had no effect on the brushborder membrane uptake of cystine. Similarly, there was no effect of gamma-glutamyl transpeptidase inhibition by AT-125 on glutamine, proline, glycine, methionine, leucine or lysine uptake by brushborder membrane vesicles. Furthermore, the uptake of cystine by isolated rat renal cortical tubule fragments, in which the complete gamma-glutamyl cycle is present, was unaffected by AT-125 inhibition of gamma-glutamyl transpeptidase. Therefore, in the two model systems studied, gamma-glutamyl transpeptidase did not appear to play a role in the transport of amino acids by the renal brushborder membrane.     

Accumulation of cystine from glutathione-cysteine mixed disulfide in cystinotic fibroblasts; blockade by an inhibitor of gamma-glutamyl transpeptidase

Cystinotic and normal skin fibroblasts in tissue culture were treated with varying concentrations of reduced glutathione, oxidized glutathione and glutathione-cysteine mixed disulfide, substrates of gamma-glutamyl transpeptidase, the catabolic enzyme of the gamma-glutamyl cycle. Cystine accumulated more rapidly and to a greater extent from the glutathione-cysteine mixed disulfide in cystinotic than in normal cells. Inhibition of gamma-glutamyl transpeptidase activity by serine in a borate buffer partially blocked this accumulation of cystine. Reduced glutathione and oxidized glutathione have lesser effects on cystine accumulation. Stored cystine in cystinotic tissues may derive in part from glutathione-cysteine mixed disulfide via transpeptidation.