The activities of the key enzymes of the gamma-glutamyl cycle in microdissected segments of the rat nephron

Distribution, Metabolism, and Toxicity of Antimony Species in Wistar Rats. A Bio-Analytical Approach

This work studied the distribution, reactivity, and biological effects of pentavalent or trivalent antimony (Sb(V), Sb(III)) and N-methylglucamine antimonate (NMG-Sb(V)) in Wistar Rats. The expression of fibrosis genes such as α-SMA, PAI-1, and CTGF were determined in Liver, and Kidney tissues. Wistar rats were treated with different concentrations of Sb(V), Sb(III), As(V) and As(III), and MA via intra-peritoneal injections. The results indicated a noteworthy elevation in mRNA levels of plasminogen activator 1 (PAI-1) in the kidneys of rats that were injected. The main accumulation site for Sb(V) was observed to be the liver, from which it is primarily excreted in its reduced form (Sb(III)) through the urine. The generation of Sb(III) in the kidneys has been found to induce damage through the expression of α-SMA and CTGF, and also lead to a higher creatinine clearance compared to As(III).

Renal glutathione transport: Identification of carriers, physiological functions, and controversies

Glutathione (GSH) is an endogenous tripeptide composed of the amino acids L-glutamate, L-cysteine, and glycine. It is found in virtually all aerobic cells and plays critical roles in maintenance of cellular redox homeostasis and drug metabolism. An important component of its regulation is transport across biological membranes. Because GSH is a charged, hydrophilic molecule, transport occurs via catalysis by specific carrier proteins rather than by simple diffusion. Although it has been clearly understood that efflux of GSH across membranes such as the canalicular and sinusoidal plasma membranes in hepatocytes and the brush-border plasma membrane in renal proximal tubules is a key step in GSH turnover and interorgan metabolism, the existence and physiological functions of uptake of GSH across various epithelial plasma membranes has been subject to some debate. Besides transport across plasma membranes, GSH transport across intracellular membranes, most notably the mitochondrial inner membrane, has received some attention in recent years because of the importance of mitochondrial redox status and the mitochondrial GSH pool in cellular physiology and pathology. This commentary will focus on renal transport processes for GSH and will discuss some of the controversies that have existed and still seem to exist in the literature, specifically regarding uptake of intact GSH by basolateral membranes of renal proximal tubular cells and uptake of intact GSH by the mitochondrial inner membrane.

Specific labelling of the hydrophobic domain of rat renal gamma-glutamyltransferase

The amphipathic form of gamma-glutamyltransferase (EC 2.3.2.2) was reconstituted into unilamellar lecithin vesicles and labelled using the membrane-soluble reagent, 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine ( [125I]TID), which can be photoactivated. Label was incorporated exclusively into the large subunit of the transferase, but no label was found in the enzyme released from the vesicles by partial proteolysis with papain. Chromatography of the papain-treated vesicles on Sephadex LH-60 indicated that the [125I]TID was bound to two peptides of low relative molecular mass. The [125I]-TID-labelled peptides should constitute the hydrophobic membrane-binding domain. The transferase was also labelled by reductive methylation (Lys residues) or with immobilized galactose oxidase and NaB3H4 (Gal residues), incorporated into lecithin vesicles and treated with papain. Both procedures yielded a single [3H]-labelled hydrophobic peptide that remained associated with the vesicles. After chromatography on Sephadex LH-60, the peptide labelled by the latter two procedures corresponds to the larger of the two [125I]TID-labelled peptides. These results suggest that papain may cleave the hydrophobic domain into two peptides. The observed labelling is also consistent with the alternative possibility that the N-terminal hydrophobic domain is preceded by an initial sequence that contains both lysine and carbohydrate residues. The smaller of the two [125I]TID-labelled peptides could be generated by removal of the hydrophilic segment from the transferase molecules that are reconstituted with the N-terminus exposed on the external surface of the vesicles.

Luminal and basolateral membrane transport of glutathione in isolated perfused S(1), S(2), and S(3) segments of the rabbit proximal tubule

Lumen-to-bath and bath-to-lumen transport rates of glutathione (GSH) were measured in isolated perfused S(1), S(2), and S(3) segments of the rabbit proximal tubule. In lumen-to-bath experiments, the perfusion solution contained 4.6 microM (3)H-GSH with or without 1.0 mM acivicin. In all three segments perfused without acivicin, luminal disappearance rate (J(DL)) and bath appearance rate (J(AB)) of (3)H-GSH were 14.5 +/- 0.5 and 2.2 +/- 0.8 fmol/min per mm tubule length, respectively. With acivicin present, J(DL) and J(AB) were reduced to 1.3 +/- 0.4 and 0.5 +/- 0.3, respectively, with no differences among segments. Cellular concentrations of (3)H-GSH in S(1), S(2), and S(3) segments when acivicin was absent were 23.1 +/- 2.0, 31.7 +/- 11.4, and 143.5 +/- 17.9 microM, respectively. With acivicin in perfusate, cellular concentrations were reduced but there was no change in the heterogeneity profile. In bath-to-lumen transport experiments (S(2) segments only), the bathing solution contained 2.3 microM (3)H-GSH. (3)H-GSH appearance in the lumen (J(AL), fmol/min per mm) and cellular accumulation from the bath were studied with and without acivicin in the perfusate. J(AL) values were 3.0 +/- 0.2 and 0.2 +/- 0.03 while cellular concentrations were 9.5 +/- 1.0 and 6.1 +/- 0.5 microM, respectively. It is concluded that: (1) GSH is primarily removed from the luminal fluid after degradation to glycine, cysteine, and glutamate, which are absorbed; (2) GSH can be absorbed intact at the luminal membrane; (3) the S(3) segment has the greatest GSH cellular concentration because its basolateral membrane has less capacity for cell-to-bath transport of GSH; and (4) GSH can be secreted intact from the peritubular compartment into the tubular lumen.

Isolation, culture and characterization of human renal proximal tubule and collecting duct cells

The complexity and heterogeneity of the human nephron with regard to cell types make well-defined in vitro systems of renal cells valuable for studies of the pathogenetic mechanisms involved in nephrotoxicity. In our laboratory renal proximal tubule cells (PTC) and collecting duct cells (CDC) have been isolated, cultured and characterized from cadaver kidneys (postmortem time <24 h) for use in studies of renal cytotoxicity induced by therapeutics and bacteria. PTC seeded at 10(6) cells/ml formed confluent monolayers within 7 days. Histochemical markers were used to determine the origin of the cell cultures. Cells were negative for factor VIII, positive for cytokeratin and gamma-glutamyltransferase (GGT), and bound winged pea lectin. CDC, isolated from the renal papillae, formed monolayers within 14 days of seeding. CDC were negative for factor VIII and GGT, positive for cytokeratin and bound peanut lectin. PTC and CDC isolates and cultures exhibited typical epithelial cell ultrastructure: cell junctions, intermediate filaments, microvilli, and numerous mitochondria. The morphological and histochemical evidence confirms that PTC and CDC can be isolated and cultured for use in in vitro studies.

Regulation of type II renal Na+-dependent inorganic phosphate transporters by 1,25-dihydroxyvitamin D3. Identification of a vitamin D-responsive element in the human NAPi-3 gene

Vitamin D is an important regulator of phosphate homeostasis. The effects of vitamin D on the expression of renal Na+-dependent inorganic phosphate (Pi) transporters (types I and II) were investigated. In vitamin D-deficient rats, the amounts of type II Na+-dependent Pi transporter (NaPi-2) protein and mRNA were decreased in the juxtamedullary kidney cortex, but not in the superficial cortex, compared with control rats. The administration of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) to vitamin D-deficient rats increased the initial rate of Pi uptake as well as the amounts of NaPi-2 mRNA and protein in the juxtamedullary cortex. The transcriptional activity of a luciferase reporter plasmid containing the promoter region of the human type II Na+-dependent Pi transporter NaPi-3 gene was increased markedly by 1,25-(OH)2D3 in COS-7 cells expressing the human vitamin D receptor. A deletion and mutation analysis of the NaPi-3 gene promoter identified the vitamin D-responsive element as the sequence 5'-GGGGCAGCAAGGGCA-3' nucleotides -1977 to -1963 relative to the transcription start site. This element bound a heterodimer of the vitamin D receptor and retinoid X receptor, and it enhanced the basal transcriptional activity of the promoter of the herpes simplex virus thymidine kinase gene in an orientation-independent manner. Thus, one mechanism by which vitamin D regulates Pi homeostasis is through the modulation of the expression of type II Na+-dependent Pi transporter genes in the juxtamedullary kidney cortex.

Understanding renal toxicity of heavy metals

The mechanisms by which metals induce renal injury are, in general, poorly understood. Characteristic features of metal nephrotoxicity are lesions that tend to predominate in specific regions of the nephron within specific cell types. This suggests that certain regions of the nephron are selectively sensitive to specific metals. Regional variability in sensitivity could result from the localization of molecular targets in certain cell populations and/or the localization of transport and binding ligands that deliver metals to targets within the nephron. Significant progress has been made in identifying various extracellular, membrane, and intracellular ligands that are important in the expression of the nephrotoxicity of metals. As an example, mercuric chloride induces a nephropathy that, at the lowest effective doses, is restricted primarily to the S3 segment of the proximal tubule, with involvement of the S2 and S1 segments at higher doses. This specificity appears to be derived, at least in part, from the distribution of enzymes and transport proteins important for the uptake of mercury into proximal tubule cells: apical gamma-glutamyltranspeptidase and the basolateral organic anion transport system. Regional distributions of transport mechanisms for binding proteins appear to be important in the expression of nephrotoxicity of metals. These and other new research developments are reviewed.

Characterization of homocysteine metabolism in the rat kidney

Epidemiological studies have provided strong evidence that an elevated plasma homocysteine concentration is an important independent risk factor for cardiovascular disease. We have shown, in the rat, that the kidney is a major site for the removal and subsequent metabolism of plasma homocysteine [Bostom, Brosnan, Hall, Nadeau and Selhub (1995) Atherosclerosis 116, 59-62]. To characterize the role of the kidney in homocysteine metabolism further, we measured the disappearance of homocysteine in isolated renal cortical tubules of the rat. Renal tubules metabolized homocysteine primarily through the transulphuration pathway, producing cystathionine and cysteine (78% of homocysteine disappearance). Methionine production accounted for less than 2% of the disappearance of homocysteine. Cystathionine, and subsequently cysteine, production rates, as well as the rate of disappearance of homocysteine, were sensitive to the level of serine in the incubation medium, as increased serine concentrations permitted higher rates of cystathionine and cysteine production. On the basis of enrichment profiles of cystathionine beta-synthase and cystathionine gamma-lyase, in comparison with marker enzymes of known location, we concluded that cystathionine beta-synthase was enriched in the outer cortex, specifically in cells of the proximal convoluted tubule. Cystathionine gamma-lyase exhibited higher enrichment patterns in the inner cortex and outer medulla, with strong evidence of an enrichment in cells of the proximal straight tubule. These studies indicate that factors that influence the transulphuration of homocysteine may influence the renal clearance of this amino acid.

Glutathione and gamma-glutamyl cycle enzymes in rat testis during sexual maturation

The main enzymes of the gamma-glutamyl cycle in the testis were studied during the onset of spermatogenesis. The activities of gamma-glutamyl transpeptidase, 5-oxoprolinase, and gamma-glutamyl cysteine synthetase, and levels of glutathione were measured in testis homogenates and Sertoli cell preparations obtained from 10-, 18-, and 26-day-old rats. A significant increase of all enzyme activities with the animal age was observed. Level of glutathione also increased in an age-dependent manner. Since the gamma-glutamyl cycle is involved in the cellular incorporation of amino acids, the present findings suggest that this uptake mechanism may be relevant during spermatogenic onset in which synthesis and secretion of specific proteins are essential for germ cell development.

Studies on the effects of L(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125) on 4-aminophenol-induced nephrotoxicity in the Fischer 344 rat

4-Aminophenol (para-aminophenol; PAP) causes selective necrosis to the S3 segment of the proximal tubule in experimental animals. The mechanism of PAP nephrotoxicity has not been fully elucidated, although it has been suggested to involve glutathione (GSH)-dependent S-conjugation followed by processing by the enzyme gamma-glutamyl transpeptidase (gamma GT) to the corresponding cysteine S-conjugate. This proposed toxicity mechanism was probed further by administering L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125), a potent gamma GT inhibitor, to Fischer 344 (F344) rats before treatment with PAP (100 mg/kg). AT-125 pretreatment did not appear to protect against PAP-induced nephrotoxicity as assessed by renal histopathology, clinical chemistry and proton nuclear magnetic resonance (1H NMR) spectroscopy of urine. These data suggest that renal gamma GT activity is not a prerequisite for PAP nephrotoxicity and that the generation of a cysteine S-conjugate is not a unique requirement for the induction of PAP nephrotoxicity.

Cellular and subcellular localization of enzymes of arginine metabolism in rat kidney

Rat kidneys extract citrulline derived from the intestinal metabolism of glutamine and convert it stoichiometrically into arginine. This pathway constitutes the major endogenous source of arginine. We investigated the localization of enzymes of arginine synthesis, argininosuccinate synthase and lyase, and of breakdown, arginase and ornithine aminotransferase, in five regions of rat kidney, in cortical tubule fractions and in subcellular fractions of cortex. Argininosuccinate synthase and lyase were found almost exclusively in cortex. Arginase and ornithine aminotransferase were found in inner cortex and outer medulla. Since cortical tissue primarily consists of proximal convoluted and straight tubules, distal tubules and glomeruli, we prepared cortical tubule fragments by collagenase digestion of cortices and fractionated them on a Percoll gradient. Argininosuccinate synthase and lyase were found to be markedly enriched in proximal convoluted tubules, whereas less than 10% of arginase and ornithine aminotransferase, were recovered in this fraction. Arginine production from citrulline was also enriched in proximal convoluted tubules. Subcellular fractionation of kidney cortex revealed that argininosuccinate synthase and lyase are cytosolic. We therefore conclude that arginine synthesis occurs in the cytoplasm of the cells of the proximal convoluted tubule.

Stereospecific glutathione conjugation of (R)- and (S)-2-bromoisovalerylurea in freshly isolated rat kidney proximal tubular cells

The glutathione conjugation of 2-bromoisovalerylurea (BIU) was studied in isolated proximal tubular kidney cells of the rat. Racemic (R,S)-BIU was incubated with the cell suspension, and the incubation medium was analysed for the diastereomeric glutathione (GSH) conjugates, cysteine conjugates and mercapturates that can be formed from (R)- and (S)-BIU. Only the mercapturate formed from (R)-BIU was found, as well as its cysteine precursor. No GSH conjugates were detected. These results indicate that these cells conjugate only the (R)-BIU enantiomer, and that the GSH conjugate is immediately further metabolized to its cysteine conjugate and mercapturate.

Inhibition of gamma-glutamyl transpeptidase potentiates the nephrotoxicity of glutathione-conjugated chlorohydroquinones

Administration of either 2,5-dichloro-3-(glutathion-S-yl)-1, 4-benzoquinone (DC-[GSyl]BQ) or 2,5,6-trichloro-3-(glutathion-S-yl)-1,4-benzoquinone (TC-[GSyl]BQ) to male Sprague-Dawley rats caused dose-dependent (50-200 mumol/kg; iv) renal proximal tubular necrosis, as evidenced by elevations in blood urea nitrogen (BUN), and in the urinary excretion of lactate dehydrogenase (LDH), gamma-glutamyl transpeptidase (gamma-GT) and glucose. Renal proximal tubular necrosis was also confirmed by histological examination of kidney slices prepared from DC-(GSyl)BQ- and TC-(GSyl)BQ-treated animals. Administration of the corresponding hydroquinone conjugates (DC-[GSyl]HQ and TC-[GSyl]HQ), prepared by reducing the quinones with a threefold molar excess of ascorbic acid, resulted in a substantial increase in nephrotoxicity. Moreover, in contrast to other glutathione (GSH)-conjugated hydroquinones, the nephrotoxicity of both DC-(GSyl)HQ and TC-(GSyl)HQ was potentiated when rats were pretreated with AT-125, an irreversible inhibitor of gamma-GT. Neither the quinone-GSH nor the hydroquinone-GSH conjugates caused any effect on liver histology or serum glutamate-pyruvate transaminase levels. The results suggest that coadministration of ascorbic acid with DC-(GSyl)BQ or TC-(GSyl)BQ decreases their interactions with extrarenal nucleophiles, including plasma proteins, and thus increases the concentration of the conjugates delivered to the kidney, and hence toxicity. Furthermore the ability of AT-125 to potentiate the nephrotoxicity of DC-(GSyl)HQ and TC-(GSyl)HQ suggests that metabolism of these conjugates by gamma-GT constitutes a detoxication reaction.

Effect of different amino acidic pretreatments that protect the kidney against papillary necrosis induced by bromoethylamine on differential distribution of renal nonprotein sulfhydryls

Content of nonprotein sulfhydryls (NPSH) was found to be higher in rat renal cortex than in external medulla and papilla. Administration of bromoethylamine (BEA), at a dose that produces extensive papillary necrosis and minor effects in the other renal segments, induced a significant reduction in NPSH levels of renal cortex and external medulla, with no changes in the papilla. Treatment with N-acetyl-L-cysteine (NAC) elicited an increase in papillary NPSH and a decrease in the cortex, with opposite changes being observed with an amino acid mixture of glutamine, glycine, and cystine (AM). Similar results were found in animals pretreated with NAC or AM prior to BEA intoxication. These pretreatments protect the cortex, external medulla, and papilla from the necrosis induced by BEA. It is suggested that protection of BEA-induced renal necrosis by NAC or AM pretreatments might be due to different mechanisms, with NPSH playing direct or indirect roles, respectively.

Metabolism and vascular effects of gamma-L-glutamyl-L-dopa on the isolated rat kidney

Gamma-L-glutamyl-L-dopa (or gludopa), a dopamine (DA) prodrug, is selectively metabolized in vivo by the kidney through the sequential action of two renal enzymes, gamma-glutamyl transpeptidase (gamma-GT) and aromatic L-amino acid decarboxylase (AADC). This study was designed to analyze, in vitro, the factors regulating gludopa metabolism and its renal vascular effects. Rat kidneys were perfused in closed circuit with a cell-free perfusion buffer containing 6% bovine serum albumin (BSA). Adding gludopa (final concentration 10(-5) M in the perfusate) led to the release of DA both into urine and perfusate (0.53 +/- 0.21 and 1.38 +/- 0.28 nmol/min/g kidney wt, respectively, during the first 5 min after substrate addition, N = 5, mean +/- SEM). Total DA release (urine plus perfusate) was 73.7 +/- 15.8 nmol/g kidney wt within 30 minutes of recirculation. In non-filtering kidneys, total DA release in the recirculating medium was lower (12.5 +/- 1.4 nmol/g kidney wt, P less than 0.01). Glomerular filtration and access to the gamma-GT on the brush border membrane of proximal tubular cells are therefore required for the maximal conversion rate of gludopa. On filtering kidneys, L-dopa was also converted to DA, but at a higher rate than gludopa (total DA formed within 30 min of recirculation = 131.2 +/- 31.9 nmol/g kidney wt) and this rate was not reduced in non-filtering kidneys (224.2 +/- 41.7 nmol/g kidney wt DA formed within 30 min). Metabolic conversion of L-dopa by AADC is thus preserved in the case of an approach via the basolateral side of the proximal tubular cells. The renal vascular effects of gludopa were studied after vascular tone had been restored by continuous perfusion of PGF2 alpha and after the inhibition of alpha- and beta-adrenoceptors. Gludopa (3.10(-6) to 4.10(-5) M) elicited concentration-dependent renal vasodilatation.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal tubular transport of glutathione in rat kidney

Filtered glutathione (gamma-glutamyl-cysteinyl-glycine or GSH) is rapidly hydrolyzed by brush-border enzymes facing the tubular lumen and is reabsorbed in the form of the constituent amino acids. The first step of hydrolysis is catalyzed by gamma-glutamyltransferase (gamma-GT). We investigated localization and capacity of the rat renal glutathione degradation/reabsorption during elevation of the filtered load (intravenous infusion of 12 resp. 18 mumol GSH/min). Fractional excretion went up from about 0.003 to 0.31 +/- 0.02 SEM during infusion of the lower and to 0.49 +/- 0.03 SEM during infusion of the higher glutathione dose. GSH degradation/reabsorption took place along the entire proximal tubule and was partially saturated by a 150-200-fold elevation of the normal filtered load. Net reabsorption of GSH up to the last accessible superficial loop was significantly lower during infusion of 18 mumol GSH/min (0.3 mumol/min) than during infusion of 12 mumol GSH/min (1.6 mumol/min). In further experiments, infusion of 18 mumol GSH/min was preceded by the i.v. administration of acivicin (0.5 mmol/kg body wt.), an inhibitor of gamma-GT. In these experiments, fractional glutathione deliveries to late proximal and early distal tubules did not significantly differ from 1, fractional excretion of GSH at the same time was 1.46 +/- 0.11 SEM, revealing net secretion of GSH with the final urine. Tubular secretion of GSH in the acivicin-treated animals occurred either in distal tubules and/or collecting ducts or in the proximal tubules of deep nephrons which are not accessible to micropuncture.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma membrane lipids modulate the response to water deprivation in rat kidney

The lipid composition of biological membranes determines many of the cellular responses to stimuli such as hormones and drugs. It is known that the neonate has differences in renal function compared to the adult individual, which determine the characteristics of pharmacokinetics and pharmacodynamics at this age. We studied the lipid composition of renal plasma membranes, the renal response to water deprivation, and the activity of plasma membrane gamma-glutamyl transpeptidase (GGTP) in newborn, 21-d-old, and adult rat. It was found that the cholesterol/phospholipid (CH/PL) ratio of newborn membranes was significantly higher than that of membranes from 21-d-old and adult rats, which were similar. These findings paralleled low ability to concentrate urine in the newborn and higher sensitivity of the neonatal kidney to water deprivation. Kidney membranes from 21-d-old and adult rats showed similar lipid composition, and the response to water deprivation was also similar. The changes observed in GGTP activity at different ages varied with the cholesterol content of kidney plasma membranes. These findings suggest participation of the lipid composition of kidney membranes in modulation of the renal response to water deprivation.

Adaptation of a gamma-glutamyl transpeptidase assay to microtiter plates

A kinetic assay for measuring gamma-glutamyl transpeptidase (GGT) activity has been adapted to microtiter plates and an automated microtiter plate reader. This method permits the simultaneous analysis of enzyme activity in a large number of samples incubated with the chromogenic GGT substrate gamma-glutamyl-p-nitroanilide. A major advantage of this assay over previously reported methods is the substantial reduction in the time needed for measuring sample enzyme activity. In addition, reduction of the total assay volume to 0.28 ml conserves both sample and reagents. This method has been calibrated at 23 degrees C using purified GGT, and used to analyze GGT activity in human sera. The assay is sensitive over a range of 3-200 U/liter.

gamma-Glutamyl transpeptidase excretion in cisplatin-induced acute renal failure

Cisplatin (cisdiamminedichloroplatinum), an important antineoplastic agent, possesses nephrotoxicity as its major side effect. A mild partially reversible nonoliguric form of acute renal failure (ARF) is generally the most common form of nephrotoxicity and occurs in experimental animals following a single dose. gamma-Glutamyl transpeptidase (gamma GT) is an enzyme with maximal activity located in the brush border of proximal renal tubular epithelium. To test the relationship between cisplatin nephrotoxicity and urinary gamma GT excretion, rats received a single 5.5 mg/kg dose of cisplatin and gamma GT excretion was evaluated and compared to anatomic and functional damage. Twenty-four hours following cisplatin administration, there was a marked enhancement of urinary gamma GT excretion, prior to the onset of azotemia. Urinary gamma GT excretion peaked at day 4, then returned to baseline, and decreased to values below baseline on days 8 through 10. By days 11 through 12, renal function and urinary gamma GT excretion had returned to normal. The correlation between nephrotoxicity, changes in urinary gamma GT excretion, and anatomic damage was excellent. Morphologically, increased gamma GT excretion was associated with loss of microvilli, and the return of urinary gamma GT excretion to normal correlated with their regeneration. We conclude that cisplatin administration results in increased urinary gamma GT excretion. This early enhancement, prior to the onset of azotemia, may provide a useful noninvasive marker of early cisplatin nephrotoxicity.

Localization of nucleotide pyrophosphatase in the rat kidney

Hydrolysis of NAD by a nucleotide pyrophosphatase of renal membrane fractions has been reported previously. The aim of the present study was to localize this enzyme in the rat kidney. Nucleotide pyrophosphatase was assayed in glomeruli, in three parts of the proximal tubule and in four parts of the distal tubule dissected form freeze-dried sections. Nucleotide pyrophosphatase activity, expressed in mumol X min-1 X mg protein-1, ranged between 9.8 and 32.3 in the proximal tubular segments and between 1.1 and 2.7 in the distal tubular segments. It was 3.4 in the glomeruli. The enrichment of the activity during the purification of brush border vesicles was measured. A ten-fold higher specific activity was found in the brush border vesicles as compared to the renal cortical homogenates. Thus, most of the renal nucleotide pyrophosphatase appears to be localized in the luminal membrane of the proximal tubule. A permeabilization of the membrane did not increase the activity of brush border vesicles. This indicates that all catalytic sites are accessible at the outer surface of the membrane.

Tubulopathy in nephrolithiasis: consequence rather than cause

To address whether a renal tubular dysfunction is encountered in a particular patient subgroup with urolithiasis, the following parameters of tubular function were measured in urine taken in the morning from 214 stone formers after fasting: pH, excretion of lysozyme and gamma-glutamyl transferase (gamma-GT); fractional excretion (FE) of glucose, insulin, Mg, K, and HCO3 after an alkali loading; and the renal threshold for phosphate (TmP/GFR). The following diagnoses were made in the patient group: primary hyperparathyroidism (N = 8), medullary sponge kidneys (N = 21), hyperuricemia (N = 10), cystinuria (N = 2), struvite stone disease (N = 6), idiopathic hypercalciuria of the absorptive (N = 25), dietary (N = 69) or renal (N = 7) type, and normocalciuric idiopathic urolithiasis (N = 66). In 31% of the patients TmP/GFR was below 0.80 mmole/liter and in 13% of the patients, FE HCO3 after alkali loading was above normal. Urinary excretion of lysozyme and that of gamma-GT both were elevated in 17% of the patients. FE glucose, FE insulin, FE Mg, and FE K were elevated in 8, 9, 3, and 7% of the patients, respectively. This study demonstrates that a significant number of stone formers present with signs of renal tubular dysfunction, primarily involving the proximal tubule since apparent leaks of phosphate and of bicarbonate were most frequently encountered. The defects were not specific for a given etiologic group of patients; on the other hand, occurrence was related to the presence of large stones in the pyelocaliceal system at the time data were gathered. Taken together these data suggest that the tubulopathy in nephrolithiasis is the consequence rather than the cause of the stone.

Ammoniagenesis catalyzed by hippurate-activated gamma-glutamyltransferase in the lumen of the proximal tubule. A microperfusion study in rat kidney in vivo

gamma-Glutamyltransferase (gamma-GT) is located in the brushborder membrane of the proximal tubule where the catalytic site of the enzyme faces the lumen. The (phosphate-independent) glutaminase activity of gamma-GT in vitro is activated by hippurate. In order to investigate glutamine deamidation in the tubule lumen in vivo, 14C-L-glutamine-containing solutions were continuously microperfused through sections of the proximal convoluted tubule in vivo and in situ. D-aspartate and L-phenylalanine (10 mmol/l, each) were added to the perfusate in order keep the reabsorption of L-glutamine as such low and to block reabsorption of any glutamate possibly formed, respectively. Intraluminal formation of glutamate from glutamine in the absence of hippurate is small. In presence of 10 mmol/l hippurate, 5%-70% of the recovered 14C-activity was 14C-glutamate at an initial 14C-L-glutamine concentration of 1 mmol/l. The respective absolute rate (+/- SEM) of glutamate formation, i.e., 36 +/- 5 pmol X s-1 X m-1, was increased 1.4-fold at an initial L-glutamine concentration of 3 mmol/l, but dropped to one third at initially 0.3 mmol/l. A rough estimate of the apparent kinetic constants resulted in a Km of 0.58 (0.19-0.97) mmol/l and a Vmax of 56 (40-93) pmol X s-1 X m-1. Deamidation of glutamine occurred also in the absence of L-phenylalanine. Acivicin (AT 125), a gamma-GT inhibitor, completely blocked glutamate formation. Endogenous hippurate concentrations determined by free flow micropuncture and HPLC were 0.16 mmol/l in the late proximal convolution, 0.6 mmol/l in the early distal convolution, and 4.9 mmol/l in the final urine.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydroxyproline metabolism by the rat kidney: distribution of renal enzymes of hydroxyproline catabolism and renal conversion of hydroxyproline to glycine and serine

The metabolism of hydroxyproline by the rat kidney leads to the production of significant quantities of both glycine and serine. This process was observed in both the isolated perfused kidney and in isolated cortical tubule suspensions. The rate of hydroxyproline metabolism was increased in both preparations by the addition of alanine. The distribution of hydroxyproline oxidase, hydroxyoxoglutarate aldolase and alanine-glyoxalate transaminase were determined in detail. All three enzymes were found exclusively in the renal cortex where they were restricted to the mitochondria. Cortical tubule fractionation studies indicated that the enzymes are located in the proximal convoluted and proximal straight segments at the nephron. The results suggest that hydroxyproline degradation could contribute significantly to the renal synthesis of serine.

Phosphate transport by brushborder membranes from superficial and juxtamedullary cortex

In vivo studies indicate that the extent of phosphate (Pi) reabsorption differs in proximal tubules of superficial (SC) and juxtamedullary (JM) nephrons. Since Na-gradient (Nao greater than Nai) dependent uptake of Pi by the luminal brushborder membrane (BBM) may be the rate-determining step in proximal tubular reabsorption, we studied this transport system in brushborder membrane vesicles (BBMV) prepared from SC and JM renal cortex of dogs fed either a low phosphorus diet (LPD, 0.07% Pi) or high phosphorus diet (HPD, 1.2% Pi). In the initial uphill phase (that is, "overshoot"), the rate of Na-gradient dependent uptake of Pi was significantly greater [delta + 35%] in BBMV from the SC cortex (BBMV-SC) than in BBMV from the JM cortex (BBMV-JM) of the dogs fed LPD. Higher Na-dependent Pi uptake was due to significantly (P less than 0.05) higher apparent Vmax (mean +/- SEM, nmoles Pi/0.5 min/mg protein) for Pi in BBMV-SC (7.5 +/- 1.57) compared with Vmax in BBMV-JM (6.05 +/- 1.74). Higher transport of Pi in BBMV-SC compared with BBMV-JM of dogs fed LPD was a difference relatively specific for the Na-dependent Pi uptake system; Na+ independent uptake of Pi and Na-dependent uptake of D-glucose were lower in BBMV-SC than in BBMV-JM. The size of BBMV or rate of Na+ uptake did not differ between BBMV-SC and BBMV-JM. The Na-gradient dependent uptake of Pi was no different between BBMV-SC and BBMV-JM from dogs stabilized on HPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation, culture and characterization of human renal tubular cells

Tubular cells have been isolated, characterized and cultured from more than 70 adult cadaver kidneys (postmortem time less than or equal to 12 hr.). Confluent monolayers were observed at 7 days after seeding (10(6) cells/ml.) and cells demonstrating normal human karyotypes have been passaged up to 6 times. Primary isolates and monolayer cultures were negative for Factor VIII activity, and strongly positive for gamma-glutamyltransferase activity and keratin. Ultrastructurally primary isolates consisted of cells with numerous mitochondria, microvilli, cytoplasmic filaments and well-developed endocytotic apparati. Monolayer cultures examined at 7, 14, 21 and 72 days demonstrated less prominent microvilli and the additional structures of desmosomes and cell junctions. Membrane-associated and cytosolic enzyme activities were measured up to 28 days in culture. The membrane-associated enzymes gamma-glutamyltransferase and alkaline phosphatase both exhibited approximately 10-fold decreases in activity during the 1st 7 days in culture. There was an approximately 5-fold increase in pyruvate kinase activity during the same time period, while fructose-1,6-bisphosphatase activity exhibited a 5-fold decrease. Glucose-6-phosphatase activity did not change during the 28 day culture period examined. From 7 to 28 days no further changes were noted in any of the enzyme activities measured. Decreased membrane-associated enzyme activity corresponded to the ultrastructural observation of less prominent microvilli. Increases in glycolytic enzyme activity and decreases in gluconeogenic enzyme activity may reflect the presence of glucose in the culture medium. The morphologic and biochemical evidence suggests that primary isolates and cultures are proximal tubule cells which should provide a well-defined in vitro human system for future studies.

Identification of proximal tubule segments in the mouse nephron by simultaneous visualization of alkaline phosphatase and gamma-glutamyl transpeptidase

ALP and gamma-GT are 2 brush border enzymes that can be individually demonstrated on adjacent sections by the histochemical methods of Mayahara (ALP) and Rutenberg (gamma-GT). On the basis of each enzyme activity, it was possible to recognize different categories of tubules in the mouse nephron. In fact, both enzymes were heterogeneously distributed along the proximal tubule, but in opposite gradients. The various staining intensities probably corresponded to proximal segmentation, but were sometimes difficult to evaluate. A technique was perfected to localize both enzymes in the same tissue section. Since each enzyme produced a distinct type of colored precipitates (ALP: black, gamma-GT: red), 4 categories of tubules could be identified, according to staining characteristics: 1. black tubules where ALP activity was predominant, corresponded to S1 segments, 2. black and red tubules where the 2 activities were about equivalent, were considered as parts of S2, 3. red ones where gamma-GT activity was high, were identified as portions of S3, 4. negative tubules where no activity was apparent, represented distal and straight collecting tubules. In addition to economize time and tissue, this simple technique permits to easily estimate variations in enzyme activities that probably correspond to structural and functional differences in the segments of the proximal tubule.

Comparative effects of antioxidants on enzymes involved in glutathione metabolism

The present study was designed to examine changes in glutathione metabolism in the liver of mice as influenced by supplementation of their diet with 1 of 4 antioxidants: butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), vitamin E and selenium. In addition to determination of the acid-soluble thiol levels, 5 different enzymes involved with glutathione utilization and synthesis were measured: glutathione transferase, gamma-glutamyl transpeptidase, selenium-dependent glutathione peroxidase, gamma-glutamylcysteine synthetase and glutathione reductase. All 4 antioxidants produced significant increases in glutathione transferase activity, with BHA and BHT being much more effective than the other two. With the exception of vitamin E, BHA, BHT and selenium all resulted in a slight enhancement in the activity of glutathione reductase as well as in the acid-soluble thiol level. On the other hand, the induction of gamma-glutamyl transpeptidase and gamma-glutamylcysteine synthetase was responsive to only vitamin E and selenium supplementation, respectively. Although the influence of each of these antioxidants in glutathione metabolism appears to be specific and somewhat compartmentalized, the overall impression is that of an increased capacity for glutathione-conjugate formation and recovery of reduced glutathione. These biochemical changes in glutathione metabolism may be relevant to the anticarcinogenic effects observed with BHA, BHT and selenium.

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

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-[alpha S, 5S]-alpha-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 degrees C with rat brushborder membrane vesicles. AT-125 given to rats in 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 both in vivo and in 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.

Immunocytochemical localization of gamma-glutamyl-transferase on isolated renal cortical tubular fragments

The localization of gamma-Glutamyltransferase (gamma-GT, E.C.2.3.2.2) was studied on isolated tubular fragments from rat kidney cortex immunocytochemically. Monospecific antibodies raised in the goat against rat kidney gamma-GT were used. Antigoat immunoglobulin from the rabbit conjugated with ferritin was used for visualisation of the antibody binding sites. The enzyme was found to be localized at the brush border membrane of proximal tubules, the luminal membrane of distal tubules and collecting duct segments. The enzyme could further be localized on the antiluminal or basolateral cell membranes of proximal and distal tubular fragments, whereas no such localization was verified for collecting duct segments. The role of this basolateral gamma-GT localization in context with the kidney's ability to extract over 83% of the renal arterial glutathione (GSH) input during a single passage is discussed.

Characterization of primary rabbit kidney cultures that express proximal tubule functions in a hormonally defined medium

Primary cultures of rabbit-kidney epithelial cells derived from purified proximal tubules were maintained without fibroblast overgrowth in a hormone-supplemented serum-free medium (Medium RK-1). A hormone-deletion study indicated that the primary cultures derived from purified rabbit proximal tubules required all of the three supplements in Medium RK-1 (insulin, transferrin, and hydrocortisone) for optimal growth but did not grow in response to EGF and T3. In contrast, the epithelial cells in primary cultures derived from an unpurified preparation of rabbit kidney tubules and glomeruli grew in response to EGF and T3, as well as insulin, transferrin, and hydrocortisone. These observations suggest that kidney epithelial cells derived from different segments of the nephron grow differently in response to hormones and growth factors. Differentiated functions of the primary cultures derived from proximal tubules were examined. Multicellular domes were observed, indicative of transepithelial solute transport by the monolayers. The proximal tubule cultures also accumulated alpha-methylglucoside (alpha-MG) against a concentration gradient. However, little or no alpha-MG accumulation was observed in the absence of Na+. Metabolic inhibitor studies also indicated that alpha-MG uptake by the primaries is an energy-dependent process, and depends upon the activity of the Na+/K+ ATPase. Phlorizin at 0.1 mM significantly inhibited 1 mM alpha-MG uptake whereas 0.1 mM phloretin did not have a significant inhibitory effect. Similar observations have been made concerning the Na+-dependent sugar-transport system located on the lumenal side of the proximal tubule, whereas the Na+-independent sugar transporter on the peritubular side is more sensitive to inhibition by phloretin than phlorizin. The cultures also exhibited PTH-sensitive cyclic AMP synthesis and brush-border enzymes typical of proximal cells. However, the activities of the enzymes leucine aminopeptidase, alkaline phosphatase, and gamma-glutamyl-transpeptidase were lower in the cultures than in purified proximal-tubule preparations from which they are derived.

Characterization of neutral amino acid uptake by cultured epithelial cells from pig kidney

Two transport systems for neutral amino acids have been characterised in LLC-PK, cells. The first, which transports alanine in a sodium-dependent manner, also mediates alanine exchange and is perferentially inhibited by serine, cysteine, and alpha-amino-n-butyric acid. This system resembles the ASC system in Ehrlich ascites and some other cell types. There is only a small contribution of other systems to alanine uptake. The second, which transports leucine with no requirement for sodium and mediates leucine exchange, is blocked by 2-aminonorbornane-2-carboxylic acid and hydrophobic amino acids. This system is similar to the L system described in other cell types. LLC-PK1 cells retain several other features implying renal proximal tubule origin; our results thus suggest that these transport systems may be involved in the reabsorption of neutral amino acids by the nephron in vivo.

Glutathione and gamma-glutamyl cycle enzymes in rat mammary gland

The main enzymes of the gamma-glutamyl cycle during the lactogenic cycle in rat mammary gland were studied. A significant increase was found in all of them with the onset of lactogenesis. The effect of methionine sulfoximine on reduced glutathione concentration was studied in tissue slices of lactating mammary gland. The findings suggest that this compound effects glutathione synthesis by inhibiting gamma-glutamylcysteine synthetase.

Function of renal gamma-glutamyltransferase: significance of glutathione and glutamine interactions

The membrane bound gamma-glutamyltransferase (gamma-GT) is capable of utilizing both glutathione, GSH, and glutamine, gln, as the natural gamma-glutamyl donor. The enzyme is oriented in the membrane to react with extracellular substrates and is present on both the brush border and peritubular capillaries. The reaction catalyzed by gamma-GT is critically dependent upon the ratio of gamma-glutamyl donor/gamma-GT which under near physiological conditions results in the formation of gamma-glutamyl peptide and glu on the blood and urine side respectively; this effectively establishes an osmotic gradient which could contribute to transepithelial and transcapillary water fluxes. Interestingly, utilization of extrarenal gamma-glutamyl substrates are quantitatively more significant in the microvascular than brush border location. Delivery of gln to these enzymes sites is some 20 times greater than GSH. Gln utilization unlike GSH is limited by the reaction with the gamma-glutamyl donor site; thus reactivity is greatly enhanced by the maleate like activator hippurate which may account for the acidosis-induced adaptation in ammonia formation from gln by this enzyme. Coupled to a role in ammoniagenesis the brush border enzyme appears to play a role in the reabsorption of filtered gln. The hydrolysis of filtered GSH as well as its utilization in transfer reactions involved in amino acid reabsorption at the brush border may reflect the role of the enzyme in eliminating osmotically active solutes from the urine and thereby facilitating water fluxes. However the role of gln as a gamma-glutamyl donor relative to GSH will depend upon the quantitative significance of tubular GSH synthesis and secretion.

Evidence for different localization of glutathione oxidase and gamma-glutamyltransferase activities during extracellular glutathione metabolism in isolated perfused rat kidney

The metabolism of extracellular glutathione was studied in the isolated, perfused rat kidney. The results indicate different localization of glutathione oxidase and gamma -glutamyltransferase (5-glutamyl)-peptide: aminoacid 5-glutamyltransferase, EC 2.3.2.2) activities, since glutathione oxidase activity was observed only with glutathione present in the perfusate, whereas gamma -glutamyltransferase-mediated metabolism of glutathione was restricted to glutathione present to the localization of renal gamma -glutamyltransferase in the brush border membranes of the tubular epithelium, but suggest an opposite localization of renal glutathione oxidase activity, i.e., in the basal plasma membrane fraction of the tubular cells, facing the capillary bloodstream. Furthermore, the existence of the tubular glutathione extraction mechanism operating in addition to glomerular filtration is confirmed.

gamma-Glutamyl transpeptidase: catalytic, structural and functional aspects

gamma-Glutamyl transpeptidase catalyzes transfer of the gamma-glutamyl moiety of glutathione to amino acids, dipeptides, and to glutathione itself; the enzyme also catalyzes the hydrolysis of glutathione to glutamate and cysteinyl-glycine. This review deals with the tissue distribution and localization of the enzyme in mammals, the catalytic properties of the enzyme (including its inhibition by reversible and irreversible inhibitors), structural studies on the enzyme, and new findings about its physiological function.

Human renal gamma-glutamyltransferase hydrolysis and transfer reactions with glutathione as substrate

The hydrolysis and transfer reactions of purified human renal gamma-glutamyltransferase were studied in vitro with glutathione as substrate at pH and substrate concentration reflecting the physiological conditions. The pH optimum ranged from 7.48 to 8.44 for hydrolysis and 7.90 to 8.92 for transfer with glutamine as acceptor. The Michaelis constants for glutathione were 13 microM in hydrolysis and 58 microM in transfer reactions respectively. Inhibition of transfer occurred for glutathione concentrations above 0.4 mM. Various ions, urea, creatinine, uric acid and L-amino acids were shown to have no appreciable effect on both reactions except L-glutamine which acts as an activator on the hydrolysis activity. Taken together, our results, if they are transposable in vivo would be relevant of an enzyme acting like an hydrolase rather than like a transferase.

Active-site amino acid residues in gamma-glutamyltransferase and the nature of the gamma-glutamyl-enzyme bond

Active-site residues in rat kidney gamma-glutamyltransferase (EC 2.3.2.2) were investigated by means of chemical modification. 1. In the presence of maleate, the activity was inhibited by phenylmethanesulphonyl fluoride, and the inhibition was not reversed by beta-mercaptoethanol, suggesting that a serine residue is close to the active site, but is shielded except in the presence of maleate. 2. Treatment of the enzyme with N-acetylimidazole modified an amino group, exposed a previously inaccessible cysteine residue and inhibited hydrolysis of the gamma-glutamyl-enzyme intermediate, but not its formation. 3. After reaction of the enzyme successively with N-acetylimidazole and with non-radioactive iodoacetamide/serine/borate, two active-site residues reacted with iodo[(14)C]acetamide. One of these possessed a carboxy group, which formed a [(14)C]glycollamide ester, and the other was cysteine, shown by isolation of S-[(14)C]carboxymethylcysteine after acid hydrolysis. When N-acetylimidazole treatment was omitted, only the carboxy group reacted with iodo[(14)C]acetamide. 4. Isolation of the gamma-[(14)C]glutamyl-enzyme intermediate was made easier by prior treatment of the enzyme with N-acetylimidazole. The gamma-glutamyl-enzyme bond was stable to performic acid, and to hydroxylamine/urea at pH10, but was hydrolysed slowly at pH12, indicating attachment of the gamma-[(14)C]glutamyl group in amide linkage to an amino group on the enzyme. Proteolysis of the gamma-[(14)C]glutamyl-enzyme after performic acid oxidation gave rise to a small acidic radioactive peptide that was resistant to further proteolysis and was not identical with gamma-glutamyl-epsilon-lysine. 5. A scheme for the catalytic mechanism is proposed.