Renal catabolism of glutathione. Characterization of a particulate rat renal dipeptidase that catalyzes the hydrolysis of cysteinylglycine

Dipeptidase 1 promotes ferroptosis in renal tubular epithelial cells in diabetic nephropathy via inhibition of the GSH/GPX4 axis

Renal tubular injury is an important pathological change associated with diabetic nephropathy (DN), in which ferroptosis of renal tubular epithelial cells is critical to its pathogenesis. Inhibition of the glutathione/glutathione peroxidase 4 (GSH/GPX4) axis is the most important mechanism in DN tubular epithelial cell ferroptosis, but the underlying reason for this is unclear. Our biogenic analysis showed that a zinc-dependent metalloproteinase, dipeptidase 1 (DPEP1), is associated with DN ferroptosis. Here, we investigated the role and mechanism of DPEP1 in DN tubular epithelial cell ferroptosis. DPEP1 upregulation was observed in the renal tubular epithelial cells of DN patients and model mice, as well as in HK-2 cells stimulated with high glucose. Furthermore, the level of DPEP1 upregulation was associated with the degree of tubular injury in DN patients and HK-2 cell ferroptosis. Mechanistically, knocking down DPEP1 expression could alleviate the inhibition of GSH/GPX4 axis and reduce HK-2 cell ferroptosis levels in a high glucose environment. HK-2 cells with stable DPEP1 overexpression also showed GSH/GPX4 axis inhibition and ferroptosis, but blocking the GSH/GPX4 axis could mitigate these effects. Additionally, treatment with cilastatin, a DPEP1 inhibitor, could ameliorate GSH/GPX4 axis inhibition and relieve ferroptosis and DN progression in DN mice. These results revealed that DPEP1 can promote ferroptosis in DN renal tubular epithelial cells via inhibition of the GSH/GPX4 axis.

Genetic studies of paired metabolomes reveal enzymatic and transport processes at the interface of plasma and urine

The kidneys operate at the interface of plasma and urine by clearing molecular waste products while retaining valuable solutes. Genetic studies of paired plasma and urine metabolomes may identify underlying processes. We conducted genome-wide studies of 1,916 plasma and urine metabolites and detected 1,299 significant associations. Associations with 40% of implicated metabolites would have been missed by studying plasma alone. We detected urine-specific findings that provide information about metabolite reabsorption in the kidney, such as aquaporin (AQP)-7-mediated glycerol transport, and different metabolomic footprints of kidney-expressed proteins in plasma and urine that are consistent with their localization and function, including the transporters NaDC3 (SLC13A3) and ASBT (SLC10A2). Shared genetic determinants of 7,073 metabolite-disease combinations represent a resource to better understand metabolic diseases and revealed connections of dipeptidase 1 with circulating digestive enzymes and with hypertension. Extending genetic studies of the metabolome beyond plasma yields unique insights into processes at the interface of body compartments.

Dipeptidase-1 governs renal inflammation during ischemia reperfusion injury

The mechanisms that drive leukocyte recruitment to the kidney are incompletely understood. Dipeptidase-1 (DPEP1) is a major neutrophil adhesion receptor highly expressed on proximal tubular cells and peritubular capillaries of the kidney. Renal ischemia reperfusion injury (IRI) induces robust neutrophil and monocyte recruitment and causes acute kidney injury (AKI). Renal inflammation and the AKI phenotype were attenuated in Dpep1^-/- mice or mice pretreated with DPEP1 antagonists, including the LSALT peptide, a nonenzymatic DPEP1 inhibitor. DPEP1 deficiency or inhibition primarily blocked neutrophil adhesion to peritubular capillaries and reduced inflammatory monocyte recruitment to the kidney after IRI. CD44 but not ICAM-1 blockade also decreased neutrophil recruitment to the kidney during IRI and was additive to DPEP1 effects. DPEP1, CD44, and ICAM-1 all contributed to the recruitment of monocyte/macrophages to the kidney following IRI. These results identify DPEP1 as a major leukocyte adhesion receptor in the kidney and potential therapeutic target for AKI.

Moonlighting in drug metabolism

Drug metabolizing enzymes catalyze the biotransformation of many of drugs and chemicals. The drug metabolizing enzymes are distributed among several evolutionary families and catalyze a range of detoxication reactions, including oxidation/reduction, conjugative, and hydrolytic reactions that serve to detoxify potentially toxic compounds. This detoxication function requires that drug metabolizing enzymes exhibit substrate promiscuity. In addition to their catalytic functions, many drug metabolizing enzymes possess functions unrelated to or in addition to catalysis. Such proteins are termed 'moonlighting proteins' and are defined as proteins with multiple biochemical or biophysical functions that reside in a single protein. This review discusses the diverse moonlighting functions of drug metabolizing enzymes and the roles they play in physiological functions relating to reproduction, vision, cell signaling, cancer, and transport. Further research will likely reveal new examples of moonlighting functions of drug metabolizing enzymes.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

The Non-Essential Amino Acid Cysteine Becomes Essential for Tumor Proliferation and Survival

The non-essential amino acid cysteine is used within cells for multiple processes that rely on the chemistry of its thiol group. Under physiological conditions, many non-transformed tissues rely on glutathione, circulating cysteine, and the de novo cysteine synthesis (transsulfuration) pathway as sources of intracellular cysteine to support cellular processes. In contrast, many cancers require exogeneous cystine for proliferation and viability. Herein, we review how the cystine transporter, xCT, and exogenous cystine fuel cancer cell proliferation and the mechanisms that regulate xCT expression and activity. Further, we discuss the potential contribution of additional sources of cysteine to the cysteine pool and what is known about the essentiality of these processes in cancer cells. Finally, we discuss whether cyst(e)ine dependency and associated metabolic alterations represent therapeutically targetable metabolic vulnerabilities.

The glutathione cycle: Glutathione metabolism beyond the γ-glutamyl cycle

Glutathione was discovered in 1888, over 125 years ago. Since then, our understanding of various functions and metabolism of this important molecule has grown over these years. But it is only now, in the last decade, that a somewhat complete picture of its metabolism has emerged. Glutathione metabolism has till now been largely depicted and understood by the γ-glutamyl cycle that was proposed in 1970. However, new findings and knowledge particularly on the transport and degradation of glutathione have revealed that many aspects of the γ-glutamyl cycle are incorrect. Despite this, an integrated critical analysis of the cycle has never been undertaken and this has led to the cycle and its errors perpetuating in the literature. This review takes a careful look at the γ-glutamyl cycle and its shortcomings and presents a "glutathione cycle" that captures the current understanding of glutathione metabolism. © 2018 IUBMB Life, 70(7):585-592, 2018.

Aminopeptidase N

The third edition of the Handbook of Proteolytic Enzymes aims to be a comprehensive reference work for the enzymes that cleave proteins and peptides, and contains over 850 chapters. Each chapter is organized into sections describing the name and history, activity and specificity, structural chemistry, preparation, biological aspects, and distinguishing features for a specific peptidase. The subject of Chapter 79 is Aminopeptidase N.

Keywords

Actinonin, amastatin, angiogenesis, angiotensin, bestatin, brush border, cancer, CD13, coronavirus, cysteinyl-glycinase, dipeptidyl peptidase IV, enkephalin, glutathione, neprilysin, puromycin, stem cells.

Structure, mechanism, and substrate profile for Sco3058: the closest bacterial homologue to human renal dipeptidase

Human renal dipeptidase, an enzyme associated with glutathione metabolism and the hydrolysis of beta-lactams, is similar in sequence to a cluster of approximately 400 microbial proteins currently annotated as nonspecific dipeptidases within the amidohydrolase superfamily. The closest homologue to the human renal dipeptidase from a fully sequenced microbe is Sco3058 from Streptomyces coelicolor. Dipeptide substrates of Sco3058 were identified by screening a comprehensive series of l-Xaa-l-Xaa, l-Xaa-d-Xaa, and d-Xaa-l-Xaa dipeptide libraries. The substrate specificity profile shows that Sco3058 hydrolyzes a broad range of dipeptides with a marked preference for an l-amino acid at the N-terminus and a d-amino acid at the C-terminus. The best substrate identified was l-Arg-d-Asp (k(cat)/K(m) = 7.6 x 10(5) M(-1) s(-1)). The three-dimensional structure of Sco3058 was determined in the absence and presence of the inhibitors citrate and a phosphinate mimic of l-Ala-d-Asp. The enzyme folds as a (beta/alpha)(8) barrel, and two zinc ions are bound in the active site. Site-directed mutagenesis was used to probe the importance of specific residues that have direct interactions with the substrate analogues in the active site (Asp-22, His-150, Arg-223, and Asp-320). The solvent viscosity and kinetic effects of D(2)O indicate that substrate binding is relatively sticky and that proton transfers do not occurr during the rate-limiting step. A bell-shaped pH-rate profile for k(cat) and k(cat)/K(m) indicated that one group needs to be deprotonated and a second group must be protonated for optimal turnover. Computational docking of high-energy intermediate forms of l/d-Ala-l/d-Ala to the three-dimensional structure of Sco3058 identified the structural determinants for the stereochemical preferences for substrate binding and turnover.

Dug1p Is a Cys-Gly peptidase of the gamma-glutamyl cycle of Saccharomyces cerevisiae and represents a novel family of Cys-Gly peptidases

GSH metabolism in yeast is carried out by the gamma-glutamyl cycle as well as by the DUG complex. One of the last steps in the gamma-glutamyl cycle is the cleavage of Cys-Gly by a peptidase to the constitutent amino acids. Saccharomyces cerevisiae extracts carry Cys-Gly dipeptidase activity, but the corresponding gene has not yet been identified. We describe the isolation and characterization of a novel Cys-Gly dipeptidase, encoded by the DUG1 gene. Dug1p had previously been identified as part of the Dug1p-Dug2p-Dug3p complex that operates as an alternate GSH degradation pathway and has also been suggested to function as a possible di- or tripeptidase based on genetic studies. We show here that Dug1p is a homodimer that can also function in a Dug2-Dug3-independent manner as a dipeptidase with high specificity for Cys-Gly and no activity toward tri- or tetrapeptides in vitro. This activity requires zinc or manganese ions. Yeast cells lacking Dug1p (dug1Delta) accumulate Cys-Gly. Unlike all other Cys-Gly peptidases, which are members of the metallopeptidase M17, M19, or M1 families, Dug1p is the first to belong to the M20A family. We also show that the Dug1p Schizosaccharomyces pombe orthologue functions as the exclusive Cys-Gly peptidase in this organism. The human orthologue CNDP2 also displays Cys-Gly peptidase activity, as seen by complementation of the dug1Delta mutant and by biochemical characterization, which revealed a high substrate specificity and affinity for Cys-Gly. The results indicate that the Dug1p family represents a novel class of Cys-Gly dipeptidases.

Gene expression levels in normal human lymphoblasts with variable sensitivities to arsenite: identification of GGT1 and NFKBIE expression levels as possible biomarkers of susceptibility

Drinking arsenic-contaminated water is associated with increased risk of neoplasias of the skin, lung, bladder and possibly other sites, as well as other diseases. Earlier, we showed that human lymphoblast lines from different normal unexposed donors showed variable sensitivities to the toxic effects of arsenite. In the present study, we used microarray analysis to compare the basal gene expression profiles between two arsenite-resistant (GM02707, GM00893) and two arsenite-sensitive lymphoblast lines (GM00546, GM00607). A number of genes were differentially expressed in arsenite-sensitive and arsenite-resistant cells. Among these, gamma-glutamyltranspeptidase 1 (GGT1) and NF kappa B inhibitor-epsilon (NFKBIE) showed higher expression levels in arsenite-resistant cells. RT-PCR analysis with gene-specific primers confirmed these results. Reduction of GGT1 expression level in arsenite-resistant lymphoblasts with GGT1-specific siRNA resulted in increased cell sensitivity to arsenite. In conclusion, we have demonstrated for the first time that expression levels of GGT1 and possibly NFKBIE might be useful as biomarkers of genetic susceptibility to arsenite. Expression microarrays can thus be exploited for identifying additional biomarkers of susceptibility to arsenite and to other toxicants.

Cisplatin-induced toxicity is associated with platinum deposition in mouse kidney mitochondria in vivo and with selective inactivation of the alpha-ketoglutarate dehydrogenase complex in LLC-PK1 cells

The anticancer drug cisplatin is nephrotoxic and neurotoxic. Previous data support the hypothesis that cisplatin is bioactivated to a nephrotoxicant. The final step in the proposed bioactivation is the formation of a platinum-cysteine S-conjugate followed by a pyridoxal 5'-phosphate (PLP)-dependent cysteine S-conjugate beta-lyase reaction. This reaction would generate pyruvate, ammonium, and a highly reactive platinum (Pt)-thiol compound in vivo that would bind to proteins. In this work, the cellular location and identity of the PLP-dependent cysteine S-conjugate beta-lyase were investigated. Pt was shown to bind to proteins in kidneys of cisplatin-treated mice. The concentration of Pt-bound proteins was higher in the mitochondrial fraction than in the cytosolic fraction. Treatment of the mice with aminooxyacetic acid (AOAA, a PLP enzyme inhibitor), which had previously been shown to block the nephrotoxicity of cisplatin, decreased the binding of Pt to mitochondrial proteins but had no effect on the amount of Pt bound to proteins in the cytosolic fraction. These data indicate that a mitochondrial enzyme catalyzes the PLP-dependent cysteine S-conjugate beta-lyase reaction. PLP-dependent mitochondrial aspartate aminotransferase (mitAspAT) is a mitochondrial enzyme that catalyzes beta-elimination reactions with cysteine S-conjugates of halogenated alkenes. We reasoned that the enzyme might also catalyze a beta-lyase reaction with the cisplatin-cysteine S-conjugate. In this study, mitAspAT was stably overexpressed in LLC-PK(1) cells. Cisplatin was significantly more toxic in confluent monolayers of LLC-PK(1) cells that overexpressed mitAspAT than in control cells containing vector alone. AOAA completely blocked the cisplatin toxicity in confluent mitAspAT-transfected cells. The Pt-thiol compound could rapidly bind proteins and inactivate enzymes in close proximity of the PLP-dependent cysteine S-conjugate beta-lyase. Treatment with 50 or 100 microM cisplatin for 3 h, followed by removal of cisplatin from the medium for 24 h, resulted in a pronounced loss of alpha-ketoglutarate dehydrogenase complex (KGDHC) activity in both mitAspAT-transfected cells and control cells. Exposure to 100 microM cisplatin resulted in a significantly greater loss of KGDHC activity in the cells overexpressing mitAspAT than in control cells. Aconitase activity was diminished in both cell types, but only at the higher level of exposure to cisplatin. AspAT activity was also significantly decreased by cisplatin treatment. By contrast, several other enzymes (both cytosolic and mitochondrial) involved in energy/amino acid metabolism were not significantly affected by cisplatin treatment in the LLC-PK(1) cells, whether or not mitAspAT was overexpressed. The susceptibility of KGDHC and aconitase to inactivation in kidney cells exposed to cisplatin metabolites may be due to the proximity of mitAspAT to KGDHC and aconitase in mitochondria. These findings support the hypothesis that a mitochondrial cysteine S-conjugate beta-lyase converts the cisplatin-cysteine S-conjugate to a toxicant, and the data are consistent with the hypothesis that mitAspAT plays a role in the bioactivation of cisplatin.

Membrane alanyl aminopeptidase

This chapter focuses on the structural chemistry of membrane alanyl aminopeptidase (mAAP). The early history of mAAP relates to its role as Cys-Gly dipeptidase or cysteinyl-glycinase. It was proposed that this peptidase activity present in apparently purified RNA preparations contributed to polypeptide biosynthesis by acting in reverse in a sequential fashion. mAAP has a broad substrate specificity removing N-terminal amino acids (Xaa-Xbb-) from almost all unsubstituted oligopeptides and from an amide or arylamide. mAAP is a type II integral membrane protein located on the plasma membrane as an ectoenzyme. The pI is approximately 5. mAAP is widely distributed among species and tissues although it is of greatest abundance in brush border membranes of the kidney, in the mucosal cells of the small intestine and in the liver. It is also present in the lung where it is identical to the pI46 type II alveolar epithelial cell antigen and is located on endothelial cells in blood vessels. On polarized epithelial cells, mAAP is localized to the apical domain and is targeted there through an apical sorting signal thought to be located in the catalytic head group region of the protein.

Mammalian osmolytes and S-nitrosoglutathione promote Delta F508 cystic fibrosis transmembrane conductance regulator (CFTR) protein maturation and function

In cystic fibrosis, the absence of functional CFTR results in thick mucous secretions in the lung and intestines, as well as pancreatic deficiency. Although expressed at high levels in the kidney, mutations in CFTR result in little or no apparent kidney dysfunction. In an effort to understand this phenomenon, we analyzed Delta F508 CFTR maturation and function in kidney cells under conditions that are common to the kidney, namely osmotic stress. Kidney cells were grown in culture and adapted to 250 mM NaCl and 250 mM urea. High performance liquid chromatography analysis of lysates from kidney cells adapted to these conditions identified an increase in the cellular osmolytes glycerophosphorylcholine, myo-inositol, sorbitol, and taurine. In contrast to isoosmotic conditions, hyperosmotic stress led to the proper folding and processing of Delta F508 CFTR. Furthermore, three of the cellular osmolytes, when added individually to cells, proved effective in promoting the proper folding and processing of the Delta F508 CFTR protein in both epithelial and fibroblast cells. Whole-cell patch clamping of osmolyte-treated cells showed that Delta F508 CFTR had trafficked to the plasma membrane and was activated by forskolin. Encouraged by these findings, we looked at other features common to the kidney that may impact Delta F508 maturation and function. Interestingly, a small molecule, S-nitrosoglutathione, which is a substrate for gamma glutamyltranspeptidase, an abundant enzyme in the kidney, likewise promoted Delta F508 CFTR maturation and function. S-Nitrosoglutathione-corrected Delta F508 CFTR exhibited a shorter half-life as compared with wild type CFTR. These results demonstrate the feasibility of a small molecule approach as a therapeutic treatment in promoting Delta F508 CFTR maturation and function and suggest that an additional treatment may be required to stabilize Delta F508 CFTR protein once present at the plasma membrane. Finally, our observations may help to explain why Delta F508 homozygous patients do not present with kidney dysfunction.

Glutathione conjugates and their synthetic derivatives as inhibitors of glutathione-dependent enzymes involved in cancer and drug resistance

Alterations in levels of glutathione (GSH) and glutathione-dependent enzymes have been implicated in cancer and multidrug resistance of tumor cells. The activity of a number of these, the multidrug resistance-associated protein 1, glutathione S-transferase, DNA-dependent protein kinase, glyoxalase I, and gamma-glutamyl transpeptidase, can be inhibited by GSH-conjugates and synthetic analogs thereof. In this review we focus on the function of these enzymes and carriers in cancer and anti-cancer drug resistance, in relation to their inhibition by GSH-conjugate analogs.

Glutathione-dependent bioactivation of haloalkenes

Several halogenated alkenes are nephrotoxic in rodents. A mechanism for the organ-specific toxicity of these compounds to the kidney has been elucidated. The mechanism involves hepatic glutathione conjugation to dihaloalkenyl or 1,1-difluoroalkyl glutathione S-conjugates, which are cleaved by gamma-glutamyltransferase and dipeptidases to cysteine S-conjugates. Haloalkene-derived cysteine S-conjugates may have four fates in the organism: (a) They may be substrates for renal cysteine conjugate beta-lyases, which cleave them to form reactive intermediates identified as thioketenes (chloroalkene-derived S-conjugates), thionoacyl halides (fluoroalkene-derived S-conjugates not containing bromide), thiiranes, and thiolactones (fluoroalkene-derived S-conjugates containing bromine); (b) cysteine S-conjugates may be N-acetylated to excretable mercapturic acids; (c) they may undergo transamination or oxidation to the corresponding 3-mercaptopyruvic acid S-conjugate; (d) finally, oxidation of the sulfur atom in halovinyl cysteine S-conjugates and corresponding mercapturic acids forms Michael acceptors and may also represent a bioactivation reaction. The formation of reactive intermediates by cysteine conjugate beta-lyase may play a role in the target-organ toxicity and in the possible renal tumorigenicity of several chlorinated olefins widely used in many chemical processes.

gamma-Glutamyl transpeptidase, a glutathionase: its expression and function in carcinogenesis

gamma-Glutamyl transpeptidase (GGT) is found throughout the plant and animal kingdoms. It is a cell surface glycoprotein that cleaves gamma-glutamyl amide bonds. The most abundant physiologic substrates for the enzyme are glutathione and glutathione-conjugated compounds. GGT initiates the cleavage of extracellular glutathione into its constituent amino acids which can then be transported into the cell. It also catalyzes the initial step in the conversion of glutathione-conjugated compounds to mercapturic acids. GGT is expressed at high levels in many human tumors and in many carcinogen-induced tumors in animals. These observations have lead an increased focus on the role of the enzyme in the development and treatment of tumors. This chapter begins with an overview of the structure and function of GGT in normal tissues. A summary of its expression in neoplastic tissues and the ways in which GGT effects the response of tumors to chemotherapy follows. The chapter concludes with a discussion of strategies for using GGT to activate and target chemotherapy drugs to tumors as a means of improving treatment for common human malignancies.

Glutathione conjugation and conversion to mercapturic acids can occur as an intrahepatic process

By catalyzing the reaction of electrophilic compounds with the sulfhydryl group of glutathione, the glutathione S-transferases play physiologically important roles in the detoxication of potential alkylating agents. The glutathione S-conjugates thus formed are transported out of cells for further metabolism by gamma-glutamyltransferase and dipeptidases, ectoproteins that catalyze the sequential removal of the glutamyl and glycyl moieties, respectively. These ectoproteins are not found in all cells, but are localized predominantly to the apical surface of epithelial tissues. The resulting cysteine S-conjugates can be reabsorbed by specific cell types, and acetylated on the amino group of the cysteinyl residue by intracellular N-acetyl-transferases, to form the corresponding mercapturic acids (N-acetylcysteine S-conjugates). Mercapturic acids are then released into the circulation and delivered to the kidney for excretion in urine, or they may undergo further metabolism. Mercapturic acid biosynthesis is generally considered to be an interorgan process, with the liver serving as the major site of glutathione conjugation, and the kidney as the primary site for conversion of glutathione conjugates to cysteine conjugates. Cysteine conjugates formed in the kidney appear to be transported back to the liver for acetylation. This interorgan model of mercapturic acid synthesis is based largely on the interorgan distribution of the enzymes involved in their formation, and in particular of the enzyme gamma-glutamyltransferase. Rats have relatively low hepatic and high renal activities of gamma-glutamyltransferase, the only protein known to initiate the breakdown of glutathione S-conjugates. The low gamma-glutamyltransferase activity in rat liver limits the hepatic degradation of glutathione S-conjugates, particularly after large doses of xenobiotic. In contrast, hepatic gamma-glutamyltransferase is significantly higher in species such as rabbit, guinea pig, and dog, and as a consequence, nearly all of the glutathione and glutathione S-conjugates released by liver cells of these species is degraded within the liver. Recent studies demonstrate that glutathione S-conjugates synthesized within hepatocytes are secreted preferentially across the canalicular membrane into bile, and are broken down within biliary spaces to form cysteine S-conjugates. The latter are then reabsorbed by the liver, N-acetylated to form mercapturic acids, and reexcreted into bile, completing an intrahepatic pathway for mercapturic acid biosynthesis. The contribution of this intrahepatic pathway to overall mercapturate formation is dependent on dose of the electrophile, route of exposure, and the physicochemical properties of the glutathione S-conjugate formed, as well as the tissue distribution and activity of gamma-glutamyltransferase.(ABSTRACT TRUNCATED AT 400 WORDS)

On the reactions of the carbapenem, meropenem, in the soluble fraction of rat kidney

1. When meropenem (I) is incubated with the soluble fraction of rat kidney or its ultrafiltrate the carbapenem disappears at a rate of 50 nmol/min per g of kidney at pH 8.4 and 37 degrees C. 2. In both cases the kinetics are inconsistent with the reaction being enzymic and the product has been shown to be a thiol. 3. The same reaction occurs in similar preparations from the kidney of several other mammalian species. 4. The reaction of meropenem with thiols has been investigated: only those having a vicinal amino group react rapidly. 5. The product of the reaction with cysteine has been characterized as the amide(III) of the latter with meropenoic acid (II), the carboxylic acid formed by the hydrolysis of the beta-lactam ring of meropenem. 6. It is postulated that the destruction of meropenem in the soluble fraction of rat kidney occurs by the non-enzymic reaction of the carbapenem with endogenous cysteine or related thiols. 7. There is no evidence that this reaction can occur in vivo but it should be considered in the future design of carbapenems.

Effect of ascorbic acid, acivicin and probenecid on the nephrotoxicity of 4-aminophenol in the Fischer 344 rat

4-Aminophenol (p-aminophenol, PAP) causes selective necrosis to the pars recta of the proximal tubule in Fischer 344 rats. The basis for this selective toxicity is not known but PAP can undergo oxidation in a variety of systems to form the 4-aminophenoxy free radical. Oxidation or disproportionation of this radical will form 1,4-benzoquinoneimine which can covalently bind to cellular macromolecules. We have recently reported that a glutathione conjugate of PAP, 4-amino-3-S-glutathionylphenol, is more toxic to the kidney than the parent compound itself. In this study we have examined the distribution and covalent binding of radiolabel from 4-[ring 3H]-aminophenol in the plasma, kidney and liver of rats 24 h after dosing and related these findings to the extent of nephrotoxicity. In addition, we have examined the effect of ascorbic acid which will slow the oxidation of PAP; acivicin, an inhibitor of gamma-glutamyltransferase and hence the processing of glutathione-derived conjugates; and probenecid, an inhibitor of organic anion transport on the nephrotoxicity produced by PAP. Administration of a single dose of PAP at 458 or 687 mumol kg-1 produced a dose-related alteration in renal function within 24 h which was associated with proximal tubular necrosis. The lesion at the lower dose was restricted to the S3 proximal tubules in the medullary rays, while at the higher dose it additionally affected the S3 tubules in the pars recta region of the cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Further study of effects of chelating agents on excretion of inorganic mercury in rats

The effects of three chelating agents, N-benzyl-D-glucamine dithiocarbamate (BGD), 2,3-dimercaptopropanol (BAL) and D-penicillamine (D-PEN), on the excretion of mercury in rats exposed to mercuric chloride (HgCl2), the chemical forms of mercury compounds excreted in the bile and urine and the intestinal reabsorption of mercury compounds in the bile were studied. Rats were injected intraperitoneally with 203HgCl2 (300 micrograms Hg and 74 kBq of 203Hg/kg) and 24 h later, they were injected intraperitoneally with a chelating agent (a quarter of an LD50). The injection of the chelating agents significantly enhanced the biliary and urinary excretions of mercury. The enhancing effect of BGD on the excretions of mercury was almost the same as that of BAL and much larger than that of D-PEN. The major chemical form of mercury in the bile and urine of rats injected with BGD after HgCl2 treatment was Hg-BGD compounds. The chemical form of mercury in the bile and urine of rats injected with BAL after HgCl2 treatment was mainly Hg-GSH compound. The mercury after HgCl2 and D-PEN treatment was excreted mainly via the urine in the form of Hg-D-PEN compound. The intestinal reabsorption of mercury from the bile of rats injected with BGD or D-PEN was only 0.18% or 0.38% of the dose, respectively. The intestinal reabsorption of mercury from the bile of rats injected with BAL was 27.38% of the dose. It was suggested that the Hg-GSH compound excreted in the bile after HgCl2 and BAL treatment is partly degraded to Hg-cysteine (Cys) by the intestinal membranous enzymes and that the ligand of Hg-Cys is replaced by BAL in the bile, resulting in the effective reabsorption of Hg-BAL compound from the intestine.

Glutathione and glycine in acute renal failure

Glutathione is an important intracellular antioxidant in virtually all tissues, including the kidney. In the kidney, it has a rapid turnover in tubule cells and likely plays a role in any oxidant-related events which contribute to the tubule cell injury which occurs during acute renal failure. It was surprising, therefore, to find that the component amino acid, glycine, rather than glutathione itself, most strongly modulated the sensitivity of tubules cells to a variety of insults in several in vitro systems where these processes can be studied most directly. This paper reviews available evidence concerning the nature of both glutathione and glycine effects, their expression in vivo in in vitro, and their implications for understanding acute renal failure.

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)

Glutathione-dependent inactivation of sodium-dependent phosphate transport across rat renal brush-border membrane

Thiol/disulfide is fundamental in protein function; we previously observed an inhibitory effect of thiol oxidants on the Na-dependent phosphate (Pi) uptake into renal brush border membrane vesicles (BBMV). We examined whether oxidation of glutathione (GSH) is involved in the mechanism. Vesicular thiols were measured by liquid chromatography. BBMV were incubated with reagents before an influx of Pi. Diamide (5 mM) reduced the capacity of the Pi uptake. Subsequent treatment with dithiothreitol (5 mM) blocked the inhibitory effect of diamide. Vesicular GSH was not modified only by the incubation, whereas it was oxidized by the treatment with diamide, and reduced by dithiothreitol. Furthermore, in vivo treatment with cAMP provided GSH-depleted BBMV without any influence on Pi uptake. Diamide did not inhibit the transport of Pi into GSH-depleted vesicles, but it did inhibit the uptake when GSH was introduced into the vesicles. In conclusion, a GSH-dependent mechanism is involved in the inhibitory effect of diamide on sodium-dependent Pi transport across the renal brush-border membrane.

Specificity and inhibition studies of human renal dipeptidase

Purified human renal dipeptidase was shown to exhibit no detectable activity against substrates that are characteristic for other known mammalian peptidases. The enzymic activities that were assayed were: aminopeptidase A, aminopeptidase B, aminopeptidase M, aminopeptidase P, and tripeptidase. A quantitative assay for renal dipeptidase was developed which measures the rate of release of glycine from glycylpeptides by pre-column derivatization of the amino acid with phenylisothiocyanate followed by high-performance liquid chromatography. The ratio of Vmax/Km for a series of dipeptides was used as an index of the enzyme's preference for substrates. According to the data obtained, the enzyme prefers that a bulky, hydrophobic group of the dipeptide be located at the N-terminal position. This suggests that the substrate-binding site of the enzyme may provide a hydrophobic pocket to accommodate the hydrophobic moiety at the N-terminus of the dipeptide. The unsaturated dipeptide substrate, glycyldehydrophenylalanine, was employed in spectrophotometric assays to provide kinetic analyses of enzymic inhibition. The inhibitory effect of dithiothreitol was immediate, and the kinetic data indicated reversible, competitive inhibition. These results suggest that the inhibitor competes with substrate for a coordination site of zinc within the active site of the enzyme. The reaction of renal dipeptidase with the transition-state peptide analog, bestatin, was time dependent, and velocity measurements were made after the inhibitor had been incubated with the enzyme until constant rates were observed. These steady-state rate measurements, made following preincubation of enzyme with inhibitor, were employed to show that bestatin caused apparent non-competitive inhibition of the enzyme. The inhibitory effect of the beta-lactam inhibitor, cilastatin, upon the oligomeric dipeptidase was shown to be competitive. Graphical analysis of this inhibition indicated that the subunits of the enzyme react independently during enzymic catalysis and that the catalytic event is not influenced by cooperativity between sites on the subunits. The conversion of leukotriene D4 to leukotriene E4 in the presence of human renal dipeptidase was demonstrated by HPLC procedures. This bioconversion reaction was quantitated by derivatizing the glycine produced by cleavage of the cysteinylglycine bond and isolating this derivative as a function of time. The relationship between the purified enzyme concentration and enzyme activity against leukotriene D4 was shown to be linear over the enzyme concentration range of 1 ng through 69 ng in this assay.(ABSTRACT TRUNCATED AT 400 WORDS)

Biosynthesis and biotransformation of glutathione S-conjugates to toxic metabolites

The material presented in this review deals with the hypothesis that the nephrotoxicity of certain halogenated alkanes and alkenes is associated with hepatic biosynthesis of glutathione S-conjugates, which are further metabolized to the corresponding cysteine S-conjugates. Some glutathione or cysteine S-conjugates may be direct-acting nephrotoxins, but most cysteine S-conjugates require bioactivation by renal, pyridoxal phosphate-dependent enzymes, such as cysteine conjugate beta-lyase (beta-lyase). The biosynthesis of glutathione S-conjugates is catalyzed by both the cytosolic and the microsomal glutathione S-transferases, although the latter enzyme is a better catalyst for the reaction of haloalkenes with glutathione. When glutathione S-conjugate formation yields sulfur mustards, as occurs with vicinal-dihaloethanes, the S-conjugates are direct-acting toxins. In contrast, the S-conjugates formed from fluoro- and chloroalkenes yield S-alkyl- or S-vinyl glutathione conjugates, respectively, which are metabolized to the corresponding cysteine S-conjugates by gamma-glutamyltransferase and dipeptidases; inhibition of these enzymes blocks the toxicity of the glutathione S-conjugates. The cysteine S-conjugates must be metabolized by beta-lyase for the expression of toxicity; the beta-lyase inhibitor aminooxyacetic acid blocks the toxicity of cysteine S-conjugates, and the corresponding alpha-methyl cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates. Homocysteine S-conjugates are also potent cyto- and nephrotoxins. The high renal content of gamma-glutamyltransferase and the renal anion transport system are probably determinants of kidney tissue as a target site. Biochemical studies indicate that renal mitochondrial dysfunction is produced by the cysteine S-conjugates. Finally, some of the glutathione and cysteine conjugates are mutagenic in the Ames test, and reactive intermediates formed by the action of beta-lyase may contribute to the nephrocarcinogenicity of certain chloroalkenes.

Studies on the leukotriene D4-metabolizing enzyme of rat leukocytes, which catalyzes the conversion of leukotriene D4 to leukotriene E4

Leukotriene D4-metabolizing enzyme was studied using rat neutrophils, lymphocytes and macrophages. These leukocyte sonicates converted leukotriene D4 to leukotriene E4. However, the leukotriene D4-metabolizing activity varied with cell type, and macrophages showed the highest activity among these leukocytes. The subcellular localization of the leukotriene D4-metabolizing enzyme of macrophages was examined, and the leukotriene D4-metabolizing activity was found to be present in the membrane fraction, but not in the nuclear, granular and cytosol fractions. When macrophages were modified chemically with diazotized sulfanilic acid, a poorly permeant reagent which inactivates cell-surface enzymes selectively, the leukotriene D4-metabolizing activity of macrophages decreased significantly (about 95%) without any inhibition of marker enzymes of microsome, cytosol, lysosome and mitochondria. When neutrophils and lymphocytes were modified with diazotized sulfanilic acid, the leukotriene D4-metabolizing activity was also inhibited about 90% by the modification. Among various enzyme inhibitors used, o-phenanthroline, a metal chelator, remarkably inhibited the leukotriene D4-metabolizing activity of leukocytes, and the o-phenanthroline-inactivated enzyme activity was fully reactivated by Co2+ and Zn2+. These findings seem to indicate that rat neutrophils, lymphocytes and macrophages possess the leukotriene D4-metabolizing metalloenzyme which converts leukotriene D4 to leukotriene E4, on the cell surface, although macrophages have a higher enzyme activity than the other two.

Enzymatic transformation of mercapturic acids derived from halogenated alkenes to reactive and mutagenic intermediates

The metabolism of the mercapturic acids S-pentachlorobutadienyl-N-acetylcysteine (N-Ac-PCBC), S-trichlorovinyl-N-acetylcysteine (N-Ac-TCVC) and S-dichlorovinyl-N-acetylcysteine (N-Ac-DCVC) by subcellular fractions from male rat liver and kidney homogenates was studied. As a model compound, N-Ac-PCBC, 14C labelled, was synthesised. It was intensively metabolised by cytosolic but not by microsomal enzymes from rat liver and kidney. The major metabolite identified by GC/MS was pentachlorobutadienylcysteine, the amount produced being highest in kidney cytosol. Metabolic conversion of 14C-N-Ac-PCBC by kidney and liver cytosol resulted in covalent binding of radioactivity to protein, binding was strongly inhibited by the beta-lyase inhibitor aminooxyacetic acid (AOAA). N-Ac-TCVC and N-Ac-DCVC were also transformed by cytosolic enzymes to the corresponding cysteine conjugates (trichlorovinylcysteine and dichlorovinylcysteine). The three mercapturic acids tested were strong mutagens in the Ames-test after addition of rat kidney cytosol. In the absence of cytosol, N-Ac-TCVC and N-Ac-DCVC were weakly but definitely mutagenic, whereas N-Ac-PCBC was not. In contrast to N-Ac-PCBC, the "direct" mutagens N-Ac-TCVC and N-Ac-DCVC were both transformed to pyruvate by bacterial (S. typhimurium TA100) homogenate 100,000 g supernatants. It is concluded that mercapturic acids are deacetylated to the corresponding cysteine conjugates by cytosolic (N-Ac-PCBC, N-Ac-TCVC and N-Ac-DCVC) and bacterial enzymes (N-Ac-TCVC and N-Ac-DCVC) and further cleaved to reactive and mutagenic intermediates by mammalian and/or bacterial beta-lyase. The observed activation mechanisms for the mercapturic acids, whose formation from hexachlorobutadiene, tetrachloroethylene and trichloroethylene has been proven, might contribute to the nephrotoxicity and nephrocarcinogenicity of the parent alkenes.

Inhibition of leukotriene D4 catabolism by D-penicillamine

Inhibition of the catabolism of the most biologically potent cysteinyl leukotriene, LTD4, was studied in rat hepatoma cells in vitro and in the rat in vivo. LTD4 dipeptidase, an ectoenzyme on the surface of AS-30D hepatoma cells, exhibited an apparent Km value of 6.6 microM for LTD4. D-Penicillamine and L-penicillamine inhibited this enzyme activity with apparent Ki values of 0.46 mM and 0.21 mM respectively. Bestatin, an inhibitor of the aminopeptidase activity of hepatoma cells, did not affect LTD4 hydrolysis at concentrations as high as 5 mM, indicating that the aminopeptidase did not contribute to LTD4 catabolism. In the rat in vivo, D-penicillamine also inhibited LTD4 catabolism. After intravenous injection of [3H]LTC4 an accumulation of [3H]LTD4 and a retarded formation of [3H]LTE4 were observed in the circulating blood after D-penicillamine pretreatment. Within 1 h after intravenous [3H]LTC4 injection, about 80% of the administered radioactivity was recovered in bile. After D-penicillamine pretreatment [3H]LTD4 was the major biliary leukotriene metabolite, whereas in untreated controls leukotriene metabolites more polar than LTC4 predominated in bile. After stimulation of endogenous leukotriene production in vivo by platelet-activating factor, N-acetyl-LTE4 was the major cysteinyl leukotriene detected in bile. D-Penicillamine treatment prior to platelet-activating factor resulted in the accumulation of LTD4, which under these circumstances was the major endogenous leukotriene metabolite detected in bile.

Inter-organ metabolism and transport of a cysteine-S-conjugate of xenobiotics in normal and mutant analbuminemic rats

Biosynthesis of N-acetylcysteine S-conjugates of toxic electrophiles, mercapturic acids, occurs via inter-organ metabolism and transport in which liver, small intestine and kidney play an important role. Since a mercapturic acid is a hydrophobic organic anion and strongly binds to plasma albumin in vitro, the ligand-albumin interaction may affect the metabolic fate of this final metabolite in vivo. To investigate the role of the circulating albumin in detoxication and elimination of a toxic electrophile, urinary occurrence of the final metabolite was determined in normal and mutant Nagase analbuminemic rats (NAR) after administration of S-benzylcysteine, a model compound of cysteine conjugates. S-Benzylcysteine intravenously administered was excreted rapidly into urine as its N-acetyl derivative in both animal groups. However, the urinary recovery of this mercapturic acid was significantly lower in NAR than in normal animals. The lower urinary recovery in NAR was due to a rapid and random distribution of the unbound metabolite in the circulation to extrarenal tissues. In contrast, no significant difference in the urinary recovery of the final metabolite was observed between the two animal groups if S-benzylcysteine was given orally. Kinetic analysis revealed that the major part of the orally administered S-benzylcysteine was transferred to the liver and acetylated predominantly in this organ in both animal groups; the mercapturic acid which was synthesized in the liver can be transferred to the kidney and excreted into urine even in the absence of the circulating albumin. These results indicate that albumin is important for a final elimination of a mercapturic acid when animals were extraorally challenged with a large dose of toxic electrophiles by which the rate of biosynthesis and the plasma level of the amphipathic metabolites were increased.

The nonenzymatic oxidation of glutathione in the presence of plasmalike concentrations of disulfides and copper ions

The oxidation of glutathione catalyzed by a combination of low concentrations of disulfides and copper and iron ions (as they occur in the blood plasma) was investigated and compared with data obtained in vivo studies. At pH 7.4 and 37 degrees C oxidation of glutathione (3 mmol/L) in a solution saturated with oxygen could be induced from 0 to 3, 5, 10, and 21 nmol/min and mL by the addition of 0.1, 1.0, 10, and 100 mumol/L CuCl2, respectively. The presence of 50 mumol/L cystinylbisglycine as an additional component increased the rate of oxidation by a factor between two and three. Cystine was only about one third as active as cystinylbisglycine, and trans-4,5-dihydroxy-1,2-dithiane, the disulfide derivative of dithiothreitol, was even less effective in propagating glutathione oxidation. FeCl2 in combination with the disulfides was 30 times less active than copper as a catalyst. With plasmalike concentrations of the reactants, a rate of glutathione oxidation of 0.2 to 0.8 nmol/min and mL, depending on the availability of free plasma copper, could be approximated. This rate corresponds to 8% to 30% of total plasma glutathione oxidation.

Characterization of dehydropeptidase I in the rat lung

The activity of dehydropeptidase I in rat tissues decreases in the order of lung greater than kidney greater than liver-spleen greater than other tissues, while aminopeptidase activity is high in the kidney, and lower in the lung than in other tissues. Dehydropeptidase I was solubilized from the membrane fraction of rat lung by treatment with papain and purified by DEAE-cellulose column chromatography, affinity chromatography on concanavalin-A-Sepharose and high-performance liquid chromatography gel filtration. The purified preparation was found to be homogeneous on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The relative molecular mass was estimated to be 150,000 by gel filtration, comprising a homodimer of two 80,000-Mr subunits. The enzyme activity was inhibited by cilastatin, o-phenanthroline and ATP. This enzyme catalyzed the hydrolysis of S(substituent)-L-cysteinyl-glycine adducts such as L-cystinyl-bis(glycine) and N-ethylmaleimide-S-L-cysteinyl-glycine, as well as the conversion of leukotriene D4 to E4. Furthermore it catalyzed a hydrolytic splitting of L-Leu-L-Leu, but not S-benzyl-L-cysteine p-nitroanilide, which is a good substrate for aminopeptidase. Our enzyme preparation was immunologically identical to the rat renal dehydropeptidase I. The physiological significance of the pulmonary dehydropeptidase I on the metabolism of glutathione and its adducts is discussed.