Mechanism of S-(1,2-dichlorovinyl)glutathione-induced nephrotoxicity

Embryotoxicity elicited by inhibition of gamma-glutamyltransferase by Acivicin and transferase antibodies in cultured rat embryos

Acivicin (also known as AT-125) and IgG isolated from goat anti-gamma-glutamyltransferase antiserum were used to inhibit the activity of gamma-glutamyltransferase (GGT, EC 2.3.2.2) in rat conceptuses cultured from Days 10 to 11 of gestation. Inhibition of GGT by either Acivicin or anti-GGT IgG produced embryotoxicity and malformations, although each compound produced a unique spectrum of effects. Acivicin, at an initial concentration in the culture medium of 5 microM, produced a marked decrease in yolk sac vasculature and was associated with embryonic malformations such as neural tube necrosis, microophthalmia, and cephalic edema after 24 hr exposure. These malformations were accompanied by significant decreases in both embryonic and yolk sac protein, yolk sac GGT activity, as well as embryonic glutathione (GSH) levels. In contrast, anti-GGT IgG produced no apparent effects on yolk sac vasculature or protein after exposure of conceptuses to an initial concentration of 50 micrograms IgG/ml culture medium, even though equal inhibition of yolk sac GGT (30%) was achieved by each inhibitor. Exposure to IgG (50 micrograms/ml) for 24 hr was associated with decreased embryonic protein; decreased levels of GSH in the embryo were observed after both 3 and 24 hr. The dichotomy of effects on the yolk sac by the two compounds indicates that Acivicin produced these effects by mechanisms other than by GGT inhibition alone. These results demonstrate that inhibition of GGT in rat embryos undergoing organogenesis can elicit embryotoxic effects and produce alterations in GSH levels. The capacity of the anti-GGT antibody to inhibit the GGT activity in the yolk sac (while having no apparent effect on yolk sac morphology), and yet influence the embryo by decreasing protein and GSH levels, underscores the important role of the yolk sac during the highly sensitive stages of organogenesis.

Cellular effects of reactive intermediates: nephrotoxicity of S-conjugates of amino acids

Several cysteine S-conjugates are potent nephrotoxins and require enzymatic activation to produce cytotoxicity. Strategies based on the knowledge that renal cysteine conjugate beta-lyase is apparently a pyridoxal phosphate (PLP)-dependent enzyme have been exploited to test the hypothesis that a beta-lyase-dependent activation is required for the expression of cysteine S-conjugate-induced toxicity. First, the toxicity of the model conjugate S-(1,2-dichlorovinyl)-L-cysteine (DCVC) is blocked both in vivo and in isolated, renal proximal tubular cells by aminooxyacetic acid, an inhibitor of PLP-dependent enzymes. Second, the nonmetabolizable alpha-methyl analogue S-(1,2-dichlorovinyl)-DL-alpha-methylcysteine is not toxic. Third, to test the hypothesis that the toxicity of DCVC is associated with the metabolic formation of a reactive thiol, S-(1,2-dichlorovinyl)-L-homocysteine (DCVHC), which may undergo a PLP-dependent gamma-elimination reaction to produce an identical thiol, was studied. DCVHC is a potent nephrotoxin, and, similar to DCVC, its toxicity was blocked by aminooxyacetic acid and the alpha-methyl analogue S-(1,2-dichlorovinyl)-DL-alpha-methylhomocysteine was not toxic. Moreover, exposure of renal proximal tubular cells to propargylglycine, a suicide substrate for PLP-dependent enzymes that catalyze gamma-elimination reactions, blocked the toxicity of DCVHC. Fourth, the renal mitochondrial beta-lyase is localized in the outer membrane; therefore, although DCVC was toxic to mitochondria, no toxicity was produced in mitoplasts, which shows that a suborganelle site of activation is involved in the mitochondrial toxicity of DCVC. Finally, the toxicity of both DCVC and DCVHC was blocked by probenecid, indicating a role for the anion transport system. DCVC and DCVHC inhibit cellular and mitochondrial respiration, indicating that mitochondria are primary intracellular targets for nephrotoxic S-conjugates.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of epoxide metabolism by epoxide hydrolase and glutathione S-transferase associated with age and genotype in Drosophila melanogaster

Epoxide hydrolase and glutathione S-transferase activities toward trans- and cis-stilbene oxides were measured in 3 strains of Drosophila melanogaster. Differences in age dependence, substrate selectivity and subcellular location were detected suggesting the presence of multiple forms of these enzymes. In addition, interstrain differences indicate the presence of genetic variation for epoxide hydrolase and glutathione transferase activities. These results illustrate a potential use of these assays in D. melanogaster to complement existing tests (e.g. recessive lethal tests or Ames assays) for evaluating the relationship between epoxide hydrolase and glutathione S-transferase levels and the genotoxicity of epoxides.

S-(1,2-dichlorovinyl)-L-homocysteine-induced cytotoxicity in isolated rat kidney cells

Incubation of isolated, rat kidney cells with S-(1,2-dichlorovinyl)-L-homocysteine (DCVHC) caused time-dependent cell death. Cytotoxicity of DCVHC was potentiated by addition of alpha-ketobutyrate, indicating the involvement of pyridoxal phosphate-dependent enzymes. A second addition of DCVHC to cells produced increased cytotoxicity, indicating that the bioactivating ability is not lost after exposure to the conjugate. DCVHC decreased cellular glutathione concentrations by 52% and substantially inhibited glutathione biosynthesis from precursors. In contrast, the cysteine analog S-(1,2-dichlorovinyl)-L-cysteine (DCVC) failed to decrease cellular glutathione concentrations and only partially inhibited glutathione biosynthesis. As with DCVC, DCVHC did not increase cellular glutathione disulfide concentrations and did not initiate lipid peroxidation, indicating that it does not produce an oxidative stress. DCVHC and DCVC produced similar alterations in mitochondrial function: Cellular ATP concentrations were decreased by 57% and cellular ADP and AMP concentrations were increased twofold, thereby decreasing the ATP/ADP ratio from 2.8 to 0.6 and the cellular energy charge from 0.80 to 0.56; DCVHC was a potent inhibitor of succinate-dependent oxygen consumption, but had little effect on respiration linked to oxidation of glutamate + malate or ascorbate + N,N,N'N'-tetramethyl-p-phenylenediamine. DCVHC was a potent inhibitor of mitochondrial Ca2+ sequestration and, in contrast to DCVC, also inhibited microsomal Ca2+ sequestration. These DCVHC-induced alterations in cellular metabolism were prevented by addition of propargylglycine or aminooxyacetic acid, and the alpha-methyl analog S-(1,2-dichlorovinyl)-DL-alpha-methylhomocysteine was not toxic. These results support a role for pyridoxal phosphate-dependent bioactivation of DCVHC and indicate that the greater nephrotoxic potency of DCVHC as compared to DCVC is partially due to the presence of both mitochondrial and extramitochondrial targets for DCVHC.

The mechanism of pentachlorobutadienyl-glutathione nephrotoxicity studied with isolated rat renal epithelial cells

Isolated renal epithelial cells were used to study the mechanism of toxicity of pentachlorobutadienyl-glutathione (PCBG), a nephrotoxic glutathione conjugate of hexachlorobutadiene. The cytotoxicity of PCBG displayed a very steep dose-response relationship; at 10 microM PCBG no toxicity was observed whereas 25, 50, and 100 microM PCBG all resulted in a similar degree of toxicity. In all cases, loss of cell viability was observed only after a 30-min lag period and reached a plateau of 50 to 60% nonviable cells between 90 and 100 min. Toxic doses of PCBG also resulted in the depletion of cellular thiols. Blocking PCBG metabolism by inhibition of gamma-glutamyl transpeptidase [1-gamma-L-glutamyl-2-(2-carboxyphenyl)hydrazine (anthglutin), 2 mM] or renal cysteine conjugate beta-lyase (aminooxyacetic acid, 0.5 mM) resulted in complete protection against PCBG-induced cell damage. Exposure of isolated renal epithelial cells to 100 microM PCBG resulted in the rapid formation of plasma membrane blebs which appeared to be associated with a loss of Ca2+ from the mitochondrial compartment and an elevation of cytosolic Ca2+ concentration as measured by Quin-2. PCBG treatment also resulted in the inhibition of cell respiration and a marked depletion of cellular ATP content, indicating additional mitochondrial effects of the toxin. Our results support a role for renal cysteine conjugate beta-lyase in the metabolic activation of PCBG and suggest that PCBG-induced renal cell injury may be the result of selective effects on mitochondrial function.

Bacterial beta-lyase mediated cleavage and mutagenicity of cysteine conjugates derived from the nephrocarcinogenic alkenes trichloroethylene, tetrachloroethylene and hexachlorobutadiene

The metabolism of beta-lyase and the mutagenicity of the synthetic cysteine conjugates S-1,2-dichlorovinylcysteine (DCVC), S-1,2,2-trichlorovinylcysteine (TCVC), S-1,2,3,4,4-pentachlorobuta-1,3-dienylcysteine (PCBC) and S-3-chloropropenylcysteine (CPC) were investigated in Salmonella typhimurium strains TA100, TA2638 and TA98. The bacteria contained significantly higher concentrations of beta-lyase than mammalian subcellular fractions. Bacterial 100,000 X g supernatants cleaved benzthiazolylcysteine to equimolar amounts of mercaptobenzthiazole and pyruvate. DCVC, TCVC and PCBC produced a linear time-dependent increase in pyruvate formation when incubated with bacterial 100,000 X g supernatants; pyruvate formation was inhibited by the beta-lyase inhibitor aminooxyacetic acid (AOAA). CPC was not cleaved by bacterial enzymes to pyruvate. DCVC, TCVC and PCBC were mutagenic in three strains of S. typhimurium (TA100, TA2638 and TA98) in the Ames-test without addition of mammalian subcellular fractions; their mutagenicity was decreased by the addition of AOAA to the preincubation mixture. CPC was not mutagenic in any of the strains of bacteria tested. These results indicate that beta-lyase plays a key role in the metabolism and mutagenicity of haloalkenylcysteines when tested in S. typhimurium systems. The demonstrated formation in mammals of the mutagens DCVC, TCVC and PCBC during biotransformation of trichloroethylene (Tri), tetrachloroethylene (Tetra) and hexachlorobutadiene (HCBD) may provide a molecular explanation for the nephrocarcinogenicity of these compounds.

Metabolism of S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine to hydrogen sulfide and the role of hydrogen sulfide in S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine-induced mitochondrial toxicity

The nephrotoxic cysteine S-conjugate S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine (CTFC) is metabolized by kidney homogenates and subcellular fractions to pyruvate and a reactive thiol, which is cytotoxic and partially decomposes to yield hydrogen sulfide and thiosulfate. Although hydrogen sulfide is a potent mitochondrial poison, the mitochondrial toxicity of CTFC is not attributable to hydrogen sulfide formation, as shown by different sites of inhibition of mitochondrial respiration by CTFC and hydrogen sulfide. The efficient mitochondrial oxidation of hydrogen sulfide apparently serves to protect mitochondria against the toxic effects of hydrogen sulfide generated from CTFC.

Metabolic activation and detoxication of nephrotoxic cysteine and homocysteine S-conjugates

S-(1,2-Dichlorovinyl)-L-homocysteine (DCVHcy), an analogue of the nephrotoxin S-(1,2-dichlorovinyl)-L-cysteine (DCVCys), is a much more potent nephrotoxin than DCVCys both in vivo and in isolated renal proximal tubular cells. S-(1,2-Dichlorovinyl)-DL-alpha-methylhomocysteine, at equimolar doses relative to DCVHcy, is not nephrotoxic. Agents that inhibit pyridoxal phosphate-dependent enzymes (DL-propargylglycine and aminooxyacetic acid) or renal organic anion transport (probenecid) protect against DCVHcy-induced nephrotoxicity. With kidney cytosol, DCVHcy or the analogue S-(2-benzothiazolyl)-L-homocysteine (BTHcy) is not metabolized to 2-ketobutyrate, but 2-mercaptobenzothiazole is a metabolite of BTHcy and the Vmax for its formation is enhanced by addition of 2-ketobutyrate. These results are consistent with a bioactivation mechanism for DCVHcy that involves enzymatic deamination followed by a nonenzymatic beta-elimination to produce two reactive intermediates--i.e., S-(1,2-dichlorovinyl)thiol and 2-keto-3-butenoic acid. The Km values for the N-acetylation of DCVCys and DCVHcy by kidney microsomal N-acetyltransferase are similar, but the rate of DCVCys N-acetylation is 4-fold greater than the rate measured with DCVHcy as the substrate. Thus, the remarkable nephrotoxic potency of DCVHcy compared with DCVCys may be attributable to intrarenal differences in activation and detoxication.