Modification of polynucleotides by a fragment produced by enzymatic cleavage of S-(1, 2-dichlorovinyl)-L-cysteine

Frog Skin: A Computer Simulation of Experiments Performed on Frog SkinIn Vitroto Investigate the Epithelial Transport of Ions

An interactive computer-assisted learning program is described, based on experiments performed on frog skin in vitro, a preparation commonly used to teach the principles of ion transport across tight epithelia. It is aimed at undergraduate students on a variety of biomedical science courses and is designed for use on any IBM-compatible microcomputer. The program uses data derived from theoretical models to allow students to design experiments by altering certain experimental parameters. They can investigate, for example, the effects of changing the concentrations of certain ions on either side of the skin or the actions of certain drugs on either passive or active transport. Such investigations involve taking measurements from a simulated voltmeter, ammeter or radiation (scintillation) counter, as appropriate. The complete learning package includes background information for the student, tutor's notes, and suggested student assignments. The use of the program in teaching physiology and its value as an alternative to animal experiments are discussed.

Mechanisms of cysteine S-conjugate beta-lyases

Mercapturic acids are conjugates of S-(N-acetyl)-L-cysteine formed during the detoxification of xenobiotics and during the metabolism of such endogenous agents as estrogens and leukotrienes. Many mercaturates are formed from the corresponding glutathione S-conjugates. This chapter focuses on (a) the discovery of the cysteine S-conjugate beta-lyases; (b) the involvement of pyridoxal-5-phosphate; (c) the influence of the electron-withdrawing properties of the group attached to the sulfur atom; and (d) the potential of cysteine S-conjugates as pro-drugs.

Bovine Pulmonary, Hepatic and Renal Tissues: Models for the Study of Mammalian C-S Lyase Enzymes

C-S lyase (CSL) enzymes are responsible for the generation of toxicity via the cleavage of cysteine conjugates to generate reactive thiol species. In order to explore and characterise CSL activity in mammalian organs, cysteine conjugate CSL enzymes were isolated from bovine pulmonary, hepatic and renal tissues. Bovine tissue”, obtained from the abbatoir, affords a readily available source of viable CSL enzymes, without the necessity of sacrificing large numbers of laboratory animals simply to provide tissue. We have demonstrated that significant CSL activity exists in bovine tissues, and that the level of this activity is comparable with that found in human tissues. These enzymes provide an explanation for the previously reported episodes of bovine toxicity, and may provide a reasonable model for other mammalian CSL enzymes.

Mutagenicity of tetrachloroethene in the Ames test--metabolic activation by conjugation with glutathione

The mutagenicity of tetrachloroethene (tetra) and its S conjugate, S-(1,2,2-trichlorovinyl)glutathione (TCVG) was investigated using a modified Ames preincubation assay. TCVG was a potent mutagen in presence of rat kidney particulate fractions containing high concentrations of gamma-glutamyl transpeptidase (GGT) and dipeptidases. Purified tetra was not mutagenic without exogenous metabolic activation or under conditions favoring oxidative metabolism. Preincubation of tetra with purified rat liver glutathione (GSH) S-transferases in presence of GSH and rat kidney fractions resulted in a time-dependent formation of TCVG as determined by (HPLC) analysis and in an unequivocal mutagenic response in the Ames test. Experiments with tetra in the isolated perfused rat liver demonstrated TCVG formation and its excretion with the bile; bile collected after the addition of tetra to the isolated perfused liver was unequivocally mutagenic in bacteria in the presence of kidney particulate fractions. The mutagenicity was reduced in all cases by the GGT inhibitor serine borate or the beta-lyase inhibitor aminooxyacetic acid. These results support the suggestion that cleavage of the GSH S conjugate formed from tetra by the enzymes of the mercapturic acid pathway and by beta-lyase may be involved in the nephrocarcinogenic effects of this haloalkene in rats.

DNA-binding of sulfur-containing metabolites from 35S-(pentachlorobutadienyl)-L-cysteine in bacteria and isolated renal tubular cells

S-(Pentachlorobutadienyl)-L-cysteine (PCBC) is the penultimate metabolite formed from the nephrocarcinogen hexachlorobutadiene (HCBD). It is activated by cysteine conjugate beta-lyase (beta-lyase) to yield thioacylating metabolites thought to be responsible for PCBC-induced cytotoxicity and mutagenicity. We have investigated the beta-lyase dependent DNA-binding of metabolites formed from 35S-PCBC in Salmonella typhimurium (S. typhimurium) TA100 and in rat renal proximal tubule cells. 35Sulfur was found in DNA isolated from S. typhimurium (410 +/- 93 DNA-adducts per 10(6) nucleotides) and renal cells (68 or 97 DNA-adducts per 10(6) nucleotides). Enzymatic hydrolysis of the isolated DNA to yield 3'-nucleotide phosphates and fractionation of the hydrolysate by HPLC indicated the presence of 3 distinct, 35S-containing metabolites which did not coelute with unchanged 3'-nucleotide phosphates and likely represent DNA constituents modified by 35S-PCBC metabolites. Identical retention volumes were obtained for altered bases isolated from bacteria and from renal cells. The results obtained indicate that PCBC metabolites may covalently bind to DNA and implicate genotoxic mechanisms in HCBD-induced nephrocarcinogenicity.

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.

Production of DNA single strand breaks in rabbit renal tissue after exposure to 1,2-dichlorovinylcysteine

S-(trans-1,2-dichlorovinyl)-L-cysteine (DCVC) is a recognized nephrotoxin. To investigate the genotoxic effects of DCVC on the kidney, DNA strand breaks were measured as an indicator of DCVC induced damage. To ascertain if bioactivation of DCVC occurred in the kidney, 3 experimental systems were used: in vivo; isolated perfused kidneys; and isolated proximal tubules of albino male rabbits. A dose-dependent increase in strand breaks in the kidney tubular DNA occurred after in vivo dosing with 5-100 mg/kg DCVC and after in vitro exposure to 10(-5)-10(-2) M DCVC. These results demonstrate the genotoxic effect of this compound on renal tissue.

Chronic toxicity of S-(trans-1,2-dichlorovinyl)-L-cysteine in mice

S-(trans-1,2-Dichlorovinyl)-L-cysteine (DCVC) exposure causes acute renal tubular cytotoxicity. To further characterize the effects of DCVC, a chronic study was undertaken. Male Swiss-Webster mice received DCVC dissolved in their drinking water at 0.01, 0.05 and 0.1 mg ml-1. At 4, 8, 21 and 37 weeks, animals were terminated. Bladders, spleens, livers, kidneys and eyes were removed for histopathological examination. At 0.05 and 0.1 mg ml-1 DCVC, growth retardation was evident by 21 weeks. By 26 weeks, all animals in the 0.1 mg ml-1 group had developed cortical cataracts. Cytomegaly, nuclear hyperchromatism and multiple nucleoli were noted in the cells of the pars recta region of the kidney by 4 weeks and correlated to time and dose. At later time points, renal tubular atrophy and early interstitial fibrosis were evident. The epithelial cytological cellular abnormalities appear to be dose-related. Minor pathological changes were noted in the spleen, while there was no effect on the liver or bladder. Chronic ingestion of DCVC results in severe kidney injury.