Cytotoxicity of cysteine S-conjugates: structure-activity relationships

Mutagenicity of the cysteine S-conjugate sulfoxides of trichloroethylene and tetrachloroethylene in the Ames test

The nephrotoxicity and nephrocarcinogenicity of trichloroethylene (TCE) and tetrachloroethylene (PCE) are believed to be mediated primarily through the cysteine S-conjugate β-lyase-dependent bioactivation of the corresponding cysteine S-conjugate metabolites S-(1,2-dichlorovinyl)-l-cysteine (DCVC) and S-(1,2,2-trichlorovinyl)-l-cysteine (TCVC), respectively. DCVC and TCVC have previously been demonstrated to be mutagenic by the Ames Salmonella mutagenicity assay, and reduction in mutagenicity was observed upon treatment with the β-lyase inhibitor aminooxyacetic acid (AOAA). Because DCVC and TCVC can also be bioactivated through sulfoxidation to yield the potent nephrotoxicants S-(1,2-dichlorovinyl)-l-cysteine sulfoxide (DCVCS) and S-(1,2,2-trichlorovinyl)-l-cysteine sulfoxide (TCVCS), respectively, the mutagenic potential of these two sulfoxides was investigated using the Ames Salmonella typhimurium TA100 mutagenicity assay. The results show both DCVCS and TCVCS were mutagenic, and TCVCS exhibited 3-fold higher mutagenicity than DCVCS. However, DCVCS and TCVCS mutagenic activity was approximately 700-fold and 30-fold lower than DCVC and TCVC, respectively. DCVC and DCVCS appeared to induce toxicity in TA100, as evidenced by increased microcolony formation and decreased mutant frequency above threshold concentrations. TCVC and TCVCS were not toxic in TA100. The toxic effects of DCVC limited the sensitivity of TA100 to DCVC mutagenic effects and rendered it difficult to investigate the effects of AOAA on DCVC mutagenic activity. Collectively, these results suggest that DCVCS and TCVCS exerted a definite but weak mutagenicity in the TA100 strain. Therefore, despite their potent nephrotoxicity, DCVCS and TCVCS are not likely to play a major role in DCVC or TCVC mutagenicity in this strain.

S-(1,2,2-Trichlorovinyl)-l-cysteine Sulfoxide, a Reactive Metabolite ofS-(1,2,2-Trichlorovinyl)-l-cysteine Formed in Rat Liver and Kidney Microsomes, Is a Potent Nephrotoxicant

Previously, we have provided evidence that cytochromes P450 (P450s) and flavin-containing monooxygenases (FMOs) are involved in the oxidation of S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) in rabbit liver microsomes to yield the reactive metabolite TCVC sulfoxide (TCVCS). Because TCVC is a known nephrotoxic metabolite of tetrachloroethylene, the nephrotoxic potential of TCVCS in rats and TCVCS formation in rat liver and kidney microsomes were investigated. At 5 mM TCVC, rat liver microsomes formed TCVCS at a rate nearly 5 times higher than the rate measured with rat kidney microsomes, whereas at 1 mM TCVC only the liver activity was detectable. TCVCS formation in liver and kidney microsomes was dependent upon the presence of NADPH and was inhibited by the addition of methimazole or 1-benzylimidazole, but not superoxide dismutase, catalase, KCN, or deferoxamine, consistent with the involvement of both FMOs and P450s. Rats given TCVCS at 230 micromol/kg i.p. exhibited acute tubular necrosis at 2 and 24 h after treatment, and they had elevated blood urea nitrogen levels at 24 h, whereas TCVC was a much less potent nephrotoxicant than TCVCS. Furthermore, pretreatment with aminooxyacetic acid enhanced TCVC toxicity. In addition, reduced nonprotein thiol concentrations in the kidney were decreased by nearly 50% 2 h after TCVCS treatment compared with saline-treated rats, whereas the equimolar dose of TCVC had no effect on kidney nonprotein thiol status. No significant lesions or changes in nonprotein thiol status were observed in liver with either TCVC or TCVCS. Collectively, the results suggest that TCVCS may play a role in TCVC-induced nephrotoxicity.

Renal toxicity of perchloroethylene and S-(1,2,2-trichlorovinyl)glutathione in rats and mice: sex- and species-dependent differences

Suspensions of renal cells from rats and renal mitochondria from rats and mice were used to assess the sex and species dependence of acute toxicity due to perchloroethylene (Perc) and its glutathione conjugate S-(1,2,2-trichlorovinyl)glutathione (TCVG). A marked sex dependence in the acute cytotoxicity of both Perc and TCVG was observed: Perc caused significant release of lactate dehydrogenase (LDH) in isolated kidney cells from male but not female rats, and TCVG caused much more LDH release from male than female rat kidney cells. Assessment of toxicity in suspensions of isolated mitochondria from kidneys of male and female rats revealed a generally similar pattern of sensitivity, with mitochondria from males exhibiting significantly more inhibition of State 3 respiration and decrease of respiratory control ratio than mitochondria from females. Respiratory function in mitochondria from male and female mice, however, was also significantly inhibited by Perc or TCVG but exhibited little sex dependence in the degree of inhibition. Comparison with results from similar studies using the congener trichloroethylene and its glutathione conjugate suggested that Perc and TCVG are more potent nephrotoxicants. Neither Perc nor TCVG produced any significant effects on cytotoxicity or mitochondrial function in isolated hepatocytes from rats or in isolated liver mitochondria from rats or mice, suggesting that the liver is not a major acute target for Perc or its glutathione conjugate. Thus, many of the species-, sex-, and tissue-dependent differences in toxicity of Perc and TCVG that are observed in vivo are also observed in these in vitro models.

Glutathione conjugation of perchloroethylene in rats and mice in vitro: sex-, species-, and tissue-dependent differences

Perchloroethylene (Per)-induced nephrotoxicity and nephrocarcinogenicity have been associated with metabolism by the glutathione (GSH) conjugation pathway to form S-(1,2,2-trichlorovinyl)glutathione (TCVG). Formation of TCVG was determined in incubations of Per and GSH with isolated renal cortical cells and hepatocytes from male and female Fischer 344 rats and with renal and hepatic cytosol and microsomes from male and female Fischer 344 rats and B6C3F1 mice. The goal was to assess the role of metabolism in the sex and species dependence of susceptibility to Per-induced toxicity. A key finding was that GSH conjugation of Per occurs in kidney as well as in liver. Although amounts of TCVG formation in isolated kidney cells and hepatocytes from male and female rats were generally similar, TCVG formation in subcellular fractions showed marked sex, species, and tissue dependence. This may be due to the presence of multiple pathways for metabolism in intact cells, whereas only the GSH conjugation pathway is active in the subcellular fractions under the present assay conditions. TCVG formation in kidney and liver subcellular fractions from both male rats and mice were invariably higher than corresponding values in female rats and mice. Amounts of TCVG formation in rat liver subcellular fractions were approximately 10-fold higher than in corresponding fractions from rat kidney. Although rats are more susceptible to Per-induced renal tumors than mice, amounts of TCVG formation were 7- to 10-fold higher in mouse kidney subcellular fractions and 2- to 5-fold higher in mouse liver subcellular fractions of both sexes compared to corresponding fractions from the rat. Hence, although the higher amounts of TCVG formation in liver and kidney from male rats correspond to their higher susceptibility to Per-induced renal tumors compared with female rats, the markedly higher amounts of TCVG formation in mice compared with rats suggest that other enzymatic or transport steps in the handling of Per in mice contribute to their relatively low susceptibility to Per-induced renal tumors

Sulfoxidation of mercapturic acids derived from tri- and tetrachloroethene by cytochromes P450 3A: a bioactivation reaction in addition to deacetylation and cysteine conjugate beta-lyase mediated cleavage

In the present study we investigated the formation of sulfoxides from N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine (N-Ac-TCVC), N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (N-Ac-1,2-DCVC), and N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine (N-Ac-2,2-DCVC), which are formed in the glutathione dependent bioactivation of tri- and tetrachloroethene. The first aim was to elucidate the enzymes involved in these oxidation reactions. N-Ac-TCVC, N-Ac-1,2-DCVC, and N-Ac-2,2-DCVC are oxidized to the corresponding sulfoxides mainly, if not exclusively, by cytochrome P450 enzymes in liver microsomes of untreated male rats, since no role for the flavin-containing monooxygenase (FMO) could be demonstrated by heat inactivation experiments and by the use of n-octylamine. The sulfoxidation rates were increased when using liver microsomes of phenobarbital and dexamethasone pretreated male rats as well as liver microsomes of dexamethasone pretreated female rats, while no sulfoxide formation was observed in liver microsomes of untreated female rats, suggesting an involvement of cytochrome P450 3A. Also, troleandomycin, a specific chemical inhibitor for cytochrome P450 3A, drastically reduced sulfoxidation rates. The observed rates of sulfoxidation also correlated well with the rates of oxidation of testosterone at the 6-beta-position, a specific marker for P450 3A activity. The second aim of this study was to compare the cytotoxicity of the sulfoxides with the cytotoxicity of the corresponding mercapturic acids in isolated rat renal epithelial cells. Both mercapturic acids and the corresponding sulfoxides were cytotoxic. Cytotoxicity of the mercapturic acids could be blocked by (aminooxy)acetic acid (AOAA), an inhibitor of cysteine conjugate beta-lyase, while the cytotoxicity of the sulfoxides was not influenced by this treatment. Moreover, the sulfoxides were significantly more cytotoxic than the corresponding mercapturic acids at equimolar doses. The results show that mercapturic acids derived from TRI and PER are oxidized to sulfoxides by microsomal monooxygenases from rat liver. The cytotoxicity of the produced sulfoxides could not be reduced by AOAA, consistent with a role of the sulfoxides as direct acting electrophiles (i.e., Michael acceptor substrates).

Glutathione-dependent bioactivation of xenobiotics

Glutathione conjugation has been identified as an important detoxication reaction. However, in recent years several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions are discussed. 1. Dichloromethane is metabolized by glutathione conjugation to formaldehyde via S-(chloromethyl)glutathione. Both compounds are reactive intermediates and may be responsible for the dichloromethane-induced tumorigenesis in sensitive species. 2. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl)glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed. 3. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cysteine conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is responsible for the observed toxicity of the parent compounds. 4. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they undergo alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Examination of the structure-toxicity relationships of L-cysteine-S-conjugates of halogenated alkenes and their corresponding mercapturic acids in rat renal tissue slices

Rat kidney slices were produced using a modified version of a mechanical tissue slicer. The slices were incubated with various concentrations of L-cysteine conjugates and mercapturic acids of halogenated alkenes in a submersion incubation system. The slices showed a time- and concentration-dependent toxicity to the nephrotoxic conjugates. The five L-cysteine conjugates tested: S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCVC), S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), S-(1-chloro-1,2,2-trifluoroethyl)-L-cysteine (CTFEC), S-(1,1-dichloro-2,2-difluoroethyl)-L-cysteine (DCDFEC) and S-(1,1-dibromo-2,2-difluoroethyl)-L-cysteine (DBDFEC) were more toxic compared to the corresponding mercapturic acids. Comparing the in vitro toxicity data with the in vivo data for the same compounds results in similar ranking for the relative nephrotoxicity of the conjugates.

High-performance liquid chromatography-fluorescence assay of pyruvic acid to determine cysteine conjugate beta-lyase activity: application to S-1,2-dichlorovinyl-L-cysteine and S-2-benzothiazolyl-L-cysteine

An HPLC-fluorescence assay has been developed for the determination of the activity of rat renal cytosolic cysteine conjugate beta-lyase. The method is based on isocratic HPLC separation and fluorescence detection of pyruvic acid, derivatized with o-phenylenediamine (OPD), and is shown to be rapid, specific, and very sensitive. The assay has been evaluated with two model substrates for rat renal cytosolic beta-lyase, notably S-1,2-dichorovinyl-L-cysteine (DCVC) and S-2-benzothiazolyl-L-cysteine (BTC). Equimolar formation of pyruvic acid and 2-mercaptobenzothiazole, a chromophoric thiol, indicated that pyruvic acid formation actually reflects the beta-elimination activity of beta-lyase during the beta-elimination of BTC. From this it follows that the pyruvic acid assay can be applied to the measurement of the beta-elimination activity of this enzyme, independent of the presence of chromophoric groups or radiolabels in substrates. Due to the large linear range and the very high sensitivity of the present HPLC-fluorescence assay (detection limit, 7.5 pmol of pyruvic acid), both good and poor substrates of beta-lyase can be measured. Enzyme kinetic data are presented for the model substrates BTC and DCVC and for four structurally related S-2,2-difluoroethyl-L-cysteine conjugates.

Glutathione depletion, lipid peroxidation, DNA double-strand breaks and the cytotoxicity of 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone in rat renal cortical cells

The mechanisms involved in the cytotoxicity of 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone, a model compound for hydroquinone derived mercapturic acids, were investigated in rat renal proximal tubule cells. 2-Bromo-3-(N-acetylcystein-S-yl)hydroquinone induced a time- and concentration-dependent decrease in cell viability and in the levels of cellular glutathione. Antioxidants such as N,N'-diphenyl-p-phenylene diamine and ascorbic acid and the iron chelator desferrioxamine very efficiently protected the cells from 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone without influencing glutathione depletion. The acetoxymethyl ester of the Ca2+ chelator Quin-2, the inhibitor of the Ca(2+)- and Mg(2+)-dependent endonucleases, aurintricarboxylic acid and the poly(ADP-ribose)-polymerase inhibitor 3-aminobenzamide also ameliorated 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone cytotoxicity. Moreover, 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone depleted Ca2+ from isolated kidney mitochondria, increased the amount of malondialdehyde in rat kidney cells and induced DNA double-strand breaks in renal cells in culture. These results suggest that renal cells oxidize 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone to the corresponding quinone; this soft electrophile reacts rapidly with glutathione, thus depleting cellular glutathione concentrations as indicated by the tentative identification of a 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone thioether in the incubation medium of renal cells treated with the mercapturate. As a result of the massive glutathione depletion, peroxidative mechanisms then cause an elevation of the cytosolic concentrations of ionized calcium; impairment of the ability of the mitochondria to sequester Ca2+ plays an important role in the elevation of the Ca2+ concentration. Finally, activation of Ca(2+)- and Mg(2+)-dependent endonucleases results in DNA damage and cell death.

p-aminophenol nephrotoxicity: biosynthesis of toxic glutathione conjugates

p-Aminophenol causes necrosis of the pars recta of the proximal tubules in rats, and its nephrotoxicity may be due to glutathione-dependent bioactivation reactions. We have investigated the hepatic metabolism of p-aminophenol in Wistar rats and the cytotoxicity of formed glutathione S-conjugates in rat renal epithelial cells. After ip application of p-aminophenol (100 mg/kg), the following metabolites were identified in rat bile: 4-amino-2-(glutathion-S-yl)phenol, 4-amino-3-(glutathion-S-yl)-phenol, 4-amino-2,5-bis(glutathion-S-yl)phenol, 4-amino-2,3,5(or 6)-tris(glutathion-S-yl)phenol, an aminophenol conjugate (likely a sulfate or glucuronide), acetaminophen glucuronide, and 3-(glutathion-S-yl)acetaminophen. 4-Amino-3-(glutathion-S-yl)phenol, 4-amino-2,5-bis(glutathion-S-yl)phenol, and 4-amino-2,3,5(or 6)-tris(glutathion-S-yl)phenol induced a dose- and time-dependent loss of cell viability in rat kidney cortical cells. Cell killing was significantly reduced by inhibition of gamma-glutamyl transpeptidase with Acivicin. p-Aminophenol was also toxic to renal epithelial cells. Coincubation of p-aminophenol with tetraethylammonium bromide, a competitive inhibitor of the organic cation transporter, and with SKF-525A, an inhibitor of cytochrome P450, protected cells from p-aminophenol-induced toxicity. p-Aminophenol would thus be accumulated in the kidney mainly by organic cation transport systems, which are concentrated in the S-1 segment of the proximal tubule. However, p-aminophenol toxicity in vivo is directed toward the S-2 and S-3 segments, which are rich in gamma-glutamyl transpeptidase. These results and the observation that biliary cannulation and glutathione depletion reduce p-aminophenol nephrotoxicity suggest that the biosynthesis of toxic glutathione conjugates is responsible for p-aminophenol nephrotoxicity in vivo. The aminophenol glutathione S-conjugates formed induce p-aminophenol nephrotoxicity by a pathway dependent on gamma-glutamyl transpeptidase.

Bioactivation of hexachlorobutadiene by glutathione conjugation

Glutathione (GSH) conjugation reactions in the metabolism of hexachlorobutadiene (HCBD), in rats and mice, initiate a series of metabolic events resulting in the formation of reactive intermediates in the proximal tubular cells of the kidney. The GSH S-conjugate 1-(glutathion-S-yl)-1,2,3,4,4-pentachlorobutadiene (GPCB), which is formed by conjugation of HCBD with GSH in the liver, is not reactive and is eliminated from the liver in the bile or plasma, or both. GPCB may be translocated intact to the kidney and processed there by gamma-glutamyl transpeptidase and dipeptidases to the corresponding cysteine S-conjugate. Alternatively, gamma-glutamyl transpeptidase and dipeptidases present in epithelial cells of the bile duct and small intestine may catalyse the conversion of GPCB to cysteine S-conjugates. The kidney concentrates both GSH and cysteine S-conjugates and processes GSH conjugates to cysteine S-conjugates. A substantial fraction of HCBD cysteine S-conjugate thus concentrated in the kidney is metabolized by renal cysteine conjugate beta-lyase to reactive intermediates. The selective formation of reactive intermediates in the kidney most likely accounts for the organ-specific effects of HCBD. Alternatively, cysteine S-conjugates may be acetylated to yield excretable mercapturic acids.