Mutagenicity of amino acid and glutathione S-conjugates in the Ames test

Abstract

The mutagenicity of the glutathione S-conjugate S-(1,2-dichlorovinyl)glutathione (DCVG), the cysteine conjugates S-(1,2- dichlorovinyl)- L-cysteine (DCVC) and S-(1,2- dichlorovinyl)- dl-α-methylcysteine (DCVMC), and the homocysteine conjugates S-(1,2- dichlorovinyl)- L-homocysteine (DCVHC) and S-(1,2- dichlorovinyl)- dl-α-methylhomocysteine (DCVMHC) was investigated in Salmonella typhimurium strain TA2638 with the preincubation assay. DCVC was a strong, direct-acting mutagen; the cysteine conjugate β-lyase inhibitor aminooxyacetic acid decreased significantly the number of revertants induced by DCVC; rat renal mitochondria (11000 X g pellet) and cytosol (105 000 X g supernatant) with high β-lyase activity increased DCVC mutagenicity at high DCVC concentrations. DCVG was also mutagenic without the addition of mammalian activating enzymes; the presence of low γ-glutamyltransferase activity in bacteria, the reduction of DCVG mutagenicity by aminooxyacetic acid, and the potentiation of DCVG mutagenicity by rat kidney mitochondria and microsomes (105000 X g pellet) with high γ-glutamyltransferase activity indicate that γ-glutamyltransferase and β-lyase participate in the metabolism of DCVG to mutagenic intermediates. The homocysteine conjugate DCVHC was only weakly mutagenic in the presence of rat renal cytosol, which exhibits considerable γ-lyase activity; this mutagenic effect was also inhibited by aminooxyacetic acid. The conjugates DCVMC and DCVMHC, which are not metabolized to reactive intermediates, were not mutagenic at concentrations up to 1 μmole/plate. The results demonstrate that γ-glutamyltransferase and β-lyase are the key enzymes in the biotransformation of cysteine and glutathione conjugates to reactive intermediates that interact with DNA and thereby cause mutagenicity.