OXIDATION AND ACETYLATION AS DETERMINANTS OF 2-BROMOCYSTEIN-S-YLHYDROQUINONE-MEDIATED NEPHROTOXICITY

2-Bromodiglutathion-S-ylhydroquinone is a more potent nephrotoxicant t han 2-bromomonoglutathion-S-ylhydroquinones. In the present study we e xamined the activity of enzymes involved in mercapturic acid biosynthe sis toward both the glutathione conjugates and their cysteine and N-ac etylcysteine metabolites and compared the results to the relative neph rotoxicity of these conjugates. Although differences were observed in the kinetics of the gamma-glutamyl transpeptidase (gamma-GT)-mediated hydrolysis and transpeptidation of the glutathione conjugates, the con centration of this enzyme within the kidney probably precludes it from contributing to their differential toxicity. In contrast, the rate at which the cysteine and corresponding mercapturate conjugates underwen t N-deacetylation/N-acetylation cycling correlated with previously rep orted differences in toxicity. The relative rates of these two reactio ns are important because electrochemical data suggest that 2-bromodicy stein-S-ylhydroquinone is more readily oxidized to the reactive quinon e than its corresponding mercapturic acid. In addition, 2-bromodi(N-ac etylcystein-S-yl)hydroquinone, which is the most potent of the mercapt uric acid conjugates, exhibited the highest N-deacetylation/N-acetylat ion ratio. In contrast, 2-bromo-3-(N-acetylcystein-S-yl)hydroquinone, which is essentially not toxic in vivo, was not a substrate for the re nal cysteine conjugate N-deacetylase. The data suggest that the rate-d etermining step for the in vivo toxicity of these conjugates is probab ly the N-acetylation reaction and the availability of the correspondin g acetyl-CoA cofactor.