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

Abstract

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)