Role of P-450c in the formation of a reactive intermediate of chlorotrianisene (TACE) by hepatic microsomes from methylcholanthrene-treated rats.
A previous study has shown that chlorotrianisene is metabolized by hepatic microsomal cytochrome P-450 monooxygenase(s) to a reactive intermediate that binds covalently to microsomal proteins [Juedes, Bulger, and Kupfer: Drug Metab. Dispos. 15, 786 (1987)]. Covalent binding of chlorotrianisene in hepatic microsomes is dramatically stimulated by treatment of rats with methylcholanthrene (MC), which is known to induce two major P-450 isozymes, P-450c (IA1) and P-450d (IA2). To determine whether P-450c and/or P-450d are involved in catalysis of covalent binding of chlorotrianisene, antibodies to P-450c and P-450d were used. Incubations of chlorotrianisene were conducted with liver microsomes from MC-treated rats (MC microsomes) and a monoclonal antibody (mAb) raised to the major MC-induced isozyme P450c, mAb 1-7-1, or a polyclonal monospecific antibody (pAb) to P-450d, pAb anti-d (-c). At a 5:1 ratio of antibody to microsomal protein, mAb 1-7-1 inhibited covalent binding by 67%, whereas pAb anti d (-c) showed a 10% inhibition. Maximal inhibition by mAb 1-7-1 was 89% at a 100:1 ratio of antibody to microsomal protein. From these findings it was concluded that P-450c is the major isozyme responsible for the metabolism of chlorotrianisene to the covalently binding reactive intermediate in MC microsomes. Additionally, it was observed that potentiation of covalent binding occurred with the noninhibitory mAbs used in these incubations. Substituting bovine serum albumin (BSA) for antibodies showed that this increase in binding is probably due to an increase in acceptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)