Effects of Cytochrome P450 Antibodies on the Oxidative Demethylation of Methoxychlor Catalyzed by Rat Liver Microsomal Cytochrome P450 Isozymes: Isozyme Specificity and Alteration of Enantiotopic Selectivity

Abstract

Anti-CYP2B1 added to the methoxychlor reaction medium catalyzed by phenobarbital-treated rat liver microsomes altered the enantiotopic selectivity of the reaction, in favor of the (S)-mono-O-demethylated metabolite. In the reaction with untreated liver microsomes, anti-CYP2B1 did not affect the enantiotopic selectivity. Phenobarbital induces CYP2B1 and 2B2, both of which have low enantiotopic selectivity in this reaction, and both are inhibited by anti-CYP2B1. Therefore, in the presence of anti-CYP2B1, the catalytic activity of other CYPs that exhibit high enantiotopic selectivity should predominate even in the phenobarbital-treated rat liver microsomes. The findings with anti-CYP2B1 were consistent with this notion. When anti-CYP2C6 was added to the reaction medium containing purified CYP2C6, the metabolic rates markedly decreased, whereas the enantiotopic selectivity was unaltered. However, when anti-CYP2C6 was added to reactions catalyzed by both microsomal preparations, the rates sometimes significantly increased, while there were no essential changes in the metabolite composition. Anti-CYP2C6 was also added to CYP2A1-catalyzed reactions. Our CYP2A1 preparation sometimes altered the metabolic rates and the (S) percentage in the metabolites. This CYP2A1 preparation may have been contaminated with a small amount of CYP2C6, and some other CYP(s) with low affinity to methoxychlor. CYP(s) metabolizing methoxychlor with low affinity to the substrate may be present in the microsomal preparations.