BIOTRANSFORMATION OF LOVASTATIN .4. IDENTIFICATION OF CYTOCHROME-P450 3A-PROTEINS AS THE MAJOR ENZYMES RESPONSIBLE FOR THE OXIDATIVE-METABOLISM OF LOVASTATIN IN RAT AND HUMAN LIVER-MICROSOMES

Previous studies from our laboratories have shown that the metabolism of the cholesterol-lowering drug lovastatin by rat and human liver microsomes occurs primarily at the 6′-position, giving 6′β-hydroxy- and 6′-exomethylene-lovastatin and that these oxidations are catalyzed by cytochrome P450-dependent monooxygenases. In the present study, the specific cytochrome P450 form involved in lovastatin oxidation was identified through immunoinhibition studies. Among several antibodies prepared against various cytochrome P450s, only anti-rat P450 3A IgG inhibited lovastatin metabolism in liver microsomes from untreated, phenobarbital treated, and pregnenolone-16α-carbonitrile-treated rats. Lovastatin metabolism at the 6′-position was markedly inhibited (6′β-hydroxy, greater than 95%; 6′-exomethylene, 70-80%) by this antibody whereas the effect of anti-rat P450 3A on the 3″-hydroxylation was variable depending on the source of the microsomes. With human liver microsomes, both anti-rat P450 3A and anti-human P450 3A inhibited lovastatin metabolism. Correlation between lovastatin oxidation and the P450 3A content in human liver microsomes (measured by immunoblot analysis) was excellent (r2 = 0.97). In addition, preincubation of human liver microsomes with troleandomycin and NADPH inhibited metabolism by 60%. These results clearly indicate that cytochrome P450 3A enzymes are primarily responsible for the metabolism of lovastatin in rat and human liver microsomes. © 1991.