Biotransformation of lovastatin: IV. 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 ( r 2 = 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.