Selective inactivation of rat liver cytochromes P-450 by 21-chlorinated steroids

The inactivation by 21-chlorinated steroids of rat liver cytochromes P-450 involved in the hydroxylation of progesterone and androstenedione has been investigated. Preincubation of intact liver microsomes from phenobarbital-treated rats with 21-chloropregnenolone, 21,21-dichloropregnenolone, or 21,21-dichloroprogesterone in the presence of NADPH caused a time-dependent decrease in progesterone 21-hydroxylase and in progesterone or androstenedione 6β-hydroxylase activity but had negligible or only minor effects on five other steroid hydroxylases. The compounds differed, however, with regard to the relative rate constants for inactivation of the 21- and 6β-hydroxylases. For example, 21,21-dichloroprogesterone and 21,21-dichloropregnenolone inactivated the progesterone 6β-hydroxylase at similar rates, but the dichloroprogesterone was a more effective inactivator of the 21-hydroxylase. The results indicate that the introduction of a dichloromethyl group into a substrate bearing a methyl group normally hydroxylated by only one or a few isozymes of cytochrome P-450 may be a rational means of designing isozyme-selective inhibitors but that target and nontarget enzymes may not totally retain the regioselectivity they exhibit towards the underivatized substrate.