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

J. Halpert, J. Y. Jaw, L. J. Cornfield, C. Balfour, E. A. Mash

Research output: Contribution to journalArticlepeer-review

26 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)26-31
Number of pages6
JournalDrug Metabolism and Disposition
Issue number1
StatePublished - 1989

ASJC Scopus subject areas

  • Pharmacology
  • Pharmaceutical Science


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