title: Mechanism-based inhibition of rat liver microsomal diazepam C3-hydroxylase by mifepristone associated with loss of spectrally detectable cytochrome P450.
abstract: Since initial studies with the steroids norethindrone and ethynylestradiol, reported by White and Muller-Eberhard in 1977 (Biochem. J. 166, 57-64), there has been continuing interest in xenobiotics that bear terminal or sub-terminal acetylenic groups which can cause catalysis-dependent inhibition of CYP monooxygenases associated either with loss of prosthetic group heme or protein adduct formation. Mifepristone is a synthetic steroid bearing a propyne substitution on carbon 17 and this suggested to us that it may act as a mechanism-based inhibitor of the CYP isoforms responsible for its metabolism. In human and rat liver, CYP3A isoforms have been implicated in mifepristone clearance and mifepristone administration to rats has also been shown to induce CYP3A enzymes and the associated diazepam C3-hydroxylase activity (Cheesman, Mason and Reilly, J. Steroid Biochem. Mol. Biol., 58, 1996, 447-454). With microsomes prepared from the livers of untreated female rats and others in which diazepam C3-hydroxylase has been induced, we show here that mifepristone can cause catalysis-dependent inhibition of this monooxygenase. In addition, incubation of microsomes with mifepristone in the presence, but not in the absence, of NADPH caused loss of spectrally detectable cytochrome P450. These results suggest that heme adduct formation may result from mifepristone metabolism by CYP3A monooxygenases which undergo self-catalysed irreversible inactivation with this drug as substrate. Since mifepristone administration in vivo is able also to cause induction of the synthesis of hepatic CYP3A apoprotein, mifepristone may have the potential in human medicine for complex interactions with other co-administered drugs which are also substrates for CYP3A monooxygenases.
authors.affiliation: Department of Biochemistry, University of Queensland, Brisbane, Australia. reilly@florey.biosci.uq.edu.au
authors.forename: P E
authors.initials: PE
authors.lastname: Reilly
authors.forename: R J
authors.initials: RJ
authors.lastname: Gomi
authors.forename: S R
authors.initials: SR
authors.lastname: Mason
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005260:Female; D006801:Humans; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology*; D015735:Mifepristone / Q000493:pharmacokinetics / Q000494:pharmacology*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors*; D009249:NADP / Q000378:metabolism; D010089:Oxidoreductases, N-Demethylating / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013379:Substrate Specificity
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D015735:Mifepristone; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; C118840:diazepam C3-hydroxylase; D010089:Oxidoreductases, N-Demethylating