title: Differential oxidation of mifepristone by cytochromes P450 3A4 and 3A5: selective inactivation of P450 3A4.
abstract: The principal enzyme involved in the oxidation of mifepristone is cytochrome P450 3A4 (CYP3A4), which undergoes mechanism-based inactivation by the drug. However, no information is available on the interaction with CYP3A5, the second most abundant CYP3A enzyme in adult human liver. Oxidation of mifepristone by recombinant CYP3A4 produced mono- and didemethylated products and one C-hydroxylated metabolite, as reported previously. However, CYP3A5 produced only the demethylated metabolites. The apparent V(max) and K(M) values for formation of the monodemethylated product by CYP3A4 and CYP3A5 were 46 and 30 nmol/min/nmol P450, and 36 and 16 microM, respectively. Unlike CYP3A4, CYP3A5 was not inactivated by mifepristone. The basis of this differential susceptibility was explored using site-directed mutants in which a CYP3A4 residue was converted to its 3A5 counterpart. Surprisingly, none of these replacements caused a significant decrease in CYP3A4 inactivation by mifepristone. Examination of selected CYP3A4 mutants at 20 other positions indicated that the relative formation rate of the C-hydroxylated product could not account for the differential susceptibility of CYP3A4 and 3A5. Together these results indicate that mifepristone fails to orient itself in the CYP3A5 active site in such a way that its propylenic group is accessible for oxidation, thus rendering CYP3A5 unable to produce the C-hydroxylated product or putative ketene that leads to enzyme inactivation. Identification of mifepristone as a selective mechanism-based inactivation of CYP3A4 may be very useful in distinguishing between the two major CYP3A enzymes collectively responsible for the oxidative metabolism of over half of the drugs currently in use.
authors.affiliation: Department of Pharmacology and Toxicology, University of Texas Medical Branch, Route 1031, 301 University Boulevard, Galveston, Texas 77555-1031, USA. kkkhan@utmb.edu
authors.forename: Kishore K
authors.initials: KK
authors.lastname: Khan
authors.forename: You Qun
authors.initials: YQ
authors.lastname: He
authors.forename: Maria Almira
authors.initials: MA
authors.lastname: Correia
authors.forename: James R
authors.initials: JR
authors.lastname: Halpert
mesh_terms: D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry* / Q000235:genetics; D006727:Hormone Antagonists / Q000737:chemistry*; D015735:Mifepristone / Q000737:chemistry*; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D011994:Recombinant Proteins / Q000737:chemistry
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D006727:Hormone Antagonists; D011994:Recombinant Proteins; D015735:Mifepristone; D003577:Cytochrome P-450 Enzyme System; C104464:CYP3A protein, human; C510164:CYP3A5 protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human