title: Mechanism-based inactivation of cytochrome P450 3A4 by 17 alpha-ethynylestradiol: evidence for heme destruction and covalent binding to protein.
abstract: 17 alpha-Ethynylestradiol (EE), a major constituent of many oral contraceptives, inactivated the testosterone 6 beta-hydroxylation activity of purified P450 3A4 reconstituted with phospholipid and NADPH-cytochrome P450 reductase in a mechanism-based manner. The inactivation of P450 3A4 followed pseudo first order kinetics and was dependent on NADPH. The values for the K(I) and k(inact) were 18 microM and 0.04 min(-1), respectively, and the t(1/2) was 16 min. Incubation of 50 microM EE with P450 3A4 at 37 degrees C for 30 min resulted in a 67% loss of testosterone 6 beta-hydroxylation activity accompanied by a 35% loss of the spectral absorbance of the native protein at 415 nm and a 70% loss of the spectrally detectable P450-CO complex. The inactivation of P450 3A4 by EE was irreversible. Testosterone, an alternate substrate, was able to protect P450 3A4 from EE-dependent inactivation. The partition ratio was approximately 50. The stoichiometry of binding was approximately 1.3 nmol of an EE metabolite bound per nmol of P450 3A4 inactivated. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [(3)H]EE was irreversibly bound to the P450 3A4 apoprotein. After extensive dialysis of the [(3)H]EE inactivated samples, high-pressure liquid chromatography (HPLC) analysis demonstrated that the inactivation resulting from EE metabolism led to the destruction of approximately half the heme with the concomitant generation of modified heme and EE-labeled heme fragments and produced covalently radiolabeled P450 3A4 apoprotein. Electrospray mass spectrometry demonstrated that the fraction corresponding to the major radiolabeled product of EE metabolism has a mass (M - H)(-) of 479 Da. HPLC and gas chromatography-mass spectometry analyses revealed that EE metabolism by P450 3A4 generated one major metabolite, 2-hydroxyethynylestradiol, and at least three additional metabolites. In conclusion, our results demonstrate that EE is an effective mechanism-based inactivator of P450 3A4 and that the mechanism of inactivation involves not only heme destruction, but also the irreversible modification of the apoprotein at the active site.
authors.affiliation: Department of Pharmacology, University of Michigan, Ann Arbor, Michigan 48109-0632, USA.
authors.forename: Hsia-lien
authors.initials: HL
authors.lastname: Lin
authors.forename: Ute M
authors.initials: UM
authors.lastname: Kent
authors.forename: Paul F
authors.initials: PF
authors.lastname: Hollenberg
mesh_terms: D001059:Apoproteins / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry; D004591:Electrophoresis, Polyacrylamide Gel; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology; D004997:Ethinyl Estradiol / Q000494:pharmacology*; D008401:Gas Chromatography-Mass Spectrometry; D006418:Heme / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors* / Q000737:chemistry; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D011485:Protein Binding / Q000187:drug effects; D012995:Solubility; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D001059:Apoproteins; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D004997:Ethinyl Estradiol; D006418:Heme; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A