title: Methoxyestrogens exert feedback inhibition on cytochrome P450 1A1 and 1B1.
abstract: Cytochrome P450 1A1 (CYP1A1) and 1B1 (CYP1B1) catalyze the oxidative metabolism of 17 beta-estradiol (E2) to catechol estrogens (2-OHE2 and 4-OHE2) and estrogen quinones, which may lead to DNA damage. Catechol-O-methyltransferase catalyzes the methylation of catechol estrogens to methoxyestrogens (2-MeOE2, 2-OH-3-MeOE2, and 4-MeOE2), which simultaneously lowers the potential for DNA damage and increases the concentration of 2-MeOE2, an antiproliferative metabolite. In this study, we showed that CYP1A1 and CYP1B1 recognized as substrates both the parent hormone E2 and the methoxyestrogens. Using purified recombinant enzymes, we demonstrated that CYP1A1 and CYP1B1 O-demethylated the methoxyestrogens to catechol estrogens according to Michaelis-Menten kinetics. Both CYP1A1 and CYP1B1 demethylated 2-MeOE2 and 2-OH-3-MeOE2 to 2-OHE2, whereas CYP1B1 additionally demethylated 4-MeOE2 to 4-OHE2. Because the P450-mediated oxidation of E2 and the O-demethylation of methoxyestrogens both yielded identical catechol estrogens as products, we used deuterated E2 (E2-d4), unlabeled methoxyestrogens, and gas chromatography/mass spectrometry to examine both reactions simultaneously. Kinetic analysis revealed that methoxyestrogens acted as noncompetitive inhibitors of E2 oxidation with K(i) ranging from 27 to 153 micro M. For both enzymes, the order of inhibition by methoxyestrogens was 2-OH-3-MeOE2 > or = 2-MeOE2 > 4-MeOE2. Thus, methoxyestrogens exert feedback inhibition on CYP1A1- and CYP1B1-mediated oxidative estrogen metabolism, thereby reducing the potential for estrogen-induced DNA damage.
authors.affiliation: Department of Pathology, Vanderbilt University Medical Center, Nashville, Tennessee 37232, USA.
authors.forename: Sheila
authors.initials: S
authors.lastname: Dawling
authors.forename: Nady
authors.initials: N
authors.lastname: Roodi
authors.forename: Fritz F
authors.initials: FF
authors.lastname: Parl
mesh_terms: D000077584:2-Methoxyestradiol; D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D002394:Catechol O-Methyltransferase / Q000378:metabolism; D002986:Clinical Trials as Topic; D019363:Cytochrome P-450 CYP1A1 / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D065633:Cytochrome P-450 CYP1B1; D004249:DNA Damage; D003903:Deuterium / Q000378:metabolism; D004958:Estradiol / Q000031:analogs & derivatives* / Q000096:biosynthesis / Q000378:metabolism* / Q000494:pharmacology*; D025461:Feedback, Physiological; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D007700:Kinetics; D008745:Methylation; D010084:Oxidation-Reduction; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: C475868:2-hydroxy-3-methoxyestradiol; D011994:Recombinant Proteins; C045018:4-methoxyestradiol; D004958:Estradiol; D000077584:2-Methoxyestradiol; D003903:Deuterium; D001189:Aryl Hydrocarbon Hydroxylases; C585496:CYP1B1 protein, human; D019363:Cytochrome P-450 CYP1A1; D065633:Cytochrome P-450 CYP1B1; D002394:Catechol O-Methyltransferase