Identification of microsomal, organic hydroperoxide-dependent catechol estrogen formation: comparison with NADPH-dependent mechanism.
Exogenous aromatic polycyclic hydrocarbons, notably benzo[a]pyrene, can be metabolized by both NADPH-dependent monooxygenase(s) and an organic hydroperoxide (OHP)-dependent mechanism. To determine whether phenolic estrogens, endogenous aromatic polycyclic hydrocarbons, can also be hydroxylated by these alternative pathways, conversion of estradiol (E2) to 2- and 4-hydroxylated catecholestrogens (2- and 4-OH-CEs) by human placental microsomes was examined under conditions previously shown to support CE formation by NADPH- and OHP-dependent mechanisms. CEs were formed under both conditions. Properties of OHP-dependent activity were similar to CE formation by peroxidases and distinct from the NADPH-dependent monooxygenase. NADPH supported only 2-hydroxylation, whereas cumene hydroperoxide supported 2- and 4-hydroxylation equally. Monooxygenase-mediated activity had characteristics of a high-affinity, low-capacity enzyme system (apparent Km for E2 = 0.3 microM, and Vmax = 31 pmol/mg protein/30 min), whereas peroxidatic activity had properties of low affinity and high capacity (apparent Km for E2 = 55 microM, Vmax = 666 pmol/mg protein/10 min). The requirement of peroxidatic activity for oxidizing co-substrate could be met by OHPs but not by H2O2. Peroxidatic CE formation could have special functional significance for physiological and pathological consequences of estrogen action since it generates 4-OH-CEs which are both effective catechols and potent, long-acting estrogens. Moreover, it could provide a link through co-oxygenation between estrogens and diverse cellular mechanisms involving generation of OHPs.
Bui
Q
Q
Department of Obstetrics and Gynecology, Milton S. Hershey Medical Center, Pennsylvania State University, Hershey.
Weisz
J
J
D002245:Carbon Dioxide / Q000494:pharmacology; D065607:Cytochrome P-450 Enzyme Inhibitors; D004958:Estradiol / Q000378:metabolism; D002393:Estrogens, Catechol / Q000378:metabolism*; D005260:Female; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism*; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000378:metabolism*; D009249:NADP / Q000502:physiology*; D010920:Placenta / Q000201:enzymology / Q000378:metabolism; D011247:Pregnancy
D065607:Cytochrome P-450 Enzyme Inhibitors; D002393:Estrogens, Catechol; D002245:Carbon Dioxide; D004958:Estradiol; D009249:NADP; D006861:Hydrogen Peroxide