The O-methylation of 4-hydroxyestradiol is inhibited by 2-hydroxyestradiol: implications for estrogen-induced carcinogenesis.
O-Methylation of catecholestrogens catalyzed by catechol-O-methyltransferase provides a major route for the rapid metabolic clearance of these steroids. However, the metabolic clearance rate of 4-hydroxyestradiol (4-OH-E2) is considerably lower than that of 2-hydroxyestradiol, although 2- and 4-hydroxycatecholestrogens (2- and 4-OH-CE) have similar apparent affinities for the enzyme. To determine the reason for this apparent paradox we have examined whether the efficiency of O-methylation of 4-OH-E2 could be affected by other catecholestrogens or their O-methyl ethers. The ratio of 4-methoxyestradiol:4-hydroxyestradiol 3-methyl ether was 2.6 at pH 8.5, the pH optimum for the reaction. The O-methylation of 4-OH-E2 (apparent Km 10 microM) was inhibited by 2-hydroxyestradiol (2-OH-E2) but not by 2- or 4-methyoxyestrogens. The values for Km, Vmax as well as the slope for the methylation of 4-OH-E2 were altered by 2-OH-E2 indicating a mixed inhibition. The inhibition constant for the intercept 1/V'max versus 2-OH-E2 concentrations and the inhibition constant for the slope versus 2-OH-E2 concentrations were 35 and 5.7 microM, respectively. The inhibition of O-methylation of 4-OH-E2 by 2-OH-E2 increased with the pH. In target tissues of the carcinogenic action of estrogens such as the rat pituitary, hamster kidney, or mouse uterus in which 2- and 4-OH-CE are both generated in almost equal amounts, the inactivation of 4-OH-CE by O-methylation may be impeded. Consequently, 4-OH-E2 would remain available as substrate for redox cycling, generation of active radicals and DNA damage.
Roy
D
D
Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston 77550-2774.
Weisz
J
J
Liehr
J G
JG
D000818:Animals; D002394:Catechol O-Methyltransferase / Q000494:pharmacology; D004958:Estradiol / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D004967:Estrogens / Q000633:toxicity*; D002393:Estrogens, Catechol; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008297:Male; D008745:Methylation; D009369:Neoplasms / Q000139:chemically induced*; D051381:Rats; D011919:Rats, Inbred Strains
D004967:Estrogens; D002393:Estrogens, Catechol; D004958:Estradiol; C001390:2-hydroxyestradiol; C014036:4-hydroxyestradiol; D002394:Catechol O-Methyltransferase