17β-Estradiol metabolism by hamster hepatic microsomes: Implications for the catechol-O-methyl transferase-mediated detoxication of catechol estrogens

We have shown that the metabolism of 17β-estradiol in hamster liver microsomes is concentration-dependent. At low (<25 μM) concentrations of 17β-estradiol, 16α-hydroxylase activity predominated, and estriol was the major metabolite. At higher concentrations (25-75 μM), 16α-hydroxylation and aromatic hydroxylation at C2 contributed equally to 17β-estradiol metabolism. Aromatic C4-hydroxylation was maximal at 75 μM of 17β-estradiol and was always less than C2-hydroxylation. Dehydrogenation of the 17β- hydroxyl group to the ketone (estrone) was also observed, but both estrone and 2-hydroxyestrone were minor (~3%) metabolites of 17β-estradiol, only detectable at concentrations of 50 βM and above. Catechol-O-methyl transferase (COMT) effectively converted both 2- and 4-hydroxy-17β-estradiol to their corresponding monomethoxy metabolites. Effective reducing conditions are required for COMT activity, because catechol estrogens are readily oxidized to their corresponding ortho-quinones, and ascorbic acid is routinely added to assays of COMT activity. Interestingly, although ascorbic acid (1 mM) increased the recovery of 2- and 4-hydroxy-17β-estradiol from microsomal incubations, it decreased the recovery of the methoxy metabolites (~40%). Since the enediol function of ascorbate resembles that of a catechol group, ascorbate is a substrate for COMT and probably competes with the catechol estrogens for methylation. Because previous studies describing the ability of COMT to inhibit the covalent binding of electrophilic reactive metabolites of [4- ¹⁴C]17β-estradiol to microsomal protein were performed in the presence of high (100 mM) Mg ²⁺ concentrations, we also investigated the effects of Mg ²⁺ on 17β-estradiol metabolism. Concentrations of Mg ²⁺ > 10 mM inhibited the metabolism of 17β-estradiol, as evidenced by i) the increased recovery of substrate; ii) a decrease in the formation of estriol, estrone, and 2-, and 4-hydroxy-17β-estradiol; iii) a decrease in the recovery of water-soluble metabolites when incubations were performed in the presence of glutathione (GSH) to trap the reactive electrophilic metabolites; and iv) a decrease in the amount of reactive electrophilic metabolites bound to microsomal protein. GSH also decreased the covalent binding of electrophilic metabolites of [4- ¹⁴C]17β-estradiol to microsomal protein, with the concomitant formation of water-soluble metabolites. Thus, both COMT and GSH combine to limit the formation of electrophilic metabolites from 17β-estradiol. The relative importance of each of these pathways to the disposition of the catechol estrogens remains to be determined.