NADPH-dependent metabolism of estrone by human liver microsomes

We characterized the NADPH-dependent metabolism of estrone (E1) by liver microsomes of 21 male and 12 female human subjects. The structures of 11 hydroxylated or keto metabolites of E1 formed by human liver microsomes were identified by chromatographic and mass spectrometric analyses. 2-Hydroxylation of E1 was the dominant metabolic pathway with all human liver microsomes tested. E1 is more prone to form catechol estrogens (particularly 4-OH-E1) than 17beta-estradiol (E2) and the average ratio of E1 4-hydroxylation to 2-hydroxylation (0.24) was slightly higher than the ratio of E2 4- to 2-hydroxylation (0.20, P < 0.001). An unidentified monohydroxylated E1 metabolite (y-OH-E1) was found to be one of the major metabolites formed by human liver microsomes of both genders. 6beta-OH-E1, 16alpha-OH-E1, and 16beta-OH-E1 were also formed in significant quantities. 16alpha-hydroxylation was not a major pathway for E1 metabolism. The overall profiles for the E1 metabolites formed by male and female human liver microsomes were similar, and their average rates were not significantly different. Hepatic CYP3A4/5 activity in both male and female liver microsomes correlated strongly with the rates of formation of several hydroxyestrogen metabolites. The dominant role of hepatic CYP3A4 and CYP3A5 in the formation of these hydroxyestrogen metabolites was further confirmed by incubations of human CYP3A4 or CYP3A5 with [3H]E1 and NADPH. Notably, human CYP3A5 has very high relative activity for E1 4-hydroxylation, exceeding its activity for E1 2-hydroxylation by approximately 100%. It will be of interest to determine the potential biological functions associated with any of the E1 metabolites identified in our present study.