Abstract

We characterized the NADPH-dependent metabolism of 17β-estradiol (E₂) by liver microsomes from 21 male and 12 female human subjects. A large number of radioactive estrogen metabolite peaks were detected following incubations of [³H]E₂ with male or female human liver microsomes in the presence of NADPH. The structures of 18 hydroxylated or keto estrogen metabolites formed by these microsomes were identified by gas chromatography/mass spectrometry analysis. 2-Hydroxylation (the formation of 2-OH-E₂ and 2-OH-E₁) was the dominant metabolic pathway with all human liver microsomes tested. The average ratio of 4-OH-E₂ to 2-OH-E₂ formation was ∼1:6. A new monohydroxylated E₂ metabolite (chemical structure unidentified) was found to be one of the major metabolites formed by human liver microsomes of both genders. 6β-OH-E₂ and 16β-OH-E₂ were also formed in significant quantities, but products of estrogen 16α-hydroxylation (16α-OH-E₂ + 16α-OH-E₁) were quantitatively minor metabolites. In addition, many other estrogen metabolites such as 6-keto-E₂, 6α-OH-E₂, 7α-OH-E₂, 12β-OH-E₂, 15α-OH-E₂, 15β-OH-E₂, 16β-OH-E₁, and 16-keto-E₂ were also formed in relatively small quantities. The overall profiles for the E₂ metabolites formed by male and female human liver microsomes were similar, and their average rates were not significantly different. The activity of testosterone 6β-hydroxylation (a selective probe for CYP3A4/5 activity) strongly correlated with the rate of formation of 2-OH-E₂, 4-OH-E₂, and several other hydroxyestrogen metabolites by both male and female liver microsomes. The dominant role of hepatic CYP3A4 and CYP3A5 in the formation of these hydroxyestrogen metabolites was further confirmed by incubations of selectively expressed human CYP3A4 or CYP3A5 with [³H]E₂ and NADPH.