Characterization of the NADPH-dependent metabolism of 17 beta-estradiol tomultiple metabolites by human liver microsomes and selectively expressed human cytochrome p450 3A4 and 3A5

We characterized the NADPH-dependent metabolism of 17 beta -estradiol (E-2) by liver microsomes from 21 male and 12 female human subjects. A large num ber of radioactive estrogen metabolite peaks were detected following incuba tions of [H-3]E-2 with male or female human liver microsomes in the presenc e of NADPH. The structures of 18 hydroxylated or keto estrogen metabolites formed by these microsomes were identified by gas chromatography/mass spect rometry analysis. 2-Hydroxylation (the formation of 2-OH-E-2 and 2-OH-E-1) was the dominant metabolic pathway with all human liver microsomes tested. The average ratio of 4-OH-E-2 to 2-OH-E-2 formation was similar to1:6. A ne w monohydroxylated E-2 metabolite (chemical structure unidentified) was fou nd to be one of the major metabolites formed by human liver microsomes of b oth genders. 6 beta -OH-E-2 and 16 beta -OH-E-2 were also formed in signifi cant quantities, but products of estrogen 16 alpha -hydroxylation (16 alpha -OH-E-2 + 16 alpha -OH-E,) were quantitatively minor metabolites. In addit ion, many other estrogen metabolites such as 6-keto-E-2, 6 alpha -OH-E-2, 7 alpha -OH-E-2, 12 beta -OH-E-2, 15 alpha -OH-E-2, 15 beta -OH-E-2, 16 beta -OH-E-1 and 16-keto-E-2 were also formed in relatively small quantities. T he overall profiles for the E-2 metabolites formed by male and female human liver microsomes were similar, and their average rates were not significan tly different. The activity of testosterone 6 beta -hydroxylation (a select ive probe for CYP3A4/5 activity) strongly correlated with the rate of forma tion of 2-OH-E-2, 4-OH-E-2, and several other hydroxyestrogen metabolites b y both male and female liver microsomes. The dominant role of hepatic CYP3A 4 and CYP3A5 in the formation of these hydroxyestrogen metabolites was furt her confirmed by incubations of selectively expressed human CYP3A4 or CYP3A 5 with [H-3]E-2 and NADPH.