Like catechol estrogens, 16alpha-hydroxylated estrogens are hormonally active, chemically reactive, and potentially mutagenic. We report here our novel findings that human CYP3A7 has a distinct high catalytic activity for the NADPH-dependent 16alpha-hydroxylation of estrone (E(1); at 10 nM to 200 microM substrate concentrations) but not for the 16alpha-hydroxylation of 17beta-estradiol (E(2)). At a physiologically relevant low substrate concentration (10 nM), CYP3A7 had a strong catalytic activity for the 16alpha-hydroxylation of E(1), and the ratio of its 16alpha-hydroxylation to 2-hydroxylation was 107%. In addition to 16alpha-hydroxylation, CYP3A7 also had catalytic activity for the 2-, 4-, 6beta-, and 16beta-hydroxylation of E(1). However, when E(2) was the substrate, CYP3A7 had only very weak catalytic activity for its 16alpha-hydroxylation (<6% of E(1) 16alpha-hydroxylation), and the ratio of its 16alpha-hydroxylation to 2-hydroxylation was 10-33%. Enzyme kinetic analysis showed that the maximal velocity and substrate-binding affinity (1/K(m)) for CYP3A7-mediated 16alpha-hydroxylation of E(1) were both approximately 10 times higher than those for E(2), thereby giving the maximal velocity:K(m) ratio of >100 times higher for the 16alpha-hydroxylation of E(1) than for E(2). Given the recent findings that human CYP3A7 is a polymorphic isoform also expressed in adult liver and certain extrahepatic tissues (in addition to fetal tissues), our data raise the possibility that CYP3A7 may be an important catalyst for the local and/or systemic formation of the procarcinogenic 16alpha-hydroxyestrone in women.