The steady-state kinetics and specific activity of 2-, 4-, and 16alpha-hydroxylation of 17beta-estradiol (E(2)) were evaluated for human cytochrome P450 (CYP) 1A1, 1A2, 1B1, and 3A4 enzymes, using complementary DNA-expressed CYP isoforms. CYP1A2 showed the highest 2-hydroxylation activity, followed by CYP1A1, 1B1, and 3A4. CYP1B1 had the highest 4-hydroxylation activity, followed by CYP1A2, 1A1, and 3A4. The 16alpha-hydroxylation reaction was catalyzed mainly by CYP1A2 and, to a similar, slightly lower extent, CYP3A4 and 1A1, with a lesser contribution by CYP1B1. The E(2) 2-, 4-, and 16alpha-hydroxylation activities of human liver microsomes were 1.3 +/- 0.3, 0.5 +/- 0.06, and 0.3 +/- 0.05 nmol metabolite/min/nmol P450, respectively. The contribution of CYP1A1 and 1B1 (mainly extrahepatic) to the E(2) hydroxylation reactions, relative to CYP1A2 and 3A4 (predominantly hepatic), may be relevant to understanding the process of hormonal carcinogenesis both in liver and in extrahepatic tissues.
Copyright 2001 by W.B. Saunders Company