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The cytochrome P450 (P450) enzymes that catalyse metabolism of the estrogen, estrone (E1), to the putative carcinogen 16alpha-hydroxy E1 (16alpha-OHE1) in humans were determined. The potential of the most abundant circulating form of estrogen, estrone 3-sulfate (E1S), to be the substrate was also investigated. Human liver microsomal sulfatases convert E1S to E1, an essential prerequisite for formation of 16alpha-OHE1 from added E1S in this system. E1 metabolism to 16alpha-OHE1 in a panel of 15 human liver microsomal preparations correlated with total P450 concentrations (r2 = 0.63) and with activities associated with P450 forms CYP3A4 and 3A5 (r2 = 0.72). E1 16alpha-hydroxylase activity in human liver microsomes was inhibited by 75% by monoclonal anti human CYP3A4/5 antibodies at 4 mg antibody/nmol total P450, and by troleandomycin, a specific CYP3A4/5 inhibitor. Rates of E1 metabolism to 16alpha-OHE1 were 1.6-fold higher when E1 was generated in situ from E1S than when E1 was added. Microsomal preparations of cDNA expressed CYP3A4 or 3A5, with NADPH-P450-reductase co-expressed, both metabolized E1 to 16alpha-OHE1, and added cytochrome b5 increased the rates 5.1- and 7.5-fold, respectively. In these systems rates of E1 metabolism to 16alpha-OHE1 were 2.8-fold higher when E1 was generated in situ from E1S than when E1 was added. Kinetic values for E1 metabolism to 16alpha-OHE1 by human liver microsomes and for the expressed CYP3A4 system were Km 154 and 172 microM, respectively, and Vmax 238 pmol/min/nmol total P450 and 1050 pmol/min/nmol CYP3A4, respectively. Thus, formation of the putative carcinogen 16alpha-OHE1 is catalysed by CYP3A4 and 3A5 and stimulated by cytochrome b5. E1S is not a substrate but formation of E1 from E1S in situ stimulates formation of 16alpha-OHE1, possibly because E1S is more water soluble and in situ generation of E1 provides for facilitated exposure of E1 to the P450 substrate binding sites. Blocking of the pathway of E1 to 16alpha-OHE1 could provide a therapeutic approach for diminishing the risk of estrogen dependent breast cancer.