title: Cytochrome b5-mediated redox cycling of estrogen.
abstract: Previously, we have demonstrated microsomal cytochrome P450-catalyzed redox cycling of estrogens. In this study, we investigated the role of cytochrome b5 in redox cycling in order to obtain a full understanding of enzymatic contributions to redox reactions of estrogens. Pure cytochrome P450c and hydrogen peroxide or cumene hydroperoxide oxidized diethylstilbestrol (DES) to diethylstilbestrol-4',4"-quinone (DES Q). This oxidation by H2O2 was doubled by addition of cytochrome b5 to cytochrome P450c (molar ratio of 1:4), but did not proceed with cytochrome b5 alone. The stimulation by cytochrome b5 of the cytochrome P450c-catalyzed oxidation of DES to DES Q occurred via modulation of the Vmax of cytochrome P450c rather than of the Km. DES Q was reduced to DES by purified cytochrome b5 and NADH-dependent cytochrome b5 reductase. Pretreatment of microsomes with an antibody to cytochrome b5 reductase inhibited microsomal NADH-dependent reduction of DES Q to DES by 55%. Cytochrome b5 likely participates in the oxidation of DES to DES Q by interacting with cytochrome P450c and in the reduction of DES Q to DES by interacting with cytochrome b5 reductase. Thus, the study demonstrates that cytochrome b5 plays an active role in biological oxidation and reduction reactions.
authors.affiliation: Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston 77550.
authors.forename: D
authors.initials: D
authors.lastname: Roy
authors.forename: H W
authors.initials: HW
authors.lastname: Strobel
authors.forename: J G
authors.initials: JG
authors.lastname: Liehr
mesh_terms: D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003579:Cytochrome Reductases / Q000378:metabolism; D042966:Cytochrome-B(5) Reductase; D015786:Cytochromes b5 / Q000378:metabolism*; D004054:Diethylstilbestrol / Q000031:analogs & derivatives / Q000378:metabolism*; D006861:Hydrogen Peroxide / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D010084:Oxidation-Reduction; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: C039998:diethylstilbestrol quinone; D004054:Diethylstilbestrol; D015786:Cytochromes b5; D003577:Cytochrome P-450 Enzyme System; D006861:Hydrogen Peroxide; D003579:Cytochrome Reductases; D042966:Cytochrome-B(5) Reductase