title: Mechanism of androstenedione formation from testosterone and epitestosterone catalyzed by purified cytochrome P-450b.
abstract: A purified rat hepatic monooxygenase system containing cytochrome P-450b oxidizes testosterone to androstenedione and 16 alpha- and 16 beta-hydroxytestosterone at approximately equal rates. The metabolism of epitestosterone by the same system is characterized by a marked stereoselectivity in favor of 16 beta-hydroxylation (4- to 5-fold relative to 16 alpha-hydroxylation), formation of 15 alpha-hydroxyepitestosterone, and a rate of androstenedione formation which is three to five times higher than that observed with testosterone. Apparent Km values for 16 alpha- and 16 beta-hydroxylation and androstenedione formation are 20-30 microM with either substrate. Mass spectral analysis of the androstenedione formed from [16,16-2H2]testosterone and [16,16-2H2] epitestosterone indicates essentially complete retention of deuterium, thereby ruling out a mechanism of androstenedione formation via C-16 hydroxylation followed by loss of water and rearrangement. Mass spectral analysis of the C-16 hydroxylation products from incubations of testosterone or epitestosterone in 18O2 shows essentially complete incorporation of 18O (greater than 95%). Androstenedione formed from testosterone is enriched in 18O only 2-fold (5-8%) over background, while the androstenedione formed from epitestosterone shows 84% enrichment. Kinetic experiments utilizing [17-2H]testosterone and [17-2H]epitestosterone as substrates indicate that cleavage of the C-17 carbon-hydrogen bond is involved in a rate-limiting step in the formation of androstenedione from both substrates. Taken together, our results indicate that androstenedione formation from epitestosterone proceeds exclusively through the gem-diol pathway, while androstenedione formation from testosterone may proceed through a combination of gem-diol and dual hydrogen abstraction pathways.
mesh_terms: D000735:Androstenedione / Q000096:biosynthesis*; D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D000072467:Cytochrome P450 Family 2; D004845:Epitestosterone / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D013058:Mass Spectrometry; D008862:Microsomes, Liver / Q000378:metabolism*; D011865:Radioisotope Dilution Technique; D051381:Rats; D039181:Steroid 16-alpha-Hydroxylase; D013379:Substrate Specificity; D013739:Testosterone / Q000378:metabolism*; D014316:Tritium
chemical_list: D014316:Tritium; D013739:Testosterone; D000735:Androstenedione; D004845:Epitestosterone; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; C450279:CYP2C11 protein, rat; D000072467:Cytochrome P450 Family 2; D039181:Steroid 16-alpha-Hydroxylase