A purified rat hepatic monooxygenase system containing cytochrome P-450b oxidizes testosterone to androstenedione and 16α- and 16β-hydroxytestosterone at approximately equal rates. The metabolism of epitestosterone by the same system is characterized by a marked stereoselectivity in favor of 16β-hydroxylation (4- to 5-fold relative to 16α-hydroxylation), formation of 15α-hydroxyepitestosterone, and a rate of androstenedione formation which is three to five times higher than that observed with testosterone. Apparent K(m) values for 16α- and 16β-hydroxylation and androstenedione formation are 20-30 μM 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 (> 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.
CITATION STYLE
Wood, A. W., Swinney, D. C., Thomas, P. E., Ryan, D. E., Hall, P. F., Levin, W., & Garland, W. A. (1988). Mechanism of androstenedione formation from testosterone and epitestosterone catalyzed by purified cytochrome P-450b. Journal of Biological Chemistry, 263(33), 17322–17332. https://doi.org/10.1016/s0021-9258(19)77839-9
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