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The 17, 20-Lyase Activity of Cytochrome P450c17 from Human Fetal Testis Favors the ⁵ Steroidogenic Pathway

Department of Pediatrics and Metabolic Research Unit (C.E.F., W.L.M.), University of California, San Francisco, California 94143-0978; and Division of Endocrinology and Metabolism, Department of Internal Medicine (R.J.A.), University of Texas Southwestern Medical Center, Dallas, Texas 75390-8857

Address all correspondence and requests for reprints to: Richard J. Auchus, M.D., Ph.D., Division of Endocrinology and Metabolism, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-8857. E-mail: richard.auchus{at}utsouthwestern.edu.

Cytochrome P450c17 catalyzes both 17-hydroxylation and 17,20-lyase conversion of 21-carbon steroids to 19-carbon precursors of sex steroids. P450c17 can mediate testosterone biosynthesis via the conversion of pregnenolone to dehydroepiandrosterone (the ⁵ pathway) or via conversion of progesterone to androstenedione (the ⁴ pathway). In many species, the 17, 20-lyase activity of P450c17 for one pathway dominates, reflecting the preferred steroidogenic pathway of that species. All studies of recombinant human P450c17 and of human adrenal microsomes have found high 17, 20-lyase activity only in the ⁵ pathway. Because the 17, 20-lyase activities in both the ⁴ and ⁵ pathways for testicular P450c17 have not been directly compared, however, it is not known if the ⁵ pathway dominates in the human testis. To resolve this issue, we assayed the conversion of 17-hydroxypregnenolone to dehydroepiandrosterone (⁵ 17, 20-lyase activity) and of 17-hydroxyprogesterone to androstenedione (⁴ 17, 20-lyase activity) by human fetal testicular microsomes. We obtained apparent Michaelis constant (K_m) and maximum velocity (V_max) values of 1.0 µM and 0.73 pmol·min^-1·µg^-1 for ⁵ 17, 20-lyase activity and of 3.5 µM and 0.23 pmol·min^-1·µg^-1 for ⁴ 17, 20-lyase activity. Catalytic efficiencies, expressed as the ratio V_max/K_m, were 0.73 and 0.066 for the ⁵ and ⁴ reactions, respectively, indicating 11-fold higher preference for the ⁵ pathway. We conclude that the majority of testosterone biosynthesis in the human testis proceeds through the conversion of pregnenolone to dehydroepiandrosterone via the ⁵ pathway.

This work was supported by a grant from Swiss Foundation for Medical-Biological Grants (to C.E.F.) and by NIH Grants HD41958 (to W.L.M.) and K08DK02387 (to R.J.A.).

Abbreviations: ⁴A, Androstenedione; DHEA, dehydroepiandrosterone; 3ß-HSD, 3ß-hydroxysteroid dehydrogenase-^5/4-isomerase; K_m, apparent Michaelis constant; 17OHP, 17-hydroxyprogesterone; 17OHPreg, 17-hydroxypregnenolone; V_max, maximum velocity.

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