The 17, 20-lyase activity of cytochrome p450c17 from human fetal testis favors the delta5 steroidogenic pathway.

Cytochrome P450c17 catalyzes both 17alpha-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 delta(5) pathway) or via conversion of progesterone to androstenedione (the delta(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 delta(5) pathway. Because the 17, 20-lyase activities in both the delta(4) and delta(5) pathways for testicular P450c17 have not been directly compared, however, it is not known if the delta(5) pathway dominates in the human testis. To resolve this issue, we assayed the conversion of 17alpha-hydroxypregnenolone to dehydroepiandrosterone (delta(5) 17, 20-lyase activity) and of 17alpha-hydroxyprogesterone to androstenedione (delta(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 microM and 0.73 pmol.min(-1). microg(-1) for delta(5) 17, 20-lyase activity and of 3.5 microM and 0.23 pmol.min(-1). microg(-1) for delta(4) 17, 20-lyase activity. Catalytic efficiencies, expressed as the ratio V(max)/K(m), were 0.73 and 0.066 for the delta(5) and delta(4) reactions, respectively, indicating 11-fold higher preference for the delta(5) pathway. We conclude that the majority of testosterone biosynthesis in the human testis proceeds through the conversion of pregnenolone to dehydroepiandrosterone via the delta(5) pathway.

Beurtheilung der Mittel, welche man anwendet, die ausgestorbene Hebräische Sprache zu verstehen

Christian Benedikt Michaelis

Über die Verbesserung des Judeneids : ein auf Befehl d. Königl.-Kuhrfürstl. Justizkanzley zu Hannover verf. Gutachten

von Moses Philipson

Die Rechtsverhältnisse der Juden in Preußen seit dem Beginne des 19. Jahrhunderts : Gesetze, Erlasse, Verordnungen, Entscheidungen

hrsg. von Alfred Michaelis

Exercitatio academica de lingua Aramaea

Altdorf , Univ., Diss., 1739

De sacra poesi Hebræorum prælectiones academicæ Oxonii habitæ : Subjicitur metricae Harianae brevis confutatio et oratio Crewiana

Robert Lowth. Notas et Epimetra adjecit Ioannes David Michaelis ...

Anfangs-Gründe der hebräischen Accentuation : nebst einer kurtzen Abhandlung von dem Alterthum der Accente und hebräischen Puncte überhaupt ...

aufgesetzet von Johann David Michaelis u. mit e. Vorrede begleitet von Christian Benedict Michaelis