Cytochrome b 5 augments 3β-hydroxysteroid dehydrogenase/Δ 5-Δ 4 isomerase activity
During adrenal steroidogenesis the competition between 3β- hydroxysteroid dehydrogenase/Δ 5-Δ 4 isomerase (3βHSD) and cytochrome P450 17α-hydroxylase/17,20 lyase (CYP17A1) for Δ 5 steroid intermediates greatly influences steroidogenic output. Cytochrome-b 5 (Cyt-b 5), a small electron transfer hemoprotein, known to augment the lyase activity of CYP17A1, has been shown to alter the steroidogenic outcome of this competition. In this study, the influence of Cyt-b 5 on 3βHSD activity was investigated. In COS-1 cells, Cyt-b 5 was shown to significantly increase the activity of both caprine and ovine 3βHSD towards pregnenolone, 17-OH pregnenolone and dehydroepiandrosterone in a substrate and species specific manner. Furthermore, kinetic studies revealed Cyt-b 5 to have no influence on the K m values while significantly increasing the V max values of ovine 3βHSD for all its respective substrates. In addition, the activity of ovine 3βHSD in microsomal preparations was significantly influenced by the addition of either purified Cyt-b 5 or anti-Cyt-b 5 IgG. The results presented in this study indicate that Cyt-b 5 augments 3βHSD activity and represents the first documentation of such augmentation in any species. © 2010 Elsevier Ltd. All rights reserved.