Cytochrome b5 augments 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase activity.

During adrenal steroidogenesis the competition between 3 beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3 beta HSD) and cytochrome P450 17 alpha-hydroxylase/17,20 lyase (CYP17A1) for Delta(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 3 beta HSD activity was investigated. In COS-1 cells, Cyt-b(5) was shown to significantly increase the activity of both caprine and ovine 3 beta HSD 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 3 beta HSD for all its respective substrates. In addition, the activity of ovine 3 beta HSD 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 3 beta HSD activity and represents the first documentation of such augmentation in any species. (C) 2011 Elsevier Ltd. All rights reserved.