title: Cytochrome b5 augments 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase activity.
abstract: 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.
authors.affiliation: Department of Biochemistry, University of Stellenbosch, Stellenbosch 7602, South Africa.
authors.forename: Pierre
authors.initials: P
authors.lastname: Goosen
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Storbeck
authors.forename: Amanda C
authors.initials: AC
authors.lastname: Swart
authors.forename: Riaan
authors.initials: R
authors.lastname: Conradie
authors.forename: Pieter
authors.initials: P
authors.lastname: Swart
mesh_terms: D015067:17-Hydroxysteroid Dehydrogenases / Q000378:metabolism*; D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D002522:Chlorocebus aethiops; D015786:Cytochromes b5 / Q000378:metabolism*; D017931:DNA Primers; D006041:Goats; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D012756:Sheep; D013045:Species Specificity; D013251:Steroid Isomerases / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D017931:DNA Primers; D015786:Cytochromes b5; D015067:17-Hydroxysteroid Dehydrogenases; C044256:3 (or 17)-beta-hydroxysteroid dehydrogenase; D013251:Steroid Isomerases