title: Kinetic evidence for separate 3 beta-hydroxysteroid dehydrogenase-isomerases in androgen and 16-androstene biosynthetic pathways in the pig testis.
abstract: The microsomal fraction from the testes of immature pigs (less than 1 week old) contains 3 beta-hydroxysteroid dehydrogenase-isomerase (3 beta-HSD-isomerase) activities that convert dehydroepiandrosterone (DHA) to 4-androstenedione and 5,16-androstadien-3 beta-ol (andien-beta) to 4,16-androstadien-3-one (dienone). These reactions are necessary for the biosynthesis of hormonally and pheromonally active steroids. Kinetic analyses of these activities were done to determine whether they are catalysed by a single enzyme or if there is any interaction between the substrates and products of one reaction on the activity of the other enzyme. Kinetic parameters were determined and the affinities for steroid substrate were similar (7-9 mumol/l) but the Vmaxapp value for the conversion of andien-beta to dienone was 10-fold that of the DHA to 4-androstenedione reaction. In analyses of the conversion of DHA to 4-androstenedione, neither andien-beta nor dienone inhibited the reaction and especially, no effect on the Kmapp for DHA was observed which would have indicated competition between DHA and andien-beta for the same active site (Kiapp from slope and intercept replots were between 3 and 80 times the values of the kinetic constants). Similarly, DHA and 4-androstenedione had minor or negligible effects on the conversion of andien-beta to dienone (Kiapp from slope replots were the same as the Kmapp but the Kiapp from the intercept replot was 12 to 25% of the Vmaxapp). It is concluded that substrate specific 3 beta-HSD-isomerases for andien-beta and DHA exist in the immature pig testis and there is little, if any interaction between these enzymes.
authors.affiliation: Département d'Anatomie et Physiologie Animale, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, Québec, Canada.
authors.forename: P
authors.initials: P
authors.lastname: Hébert
authors.forename: G M
authors.initials: GM
authors.lastname: Cooke
mesh_terms: D000728:Androgens / Q000378:metabolism*; D000730:Androstadienes / Q000494:pharmacology; D000735:Androstenedione / Q000378:metabolism; D000736:Androstenes / Q000378:metabolism*; D000818:Animals; D003687:Dehydroepiandrosterone / Q000378:metabolism; D007527:Isoenzymes; D007700:Kinetics; D008297:Male; D008861:Microsomes / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D011284:Pregnenolone / Q000378:metabolism; D011375:Progesterone Reductase / Q000378:metabolism*; D013251:Steroid Isomerases / Q000378:metabolism*; D013552:Swine; D013737:Testis / Q000378:metabolism*
chemical_list: C021425:3 beta-hydroxysteroid oxidoreductase-delta(5) 3-ketosteroid isomerase; D000728:Androgens; D000730:Androstadienes; D000736:Androstenes; D007527:Isoenzymes; D009097:Multienzyme Complexes; C042468:16-androstene; D000735:Androstenedione; D003687:Dehydroepiandrosterone; D011284:Pregnenolone; D011375:Progesterone Reductase; D013251:Steroid Isomerases