Kinetic studies on the enzymes involved in estrogen biosynthesis and evidence for existence of a single 5-Ene-3beta-hydroxysteroid dehydrogenase complex in the ovary of the freshwater catfish, Clarias batrachus.

Substrate velocity kinetics and other characteristics of two key enzymes involved in estrogen biosynthesis in the ovary of the catfish, Clarias batrachus, were studied. Enzyme reactions were measured spectrophotometrically during the initial phase of vitellogenesis. 5-Ene-3beta-hydroxysteroid dehydrogenase (5-ene-3beta-HSD) was found to have a different Michaelis-Menten constant (Km) for each of three 5-ene-3beta-hydroxysteroids, (pregnenolone (P5), 17alpha-hydroxypregnenolone (17alpha-P5), and dehydroepiandrosterone (DHA) but the constant for 17beta-hydroxysteroid dehydrogenase (17beta-HSD) was similar for both the substrates androstendione (A-dione) and estradiol-17beta (E2). 5-Ene-3beta-HSD required exclusively NAD as cofactor for the conversion of P5, 17alpha-P5, and DHA. A-dione was converted into testosterone (T) by a NADP-dependent 17beta-HSD, whereas there was an obligatory requirement of NAD for the conversion of E2 to estrone (E1). P5 was utilized more efficiently as substrate by 5-ene-3beta-HSD than DHA, as indicated by its lower Km/Vmax ratio. The rate of the combined substrate reaction was less than the sum of the rate of reactions measured separately for each of the three sets of substrate pairs. These results indicate that a single enzyme is responsible for the oxidation of three types of 5-ene-3beta-hydroxysteroids studied.