Subcellular Localization of 3β Hydroxysteroid Dehydrogenase Isomerase in Testis ofBufo arenarumH

3β-hydroxysteroid dehydrogenase 5-ene isomerase (3βHSD/I) catalyzes an essential step in the biosynthesis of steroid hormones and is usually considered to be mainly microsomal, although there is a dual distribution of the enzyme in toad interrenals. The present study demonstrates that in the testicular tissue, as in interrenals ofBufo arenarumH., 3βHSD/I is both mitochondrial and microsomal. The conversion of dehydroepiandrosterone to androstenedione takes place only in microsomes while pregnenolone is converted to progesterone in both microsomes and mitochondria. Kinetic constants of 3βHSD/I were determined by the oxidation of pregnenolone and dehydroepiandrosterone. The preferred substrate of the microsomal 3βHSD/I enzyme was dehydroepiandrosterone (Km=0.17 μMand 0.53 μMfor dehydroepiandrosterone and pregnenolone, respectively) not only during the breeding season but also in the non breeding period (Km=0.49 μMand 2.9 μMfor dehydroepiandrosterone and pregnenolone, respectively).