Subcellular localization of 3 beta hydroxysteroid dehydrogenase isomerase in testis of Bufo arenarum H.

3 Beta-hydroxysteroid dehydrogenase 5-ene isomerase (3 beta HSD/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 of Bufo arenarum H., 3 beta HSD/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 3 beta HSD/I were determined by the oxidation of pregnenolone and dehydroepiandrosterone. The preferred substrate of the microsomal 3 beta HSD/I enzyme was dehydroepiandrosterone (K(m) = 0.17 microM and 0.53 microM for dehydroepiandrosterone and pregnenolone, respectively) not only during the breeding season but also in the non-breeding period (K(m) = 0.49 microM and 2.9 microM for dehydroepiandrosterone and pregnenolone, respectively).

Mitochondrial localization of 3 beta-hydroxysteroid dehydrogenase 5-ene isomerase in interrenals of the toad Bufo arenarum H.

The enzymatic activity of 3 beta-hydroxysteroid dehydrogenase 5-ene isomerase (3 beta HSD/I) catalyzes an essential step in the biosynthesis of steroid hormones including progesterone, mineralocorticoids, glucocorticoids, estrogens, and androgens. Its subcellular localization in steroidogenic tissues is usually considered to be mainly microsomal. The present study demonstrates that in the interrenal of Bufo aernarum H., 3 Beta HSD/I activity localizes in mitochondria and micromes. It also shows that the two distinct pathways to aldosterone previously demonstrated for interrenals of B. arenarum H. exhibited differential subcellular localizations, microsomal for the 4-ene route and mitochondrial for the 5-ene route. Kinetic constants of 3 Beta HSD/I were determined for the oxidation of pregnenolone and the recently described 3 Beta-hydroxy analogue of aldosterone (3 Beta AA). The preferred substrate of the mitochondrial 3 Beta HSD/I enzyme was 3 Beta AA (Km = 0.7 microM and 14.0 microM for 3 Beta AA and pregnenolone, respectively). However, the microsomal enzyme has a greater affinity for pregnenolone (Km = 0.8 microM) than for 3 Beta AA (Km = 17.0). Enzymes from both localizations have similar nucleotide (NAD+) requirements, activities being higher in summer. This dual localization opens novel possibilities for the regulation of interrenal functions.