Steroid biosynthesis by ovarian follicles of Xenopus laevis in vitro during oogenesis

In vitro steroid metabolism was investigated in Xenopus laevis vitellogenic follicles (0.8 mm ? diameter ? 1.0 mm) and full-grown follicles (diameter ? 1.2 mm); in both classes of follicles pregnenolone was transformed to progesterone and progesterone itself was further metabolized to 4-en-3-ketosteroids. The 3β-hydroxysteroid dehydrogenase activity was restricted to follicular cells. Xenopus laevis follicles catalyze the conversion of androstenedione and testosterone to estradiol-17β and estrone, respectively. Generally 10 to 50 times more estrogens were found to be associated with follicular envelopes. Ovarian follicles also metabolized estrone to estradiol-17β; there was only minimal conversion of estradiol to estrone. In vitro purified Acipenser gonadotropic hormone induced a 60% inhibition of the conversion of androstenedione to estrogens.