3 beta-Hydroxysteroid dehydrogenase/delta 5-4 isomerase activity in the rhesus monkey placenta and fetal adrenal, testis and ovary during late gestation.

3 beta-Hydroxysteroid dehydrogenase/delta 5-4 isomerase (3 beta-HSDH) was measured in the rhesus monkey (Macaca mulatta) placenta, fetal adrenal (whole organ minus medulla), testis and ovary during late gestation (Days 145-162). Activities were evaluated from the conversion of [3H]-pregnenolone to [3H]progesterone. The maximum enzyme velocity (Vm) in adrenal microsomes (100,000 g pellet) was significantly higher (146 nmoles progesterone/h X mg-1 protein) than in microsomes from the other tissues. Testicular Vm was greater than either ovarian or placental Vm which were not different from one another (11.5 versus 1.9, 1.2 nmoles progesterone/h X mg-1 protein, respectively). Apparent Michaelis-Menten constants in the adrenal, placenta, testis and ovary averaged 1.8, 2.5, 0.27 and 0.16 microM, respectively. In some cases, substrate inhibition was noted. Estimated dissociation constants for pregnenolone were 2.3 microM (adrenal), 2.1 microM (placenta), 0.74 microM (testis) and 0.13 microM (ovary). 3 beta-HSDH was less active in a crude mitochondrial preparation from the fetal adrenal (10,000 g pellet) than in microsomes, whereas activity in the placenta and testis appeared to be equally distributed between mitochondria and microsomes. Rate measurements were consistent with the apparent potentials of these organs to synthesize their characteristic hormones. Thus, 3 beta-HSDH activity may be an important rate determining step in hormone synthesis. The importance of substrate inhibition in progesterone formation remains to be assessed.