5α-Reductase activity in human placenta

The concentration of 5α-pregnane-3,20-dione in peripheral blood of pregnant women is higher than that found in blood obtained from nonpregnant women throughout the luteal phase of the ovulatory cycle. The in vitro synthesis of 5α-reduced pregnanes from [³H]progesterone by placental tissue was investigated using tissue minces and homogenates in the presence of added nicotinamide-adenine dinucleotide phosphate. The metabolites, [³H]5α-pregnane-3,20-dione and [³H]3β-hydroxy-5α-pregnan-20-one, were isolated and characterized employing a combination of chromatographic techniques, derivative formation, and crystallization with authentic [¹⁴C]5α-pregnane-3,20-dione and [¹⁴C]3β-hydroxy-5α-pregnan-20-one to constant ³H:¹⁴C ratios. In short-term incubations with placenta homogenates, the apparent pH optimum for the synthesis of 5α-pregnane-3,20-dione was 8.8. There was no evidence for the formation of 5β-reduced pregnanes from [³H]progesterone by placental tissue. The in vitro metabolism of [³H]progesterone by minces of five term placentas, during 1 hour incubations at pH 7.4, was studied. The rate of formation of 5α-pregnane-3,20-dione varied from 109 to 197 pmoles/gm placental tissue/hr.