The kinetics of the metabolism of progesterone by the reducing enzymes present in the human fetal membranes was evaluated utilizing tissue samples obtained from early and late pregnancies. The specific activities of 5α-reductase, 3β- and 20α-hydroxysteroid oxidoreductase were greater in amnion and chorion laeve tissue obtained from a 16 weeks pregnancy than in those obtained from a 36 1/2 week and from a term pregnancy. Amnion and chorion laeve tissue 20α-hydroxysteroid oxidoreductase enzymes utilized either NADH or NADPH as the source of reducing equivalents (Km = 0.2-0.3 mM) while amnion tissue 5α-reductase activity required NADPH as the obligatory cofactor (Km = 0.1mM). The apparent pH optima for the 20α-hydroxysteroid oxidoreductase present in amnion and chorion laeve tissues were similar, while the apparent pH optima for the 5α-reductase enzyme was slightly higher in chorion laeve than in amnion tissue. In 15 min incubations, the apparent maximal activity of amnion 5α-reductase was found at a temperature of 49°C. whereas chorion laeve tissue 3β- and 20α-hydroxysteroid oxidoreductase had maximal activity at 37°C. From progesterone saturation kinetics an apparent Km of 0.20-0.25 mM for amnion and chorion laeve 20α-hydroxysteroid oxidoreductase, and an apparent Km of 0.4-1.2 μM for amnion and chorion laeve tissue 5α-reductase activity was computed. Amnion 5α-reductase activity appeared to be particulate in nature.
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