Initiation of human parturition—VII partial characterization of progesterone metabolizing enzymes of human amnion and chorion laeve

Abstract

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 5 α-reductase, 3 β- and 20 α-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 20 α-hydroxysteroid oxidoreductase enzymes utilized either NADH or NADPH as the source of reducing equivalents ( K m = 0.2–0.3 mM) while amnion tissue 5 α-reductase activity required NADPH as the obligatory cofactor ( K m = 0.1mM). The apparent pH optima for the 20 α-hydroxysteroid oxidoreductase present in amnion and chorion laeve tissues were similar, while the apparent pH optima for the 5 α-reductase enzyme was slightly higher in chorion laeve than in amnion tissue. In 15 min incubations, the apparent maximal activity of amnion 5 α-reductase was found at a temperature of 49°C. whereas chorion laeve tissue 3 β- and 20 α-hydroxysteroid oxidoreductase had maximal activity at 37°C. From progesterone saturation kinetics an apparent K m of 0.20–0.25 mM for amnion and chorion laeve 20 α-hydroxysteroid oxidoreductase, and an apparent K m of 0.4–1.2 μM for amnion and chorion laeve tissue 5 α-reductase activity was computed. Amnion 5 α-reductase activity appeared to be particulate in nature.