Kinetics of Nicotinamide Nucleotide Transhydrogenation by Human Placenta.

1. Measurements of kinetic constants for a purified preparation of human-placenta oestradiol dehydrogenase have been made. 2. These constants have been compared with similar measurements made on crude ammonium sulphate precipitates of human-placenta homogenates. The comparison indicates that nearly half of the observed nicotinamide nucleotide-transhydrogenation activity in the crude preparations is due to a specific oestrogen-dependent transhydrogenase. 3. The remainder of the observed activity results from a substrate-mediated transhydrogenation catalysed by the oestradiol-dehydrogenase activities present in the preparation.