Abstract
Measurements of kinetic constants for a purified preparation of human-placenta estradiol dehydrogenase have been made. 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 estrogen-dependent transhydrogenase. The remainder of the observed activity results from a substrate-mediate transhydrogenation catalysed by the estradiol-dehydrogenase activities present in the preparation.