The molecular weight of the 17β-hydroxysteroid dehydrogenase of human placenta, estimated by gel filtration and reduced nicotinamide-adenine dinucleotide phosphate titration, is 62,000-65,000. Fluorescence measurements indicate that the Michaelis constants of nicotinamide-adenine dinucleotide phosphate and reduced nicotinamide-adenine dinucleotide phosphate are 0.89 and 0.83 μM, respectively, and that the dissociation constants of these pyridine nucleotides are 51 and 46 mμM, respectively. When the enzyme is cooled in the presence of 1% glycerol, it loses its activity rapidly and this loss is paralleled by a decrease in the ability of the enzyme to bind reduced nicotinamide-adenine dinucleotide phosphate. Because of the rather broad substrate specificity of this enzyme, various compounds were examined for their effect on 17β-estradiol oxidation. Structural analogs of 17β-estradiol (17-desoxy-estradiol and 17α-estradiol), as well as certain other steroid hormones (androst-4-en-3,17-dione and progesterone) and nonsteroidal compounds (2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethane and diethylstilbestrol), were found to be competitive inhibitors of 17β-estradiol oxidation, while corticosteroids did not inhibit it. Binding studies by ultracentrifugation indicate a single binding site on the enzyme for 17β-estradiol and estrone. When examined by gel filtration the binding of 17β-estradiol to the enzyme was found to be reduced by diethylstilbestrol as well as by a synthetic estrogen antagonist, 1-(2-[p-(3,4-dihydro-6-methoxy- 2-phenyl-1-naphthyl)phenoxy]ethyl)pyrrolidine. Conditions which inactivate the enzyme (cold sodium dodecyl sulfate, p-hydroxymercuribenzoate, and guanidine hydrochloride) also reduce its estrogen binding capacity.
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