Abstract
Summary The activities of the 3-β-hydroxysteroid dehydrogenase (3-β-hydroxysteroid: NAD(P) oxidoreductase, EC 1.1.1.51) and the Δ5-3-oxosteroid isomerases (3-oxosteroid Δ4-Δ5-isomerases EC 5.3.3.1), in the soluble, mitochondrial and microsomal fractions of human term placenta were determined. Both enzymic activities were found in all subcellular fractions. The greatest activities were in the particulate fractions. The optimum pH for the 3-β-hydroxysteroid dehydrogenase was in the vicinity of pH 10–10.5. The results of this study suggest that the 3-β-hydroxysteroid dehydrogenase and the isomerase might be separate enzymes and that there might be distinct isomerases for different substrates in the particulate fractions.
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