| Summary: | The 3-Β-hydroxysteroid dehydrogenase of rat liver which catalyzes the conversion of 5α-cholestan-3-one to 5α-cholestan-3Β-ol is localized mainly in the microsomal fraction. The enzyme required NADPH as hydrogen donor and differed from the known 3-Β-hydroxysteroid dehydrogenases of the C19 series in being inactive in the presence of NADH. The microsomal preparations did not reduce the 3-keto groups of cholest-4-en-3-one, cholest-5-en-3-one, or 5Β-cholestan-3-one to the corresponding 3Β-hydroxy compounds. The conversion of 5α-cholestan-3-one to 5α-cholestan-3Β-ol was only slightly inhibited by the reaction product or by other monohydroxy steroids, but a strong inhibitory effect was noted with cholest-5-en-3-one, 5α-cholestane-3Β,7α-diol and 5α-cholestan-7-on-3Β-ol.The microsomes, but not high speed supernatant solution, catalyzed the reverse of the cholestanone reductase reaction, namely the conversion of 5α-cholestan-3Β-ol to 5α-cholestan-3-one in the presence of oxygen and an NADP-generating system.The action of the microsomal preparations upon 5α-cholestan-3-one produced 5α-cholestan-3α-ol in addition to the 3Β-epimer. The 3-α-hydroxysteroid dehydrogenase involved functioned with either NADH or NADPH as hydrogen donor. The ratio of 5α-cholestan-3Β-ol to 5α-cholestan-3α-ol formed from 5α-cholestan-3-one was approximately 10:1 and was independent of the sex of the animal from which the microsomes were prepared.
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