5α-Reductase, the enzyme responsible for the conversion of testosterone to dihydrotestosterone in androgen-dependent target tissues, is a membrane-bound enzyme. In the ventral prostate of the rat its activity is found both in the nuclear membrane and in the membranes of the endoplasmic reticulum. Treatment of these membranes with digitonin (2 mg/mg of protein) plus 3 M KCl extracted the enzyme in a form that was retained on Bio-Gel A-1.5m column chromatography and failed to sediment during centrifugation at 100,000g for 1 hr. The activity in these extracts could be stabilized for as long as 4 days in the cold by either glycerol or NADPH. The pH optimum and apparent Km of the extracted enzymes were similar to those of the 5α-reductase in intact nuclei and microsomes. The 5α-reductase in the nuclear and microsomal extracts had an apparent mol wt of the order of 250,000-350,000 as estimated by gel filtration and a sedimentation coefficient of 13.5-15 S as estimated by density gradient centrifugation. The NADPH-stabilized enzyme was purified 90-fold.
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