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17 -Hydroxysteroid dehydrogenase of rat skin. Substrate specificity and inhibitors.

Abstract A 17β-hydroxysteroid dehydrogenase activity has been demonstrated in the microsomal preparations made from the skin of newborn rats. The enzyme prefers NAD to NADP as the coenzyme and utilizes 17β-estradiol as well as testosterone as substrates. The apparent Km values estimated for 17β-estradiol are 4.1 x 10-6 m, for estrone 1.0 x 10-5 m, and for testosterone 4.0 x 10-6 m. The enzyme can be stabilized by 10 to 20% by volume of glycerol when stored at -10° in 0.1 m phosphate buffer, pH 7.5. 17β-Estradiol and testosterone have been shown to inhibit each other in the enzymic dehydrogenation at C-17, and kinetic data are compatible with the explanation that these steroids probably compete with each other on the same enzyme site. A number of other steroids have been tested for their inhibitory effects on the conversion of [4-14C]17β-estradiol into [4-14C]estrone by the enzyme preparation. Of these, 4-androstene-3β,17β-diol, 5α-dihydrotestosterone, and 19-nortestosterone are effective inhibitors, whereas epitestosterone and 17α-hydroxy-progesterone are poor inhibitors, and androstene-3,17-dione, cortisone, and cortisol are not inhibitors. A common structural feature of the effective inhibitors is the presence of a 17β-hydroxyl group; the structure in ring A and the presence of the 19-methyl group do not seem to be critical for the inhibitory effect. The structural features of these inhibitors are quite different from those for steroid inhibitors of the 5α-reductase of testosterone and progesterone in the microsomes of human skin (Voigt, W., et al. (1970) J. Biol. Chem., 245, 5594), implying that the topographical features at the binding sites for steroids on the surface of the two enzymes are quite different.