3.beta.-Hydroxy-5.beta.-steroid dehydrogenase activity of human liver alcohol dehydrogenase is specific to .gamma.-subunits

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Abstract
Human liver alcohol dehydrogenase [alcohol:NAD+oxidoreductase, EC 1.1.1.1. (ADH)] catalyzes the stereospecific oxidation of different 3.beta.-hydroxy-5.beta.-steroids with ranges of Km from 46 to 320 .mu.M and values of kcat from 7.0 to 72 min-1, pH 8.5. Only the class I isozymes containing .gamma.-subunits, .gamma.1.gamma.1, .alpha..gamma.1, .beta.1.gamma.1, .gamma.2.gamma.2, .alpha..gamma.2, and .beta.1.gamma.2, catalyze oxidation of these steroids with kcat/Km ratios 4-10-fold greater than those for ethanol. In marked contrast, class I .alpha..alpha., .alpha..beta.1, and .beta.1.beta.1, class II, and class III isozymes do not oxidize 3.beta.-hydroxy-5.beta.-steroids though they readily oxidize ethanol. 1,10-Phenanthroline and 4-methylpyrazole competitively inhibit both alcohol dehydrogenase catalyzed ethanol and 3.beta.-hydroxy-5.beta.-steroid oxidation demonstrating that the catalysis of both types of substrates occurs at the same active site. The .gamma.-subunit-catalyzed oxidation of 3.beta.-hydroxy-5.beta.-steroids is the most specific catalytic function described thus far for any human liver alcohol dehydrogenase isozyme: there is no other isozyme that catalyzes this reaction. Testosterone, an allosteric inhibitor of ethanol oxidation specific for .gamma.-subunit-containing human liver ADH isozymes [Mardh, G., Falchuk, K. H., Auld, D. S., and Vallee, B. L. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 2836-2840], also noncompetitively inhibits .gamma.-subunit-catalyzed sterol oxidation. Thus, the .gamma.-isozymes display remarkable stereospecific interactions with certain steroids: 5.beta.-steroids (cis A/B ring fusion) are substrates whereas 5.alpha.- and .DELTA.4-steroids are allosteric inhibitors suggesting that the .gamma.-ADH isozymes are essential in the metabolism of these steroids.