Hydroxysteroid dehydrogenases catalyze the interconversion of hydroxyl and carboxyl groups of steroids. P.testosteroni derived hydroxysteroid dehydrogenases are of two types: 3-α-hydroxysteroid dehydrogenase (α enzyme) and 3-β-hydroxysteroid dehydrogenase (β enzyme). The α enzyme has a molecular weight of 47,000 daltons. The α enzyme oxidizes only 3-α-hydroxysteroids of the C19, C21 and C24 series. It is inhibited by heavy metals and sulfhydryl-binding agents. The β enzyme catalyzes the oxidation of 3-β-hydroxysteroids of the C19 and C21 series, 17-β-hydroxysteroids of the C18, C19 and C21 series, as well as certain 16-β-hydroxysteroids. It is inhibited by heavy metals and reducing agents. The oxidation of testosterone is inhibited by 3,17-α-estradiol and other 1,3,5-estradiene derivatives. Worthington supplies two preparations: one from the regular P. testosteroni (ATCC 11966) culture which produces both the α and the β enzymes, and a second from a mutant strain which produces almost exclusively the α enzyme. By using both, the β-hydroxysteroid can be determined by difference.

Technical Note: STDHP and STDH contain both α and β activities. STDHMP, however, contains only α activity.

Unit Definition: One Unit reduces one micromole of NAD per minute at 25°C, pH 9.0 using androsterone or testosterone as substrate.