THE MOUSE 3-BETA-HYDROXYSTEROID DEHYDROGENASE MULTIGENE FAMILY INCLUDES 2 FUNCTIONALLY DISTINCT GROUPS OF PROTEINS

The enzyme 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) plays an essential role in the biosynthesis of all steroid hormones. We previously reported the isolation, characterization, and tissue-specific expression of four distinct but highly homologous 3 beta HSD cDNAs (forms I, II, III, and IV). Enzymatic characterization of three of these isoforms demonstrated that mouse 3 beta HSD I and III function as dehydrogenase/isomerases, but 3 beta HSD IV functions exclusively as a 3-ketosteroid reductase. We now report the isolation and characterization of an additional distinct mouse 3 beta HSD cDNA, 3 beta HSD V, which is expressed in the liver of male mice beginning in late puberty. Similar to 3 beta HSD IV, 3 beta HSD V functions exclusively as a 3-ketosteroid reductase converting an active androgen, dihydrotestosterone (DHT), into an inactive androgen, 5 alpha-androstane-3 beta,17 beta-diol. Expressed 3 beta HSD V, however, exhibits a considerably lower apparent Michaelis-Menten constant (K-m) value for DHT than 3 beta HSD IV (0.47 mu M vs. 2.2 mu M, respectively). The complete predicted amino acid sequence of 3 beta HSD II is also reported. The predicted amino acid sequence of mouse 3 beta HSD V reveals that this new form is more closely related to the 3-ketosteroid reductases, mouse 3 beta HSD IV and rat III (93 and 84% identity, respectively), than to the other rodent isoforms that share less than 75% identity. The 3-ketosteroid reductases, mouse 3 beta HSD IV and V and rat III, not only differ from all of the other characterized 3 beta HSD isoforms in their substrate specificity but also in their cofactor specificity utilizing NADPH rather than NADH. The characterization of this fifth member of the mouse 3 beta HSD multigene family, as well as the report of the complete coding region for 3 beta HSD II, demonstrates that members of this highly homologous gene family fall into two functionally distinct groups of enzymes. One group, as represented by 3 beta HSD I and III (and most likely II), functions as dehydrogenase/isomerases and is essential for the biosynthesis of active steroid hormones. The other group, represented by 3 beta HSD IV and V, functions as 3-ketosteroid reductases and is most likely involved in the inactivation of active steroid hormones such as DHT.