The mouse 3β-hydroxysteroid dehydrogenase multigene family includes two functionally distinct groups of proteins

Abstract

The enzyme 3β-hydroxysteroid dehydrogenase (3βHSD) 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 3βHSD cDNAs (forms I, II, III, and IV). Enzymatic characterization of three of these isoforms demonstrated that mouse 3βHSD I and III function as dehydrogenase/isomerases, but 3βHSD IV functions exclusively as a 3-ketosteroid reductase. We now report the isolation and characterization of an additional distinct mouse 3βHSD cDNA, 3βHSD V, which is expressed in the liver of male mice beginning in late puberty. Similar to 3βHSD IV, 3βHSD V functions exclusively as a 3-ketosteroid reductase converting an active androgen, dihydrotestosterone (DHT), into an inactive androgen, 5α-androstane-3β,17β-diol. Expressed 3βHSD V, however, exhibits a considerably lower apparent Michaelis-Menten constant (K(m)) value for DHT than 3βHSD IV (0.47 μM vs. 2.2 μM, respectively). The complete predicted amino acid sequence of 3βHSD II is also reported. The predicted amino acid sequence of mouse 3βHSD V reveals that this new form is more closely related to the 3-ketosteroid reductases, mouse 3βHSD 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 3βHSD IV and V and rat III, not only differ from all of the other characterized 3βHSD 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 3βHSD multigene family, as well as the report of the complete coding region for 3βHSD II, demonstrates that members of this highly homologous gene family fall into two functionally distinct groups of enzymes. One group, as represented by 3βHSD 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 3βHSD IV and V, functions as 3-ketosteroid reductases and is most likely involved in the inactivation of active steroid hormones such as DHT.