Structure-Function Relationships of Multiple Rat Members of the 3β-Hydroxysteroid Dehydrogenase Family

Abstract

The conversion of 3β-hydroxy-5-ene steroids by the membrane-bound enzyme 3β-hydroxysteroid dehydrogenase/D⁵-D⁴-isomerase, hereafter called 3βHSD, is an essential step in the biosynthesis of all classes of hormonal steroids. We have recently characterized 3 types of cDNAs encoding rat 3βHSD (1, 2). The predicted rat type I and type II 3βHSD-expressed proteins share 94% homology (1), while they share only 80% similarity with the rat type III 3βHSD, which is also a 372-amino acid protein (2) as observed for the human (3–5), macaque (6), and bovine (7) 3βHSD predicted proteins. Using the highly sensitive RNase protection method, we have shown that the type I and type II 3βHSD mRNAs are present in several rat tissues, including the ovary, testis, adrenal, and adipose tissue (1,2), whereas the type III was only found in liver (2). In addition, we have demonstrated by computer analysis that the type I 3βHSD and the type III 3βHSD-encoded proteins possess 2 predicted transmembrane-spanning domains (MSD) (1, 2, 8), whereas type II 3βHSD is devoid of one of the 2 MSDs (1, 8). Moreover, transient expression of rat type I and type II 3βHSD cDNAs in non-steroidogenic cells revealed that these two 42-kd proteins catalyze both the oxidation and isomerization of D^5–3β-hydroxysteroid precursors into D^4–3-ketosteroids, as well as the inter-conversion of 3β-hydroxy and 3-keto-5α-androstane steroids (1, 8).