Androgenic 17 beta-hydroxysteroid dehydrogenase activity of expressed rat type I 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase

Transient expression in nonsteroidogenic mammalian cells of the rat wild type I and type II 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4-isomerase (3 beta-HSD) cDNAs shows that the encoded proteins, in addition to being able to catalyze the oxidation and isomerization of delta 5-3 beta-hydroxysteroid precursors into the corresponding delta 4-3-ketosteroids, interconvert 5 alpha-dihydrotestosterone (DHT) and 5 alpha-androstane-3 beta,17 beta-diol (3 beta-diol). When homogenate from cells transfected with a plasmid vector containing type I 3 beta-HSD is incubated in the presence of DHT using NAD+ as cofactor, a somewhat unexpected metabolite is formed, namely 5 alpha-androstanedione (A-dione), thus indicating an intrinsic androgenic 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) activity of this 3 beta-HSD isoform. Although the relative Vmax of 17 beta-HSD activity is 14.9-fold lower than that of 3 beta-HSD activity, the Km value for the 17 beta-HSD activity of type I 3 beta-HSD is 7.97 microM, a value which is in the same range as the conversion of DHT into 3 beta-diol which shows a Km value of 4.02 microM. Interestingly, this 17 beta-HSD activity is highly predominant in unbroken cells in culture, thus supporting the physiological relevance of this "secondary" activity. Such 17 beta-HSD activity is inhibited by the classical substrates of 3 beta-HSD, namely pregnenolone (PREG), dehydroepiandrosterone (DHEA), delta 5-androstene-3 beta,17 beta-diol (delta 5-diol), 5 alpha-androstane-3 beta,17 beta-diol (3 beta-diol) and DHT, with IC50 values of 2.7, 1.0, 3.2, 6.2, and 6.3 microM, respectively. Although dual enzymatic activities have been previously reported for purified preparations of other steroidogenic enzymes, the present data demonstrate the multifunctional enzymatic activities associated with a recombinant oxidoreductase enzyme. In addition to its well known 3 beta-HSD activity, this enzyme possesses the ability to catalyze DHT into A-dione thus potentially controlling the level of the active androgen DHT in classical steroidogenic as well as peripheral intracrine tissues.