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-mu-M, 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-mu-M. 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-mu-M, 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.