3β-hydroxysteroid dehydrogenase/Δ5→4-isomerase activity associated with the human 17β-hydroxysteroid dehydrogenase type 2 isoform

The type 2 isoform of human 17 beta-hydroxysteroid dehydrogenase (17 beta HSD2) efficiently catalyzes the oxidative metabolism of androgens and estrogens, and it is expressed in a large series of human peripheral tissues. To obtain a better understanding of the regulation of local steroid biosynthesis and metabolism in human tissues, we have established a dual steroidogenic activity of the 17 beta HSD2 enzyme after transfection of human 17 beta HSD2-transfected human embryonic kidney (293) cells. After transient transfection, the metabolism of testosterone, pregnenolone, and dehydroepiandrosterone (DHEA) in intact transfected 293 cells was evaluated by TLC-based radiometric assays. 17 beta HSD2-transfected cells converted 91% of testosterone (1 mu mol/L) into androstenedione in a 2-h incubation period. In addition, pregnenolone (1 mu mol/L) was converted to progesterone (18.5%), whereas DHEA (1 mu mol/L) was metabolized to androstenedione (8.3% conversion) in a 15-fi incubation period. The kinetics of the 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) and 17 beta HSD2 activities using cell homogenate protein of stably transfected 293 cells indicated that the catalytic efficiency (apparent catalytic efficiency = maximum velocity/K-m) of this 3 beta HSD activity is approximately 2000-fold (pregnenolone as substrate) or 3000-fold (DHEA as substrate) weaker than that of 17 beta HSD2 activity. It is noteworthy, however, that the apparent catalytic efficiency of the HSD3B2 gene product is only approximately 50-fold higher than that of the 3 beta HSD aspect of the 17 beta HSD2 gene product. Pregnenolone or DHEA effectively inhibited 17 beta HSD2 activity in a noncompetitive fashion. Furthermore, the potent 5 alpha-reductase/3 beta HSD inhibitor, 17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstane-3-one, inhibited neither 3 beta HSD nor 17 beta HSD2 activities. We conclude that human 17 beta HSD2 enzyme exhibits 3 beta HSD activity. Notwithstanding that this 3 beta HSD activity is reduced compared to 17 beta HSD oxidative activity, it may account for at least some of the reports of 3 beta HSD activity found in human peripheral tissues that express notable amounts of the 17 beta HSD2 isozyme as well as in individuals with severe classic 3 beta HSD deficiency.