3β-Hydroxysteroid Dehydrogenase/Δ5→4-Isomerase Activity Associated with the Human 17β-Hydroxysteroid Dehydrogenase Type 2 Isoform1

The type 2 isoform of human 17beta-hydroxysteroid dehydrogenase (17betaHSD2) 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 17betaHSD2 enzyme after transfection of human 17betaHSD2-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. 17betaHSD2-transfected cells converted 91% of testosterone (1 micromol/L) into androstenedione in a 2-h incubation period. In addition, pregnenolone (1 micromol/L) was converted to progesterone (18.5%), whereas DHEA (1 micromol/L) was metabolized to androstenedione (8.3% conversion) in a 15-h incubation period. The kinetics of the 3beta-hydroxysteroid dehydrogenase (3betaHSD) and 17betaHSD2 activities using cell homogenate protein of stably transfected 293 cells indicated that the catalytic efficiency (apparent catalytic efficiency = maximum velocity/Km) of this 3betaHSD activity is approximately 2000-fold (pregnenolone as substrate) or 3000-fold (DHEA as substrate) weaker than that of 17betaHSD2 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 3betaHSD aspect of the 17betaHSD2 gene product. Pregnenolone or DHEA effectively inhibited 17betaHSD2 activity in a noncompetitive fashion. Furthermore, the potent 5alpha-reductase/3betaHSD inhibitor, 17beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5alpha-androstane-3-one , inhibited neither 3betaHSD nor 17betaHSD2 activities. We conclude that human 17betaHSD2 enzyme exhibits 3betaHSD activity. Notwithstanding that this 3betaHSD activity is reduced compared to 17betaHSD oxidative activity, it may account for at least some of the reports of 3betaHSD activity found in human peripheral tissues that express notable amounts of the 17betaHSD2 isozyme as well as in individuals with severe classic 3betaHSD deficiency.