Characteristics of a highly labile human type 5 17 beta-hydroxysteroid dehydrogenase

17 beta-Hydroxysteroid dehydrogenases (17 beta HSDs) play an essential role in the formation of active intracellular sex steroids. Six types of 17 bet a HSD have been described to date, which only share approximately 20% homol ogy. Human type 5 17 beta HSD complementary DNA is unique among the 17 beta HSDs because it belongs to the aldo-keto reductase family, whereas the oth ers are members of the short chain alcohol dehydrogenases. The characterist ics of human type 5 17 beta HSD were investigated in human embryonic (293) cells stably transfected with human and mouse type 5 17 beta HSD, as well a s human type 3 3 alpha HSD. Using intact transfected cells, type 5 17 beta HSD shows a substrate specificity pattern comparable to those of human type 3 17 beta HSD and mouse type 5 17 beta HSD. These enzymes catalyze more ef ficiently the transformation of androstenedione (4-dione) to testosterone, whereas the transformation of dihydrotestosterone to 5 alpha-androstane-3 a lpha,17 beta-diol is much lower. In contrast, type 3 3 alpha HSD catalyzes more efficiently the transformation of dihydrotestosterone to 5 alpha-andro stane-3 alpha,17 beta-diol, whereas the transformation of 4-dione to testos terone represents only 7% of the 3 alpha HSD activity. However. upon homoge nization, human type 5 17 beta HSD activity decreases to approximately 10% of the activity in intact cells and remains stable at this level together w ith the 3 alpha HSD activity. Under the same conditions, however, the mouse Enzyme is not altered by homogenization. Indeed, using purified human 17 b eta HSD overexpressed in Escherichia coli, we could confirm that a much gre ater amount of protein is required to produce activity similar to the enzym atic activity measured in intact transfected cells, The present data provid e the answer to the question of why previous researchers could hardly detec t type 5 17 beta HSD activity. Indeed, all previous publications used cell or tissue homogenates or purified enzymes. Under such conditions, only the low level, but stable, 3 alpha BSD and 17 beta HSD activities could be meas ured, whereas the high level, but highly unstable, 17 beta HSD activity cou ld not be measured. As type 5 17 beta HSD shares 84%, 86%, and 88% amino ac id identity with types 1 and 3 3 alpha HSD and 20 alpha HSDs, respectively, Northern blot analysis used in previous studies could not provide unequivo cal information. In this report, we used a more specific ribonuclease prote ction assay and could thus show that human type 5 17 beta HSD is expressed in the liver, adrenal, and prostate; in prostatic cancer cell lines DU-145 and LNCaP: as well as in bone carcinoma (MG-63) cells. By analogy with type 3 17 beta HSD, which is responsible for the formation of androgens in the testis, the expression of type 5 17 beta HSD in the prostate and bone cells suggests that this enzyme is involved in the formation of active intracell ular androgens in these tissues.