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.