Structure-function aspects and inhibitor design of type 5 17 beta-hydroxysteroid dehydrogenase (AKR1C3)

17 beta -Hydroxysteroid dehydrogenase (17 beta -HSD) type 5 has been cloned from human prostate and is identical to type 2 3 alpha -HSD and is a membe r of the aldo-keto reductase (AKR) superfamily; it is Formally AKR1C3. In v itro the homogeneous recombinant enzyme expressed in Escherichia coli funct ions as a 3-keto-, 17-keto- and 20-ketosteroid reductase and as a 3 alpha-. 17 beta- and 20 alpha -hydroxysteroid oxidase. The enzyme will reduce 5 al pha -DHT, Delta (4)-androstene-3,17-dione, estrone and progesterone to prod uce 3 alpha -androstanediol- testosterone, 17 beta -estradiol and 20 alpha -hydroxprogesterone. respectively. It will also oxidize 3 alpha -androstane diol. testosterone, 17 beta -estradiol and 20 alpha -hydroxyprogesterone to produce 5 alpha -androstane-3,17-dione, Delta (4)-androstene-3,17-dione, a nd progesterone, respectively. Many of these properties are shared by the r elated AKR1C1, AKR1C2 and AKR1C4 isoforms. RT-PCR shows that AKR1C3 is domi nantly expressed in the human prostate and mammary gland. Examination of k( cat)/K-m for these reactions indicates that as a reductase it prefers 5 alp ha -dihydrotestosterone and 5 alpha -androstane-3,17-dione as substrates to Delta (4)-androstene-3,17-dione, suggesting that in the prostate it favors the formation of inactive androgens. Its concerted reductase activity may, however, lead to a pro-estrogenic state in the breast since it will conver t estrone to 17 beta -estradiol: convert Delta (4)-androstene-3,17-dione to testosterone (which can be aromatized to 17 beta -estradiol); and it will reduce progesterone to its inactive metabolite 20 alpha -hydroxyprogesteron e. Drawing on detailed structure-function analysis of the related rat 3 alp ha -HSD (AKR1C9), which shares 69% sequence identity with AKR1C3, it is pre dicted that AKR1C3 catalyzes an ordered bi bi mechanism, that the rate dete rmining step is k(chem), and that an oxyanion prevails in the transition st ate. Based on these relationships steroidal-based inhibitors that compete w ith the steroid product would be desirable since they would act as uncompet itive inhibitors.. With regards to transition state analogs steroid carboxy lates and pyrazoles may be preferred while 3 alpha, 17 beta or 20 alpha -sp iro-oxiranes may act as mechanism-based inactivators. (C) 2001 Elsevier sci ence Ireland Ltd. All rights reserved.