Human 3 alpha-hydroxysteroid dehydrogenase isoforms (AKR1C1-AKR1C4) of thealdo-keto reductase superfamily: functional plasticity and tissue distribution reveals roles in the inactivation and formation of male and female sexhormones

The kinetic parameters, steroid substrate specificity and identities of rea ction products were determined for four homogeneous recombinant human 3 alp ha-hydroxysteroid dehydrogenase (3 alpha-HSD) isoforms of the aldo-keto red uctase (AKR) superfamily. The enzymes correspond to type 1 3 alpha-HSD (AKR IC4), type 2 3 alpha(17 beta)-HSD (AKR1C3), type 3 3 alpha-HSD (AKR1C2) and 20 alpha(3 alpha)-HSD (AKR1C1), and share at least 84% amino acid sequence identity. All enzymes acted as NAD(P)(H)-dependent 3-, 17- and 20-ketoster oid reductases and as 3 alpha-, 17 beta- and 20 alpha-hydroxysteroid oxidas es. The functional plasticity of these isoforms highlights their ability to modulate the levels of active androgens, oestrogens and progestins. Salien t features were that AKR1C4 was the most catalytically efficient, with k(ca t)/K-m values for substrates that exceeded those obtained with other isofor ms by 10-30-fold. In the reduction direction, all isoforms inactivated 5 al pha-dihydrotestosterone (17 beta-hydroxy-5 alpha-androstan-3-one; 5 alpha-D HT) to yield 5 alpha-androstane-3 alpha,17 beta-diol (3 alpha-androstanedio l). However, only AKR1C3 reduced Delta(4)-androstene-3,17-dione to produce significant amounts of testosterone. All isoforms reduced oestrone to 17 be ta-oestradiol, and progesterone to 20 alpha-hydroxy-pregn-4-ene-3,20-dione (20 alpha-hydroxyprogesterone). In the oxidation direction, only AKR1C2 con verted 3a-androstanediol to the active hormone 5 alpha-DHT. AKR1C3 and AKR1 C4 oxidized testosterone to Delta(4)-androstene-3,17-dione. All isoforms ox id ized 17 beta-oestradiol to oestrone, and 20 alpha-hydroxyprogesterone to progesterone. Discrete tissue distribution of these AKR1C enzymes was obse rved using isoform-specific reverse transcriptase-PCR. AKR1C4 was virtually liver-specific and its high k(cat)/K-m allows this enzyme to form 5 alpha/ 5 beta-tetrahydrosteroids robustly. AKR1C3 was most prominent in the prosta te and mammary glands. The ability of AKR1C3 to interconvert testosterone w ith Delta(4)-androstene-3,17-dione, but to inactivate 5 alpha-DHT, is consi stent with this enzyme eliminating active androgens from the prostate. In t he mammary gland, AKR1C3 will convert Delta(4)-androstene-3,17-dione to tes tosterone (a substrate aromatizable to 17 beta-oestradiol), oestrone to 17 beta-oestradiol, and progesterone to 20 alpha-hydroxyprogesterone, and this concerted reductive activity may yield a. pro-oesterogenic state. AKR1C3 i s also the dominant form in the uterus and is responsible for the synthesis of 3 alpha-androstanediol which has been implicated as a parturition hormo ne. The major isoforms in the brain, capable of synthesizing anxiolytic ste roids, are AKR1C1 sand AKR1C2. These studies are in stark contrast with tho se in rat where only a single AKR with positional- and stereospecificity fo r 3 alpha-hydroxysteroids exists.