Biol. Pharm. Bull. 28(6) 1075—1078 (2005)

growing group of NAD(P)(H)-dependent oxidoreductases that metabolize carbohydrates, steroids, prostaglandins, and other endogenous aldehydes and ketones, as well as xenobiotic compounds. Currently there are more than a hundred known members of this superfamily classified into 14 families. The largest family, AKR1, is subdivided into five subfamilies: AKR1A) mammalian aldehyde reductases; AKR1B) mammalian aldose reductases; AKR1C) hydroxysteroid dehydrogenases (HSDs) and prostaglandin F synthases; AKR1D) D-3-ketosteroid-5b-reductases; and AKR1E) mouse keto-reductase. The subfamilies are defined by a 60% identity in amino acid sequence among subfamily members, but the AKR1C subfamily includes some members that have not been studied as to their enzymatic properties or functions. One such member is AKR1C19 that was found by mouse genomic analysis and cDNA isolation from the liver and gastrointestinal tract. Although AKR1C19 is postulated to be a 3(20)a-HSD-like enzyme based on its highest sequence identity (72%) with human 3(20)a-HSD (AKR1C1) in the AKR1C subfamily, a mouse counterpart of AKR1C1 is shown to be an NADP(H)-dependent 3a /3b /20a-HSD (AKR1C18), which shares a lower sequence identity (65%) with AKR1C19. To determine the functional relationship of AKR1C19 with AKR1C18, we examined the enzymatic properties of the recombinant AKR1C19.