In this study, we characterized rat and mouse aldo-keto reductases (AKR1C16 and AKR1C13, respectively) with 92 sequence identity. The recombinant enzymes oxidized non-steroidal alcohols using NAD(+) as the preferred coenzyme, and showed low 3 alpha/17 beta/20 alpha-hydroxysteroid dehydrogenase (HSD) activities. The substrate specificity differs from that of rat NAD(+)-dependent 3 alpha-HSD (AKR1C17) that shares 95 sequence identity with AKR1C16. To elucidate the residues determining the substrate specificity of the enzymes, we performed site-directed mutagenesis of Tyr24, Asp128 and Phe129 of AKR1C16 with the corresponding residues (Ser, Tyr and Leu, respectively) of AKR1C17. The double mutation (Asp128/Tyr-Phe129/Leu) had few effects on the substrate specificity, while the Tyr24/Ser mutant showed only 3 alpha-HSD activity, and the triple mutation of the three residues produced an enzyme that had almost the same properties as AKR1C17. The importance of the residue 24 for substrate recognition was verified by the mutagenesis of Ser24/Tyr of AKR1C17 which resulted in a decrease in 3 alpha-HSD activity and appearance of 17 beta- and 20 alpha-HSD activities. AKR1C16 is also 92 identical with rat NAD(+)-dependent 17 beta-HSD (AKR1C24), which possesses Tyr24. The replacement of Asp128, Phel29 and Ser137 of AKR1C16 with the corresponding residues (Glu, Ser and Phe, respectively) of AKR1C24 increased the catalytic efficiency for 17 beta- and 20 alpha-hydroxysteroids. (c) 2007 Elsevier Inc. All rights reserved.