Treatment of Chinese hamster ovary (CHO) cells by the aldehyde containing calpain inhibitor I resulted in the induction of a 35-kDa protein that was partially sequenced and shown to be a member of the aldo-keto reductase superfamily (Inoue, S., Sharma, R. C., Schimke, R. T., and Simoni, R. D. (1993) J. Biol. Chem. 268, 5894 -5898). Using rapid amplification of cDNA ends polymerase chain reaction, we have sequenced the cDNA for this protein (CHO reductase). This enzyme is a new member of the aldo-keto reductase superfamily and shows greatest amino acid sequence identity to mouse fibroblast growth factor-regulated protein and mouse vas deferens protein (92 and 80% sequence identity, respectively). The enzyme exhibits about 70% sequence identity with the aldose reductases (ALR2; EC 1.1.1.21) and about 47% with the aldehyde reductases (ALR1; EC 1.1.1.2). Northern analysis showed that it is induced in preference to either ALR1 or ALR2 and RNase protection assays showed gene expression in bladder, testis, jejunum, and ovary in descending order of expression. The cDNA for this inducible reductase was cloned into the pET16b vector and expressed in BL21(DE3) cells. Expressed CHO reductase showed kinetic properties distinct from either ALR1 or ALR2 including the ability to metabolize ketones. This protein joins a growing number of inducible aldo-keto reductases that may play a role in cellular regulation and protection.To characterize the calpain inhibitor I-sensitive protease(s) involved in the degradation of 3-hydroxy-3-methylglutaryl-CoA reductase, Simoni's group (1) attempted to isolate Chinese hamster ovary (CHO) 1 cells resistant to this peptide aldehyde (N-acetyl-leucyl-leucyl-norleucinal (ALLN)). Instead of inducing a protease, a 35-kDa protein was overexpressed which gave tryptic peptide fragments with a high degree of sequence identity to members of the aldo-keto reductase superfamily. 2 This superfamily is a rapidly growing group of monomeric oxidoreductases containing at least 40 members at present (2). This group includes the aldehyde and aldose reductases, a number of hydroxysteroid dehydrogenases, Shaker channels, and plant chalcone reductases. They are characterized by a TIM-barrel structure (3) and the preferential use of NADPH over NADH. Substrates include aliphatic and aromatic aldehydes, monosaccharides, steroids, prostaglandins, polycyclic aromatic hydrocarbons, and isoflavinoid phytoalexins.Several members of this family have been shown to be induced in response to hormonal or chemical factors. These include fibroblast growth factor-regulated protein (FR-1) (4), mouse vas deferens protein (MVDP) (5), aldose reductase (ALR2) (6 -8), and dihydrodiol dehydrogenase (9). To determine the relationship of this new reductase to the other members of the family we have used RACE PCR to isolate and sequence its mRNA from CHO cells. It shows highest sequence identity to FR-1 and MVDP at 92 and 80%, respectively. FR-1 and MVDP have not been extensively characterized kinetically but CHO reductase showed a greater ...