Department of Biochemistry, Osaka University Medical School, Japan.
We have purified a rat liver enzyme that catalyzes the NADPH-dependent reduction of 3-deoxyglucosone (3-DG), a major intermediate in the Maillard reaction and a potent cross-linker responsible for the polymerization of proteins. Comparison of the amino acid (aa) sequences of nine peptides obtained from the rat 3-DG-reducing enzyme by lysylendopeptidase digestion with the aa sequence of human aldehyde reductase (ALR) [Bohren et al., J. Biol. Chem. 266 (1991) 24031-24037] strongly suggested that the purified enzyme was rat ALR. We cloned the cDNA encoding ALR from a rat kidney cDNA library using a human ALR cDNA fragment, amplified by polymerase chain reaction, as a probe. All nine peptides identified in the purified rat 3-DG-reducing enzyme were found in the aa sequence deduced from the rat ALR cDNA. Moreover, cell extract from COS-1 cells transfected with the rat ALR cDNA exhibited NADPH-dependent 3-DG-reducing activity and cross-reacted with antiserum raised against the purified rat 3-DG-reducing enzyme. All the above data indicate clearly that the 3-DG-reducing enzyme is identical with ALR. Northern blot analysis of total mRNA from a variety of rat tissues showed fairly high levels of expression of ALR mRNA. This suggests that sufficient ALR is present to detoxify 3-DG when it is formed through the Maillard reaction in vivo.