Chicken muscle aldose reductase: purification, properties and relationship to other chicken aldo/keto reductases

A superfamily of NADPH-dependent reductases in eukaryotes and prokaryotes

Aldose reductase (AR) is implicated in some of the disabling complications of diabetes, including neuropathy, retinopathy and cataracts. Our studies are aimed at further clarifying the role of AR in diabetes and facilitating the design of new classes of potent, specific AR inhibitors by gaining an understanding of the protein structure of AR. To this end, we have determined the complete protein sequence of rat lens AR using cDNA analysis and primer extension of mRNA. By comparing protein sequences, we have found that the structural relatedness (41% to 57%) among the vertebrate proteins, aldose reductase, aldehyde reductase, prostaglandin F synthase and the frog lens protein rho-crystallin can now be extended to prokaryotes by the inclusion of Corynebacterium 2,5-diketo-D-gluconate reductase. This more distantly related protein shares 30-40% identity with the vertebrate enzymes. Sequence alignments reveal that 18% of the amino acids are completely conserved in all members of the superfamily, many of them in clusters, suggesting that they mark important structural features such as the nucleotide binding site and substrate binding site. rho-Crystallin, which is structurally related to this superfamily of NADPH-dependent reductases, does not appear to reduce PGH2, PGD2, xylose or glyceraldehyde to any appreciable extent. It does, however, bind NADPH.

Inhibition of aldose reductase from human retina

Aldose reductase was prepared from a pool of 21 male and 16 female human retinas by ammonium sulphate fractionation (40-75% saturation) and chromatography on DEAE-Sephacel and Matrex-OA. The overall purification was 132-fold with 50% recovery of enzyme activity. The concentrations of the aldose reductase inhibitors Sorbinil, Statil and M79175 required to give 50% inhibition (IC50 value) of enzyme activity with the model substrate 4-nitrobenzaldehyde (4NB) were 3.4 microM, 2.3 microM and 0.22 microM respectively. This indicated that M79175 was the most effective inhibitor tested of aldose reductase with 4NB in vitro. These inhibitors were more effective when tested against aldose reductase activity with glucose, the substrate which might play a role in the pathogenesis of diabetic complications. Sorbinil gave an IC50 (glucose) of 0.40 microM; M79175 and Statil were more effective. At an inhibitor concentration of 0.1 microM the %-inhibitions observed were: Sorbinil 20% M79175 55%, Statil 76%. Thus Statil was the most potent compound tested against human retinal enzyme using the more physiological substrate in vitro. This report provides the first direct evidence that human retinal aldose reductase is susceptible to inhibition by compounds designed for chemotherapy of diabetic complications, and indicates that the concentrations of inhibitor required for a substantial block of activity in vitro are lower than those attained in plasma in man.