Localization, isolation and properties of three NADPH-dependent aldehyde reducing enzymes from dog kidney

Three kinds of NADPH-dependent aldehyde reducing enzymes were present in the dog kidney. Aldose reductase was located in the inner medulla region and aldehyde reductase in all regions of the renal cortex, outer medulla and inner medulla. In addition, a new reductase designated tentatively as high-Km aldose reductase, which was converted into an aldose reductase-like enzyme, was present in the inner medulla region of the kidney. Aldose reductase, aldehyde reductase and high-Km aldose reductase were purified to homogeneity from each region of the dog kidney. The molecular weight of aldose reductase was estimated to be 38,500 by SDS-polyacrylamide gel electrophoresis and the isoelectric point was found to be 5.7 by chromatofocusing. Aldose reductase had activity for aldo-sugars such as D-xylose, D-glucose and D-galactose as substrates and utilized both NADPH and NADH as coenzymes. Sulfate ions resulted in over 2-fold activation of aldose reductase. All aldehyde reductases from the three regions had the same properties. The molecular weights and isoelectric points of aldehyde reductases were 40,000 and 6.1, respectively. The aldehyde reductases were inactive for D-hexose, utilized only NADPH as coenzyme and were not affected by sulfate ions. High-Km aldose reductase had a molecular weight of 38,500 and an isoelectric point of 5.4. It had activity for aldo-sugars, but showed much higher Km and lower kcat/Km values than aldose reductase. Sulfate ions inhibited high-Km aldose reductase. It was converted into an aldose reductase-like enzyme by incubation in phosphate buffer at pH 7.0. The three kinds of enzymes were strongly inhibited by the known aldose reductase inhibitors. However, aldehyde reductase and high-Km aldose reductase were, in general, less susceptible than aldose reductase.