Human kidney aldose and aldehyde reductases

Mounting experimental evidence links increased aldose reductase activity with diabetes-related kidney functional changes. To investigate the interrelationship of NADPH-dependent reductases in the human kidney, both aldose reductase and aldehyde reductase were purified from human kidney by a series of chromatographic procedures, including gel filtration on Sephadex G-100, affinity chromatography on Matrex Gel Orange A, and chromatofocusing on Mono P. Each purified enzyme appeared as a single band on polyacrylamide gel after electrophoresis or isoelectric focusing. Aldose reductase has a pI of 5.7 and apparent molecular weight of 37 kDa, calculated from SDS-polyacrylamide gel electrophoresis, while aldehyde reductase has a pI of 5.2 and molecular weight of 39 kDa. Similar molecular weights were also obtained by gel filtration, indicating that both aldose and aldehyde reductases are present as monomers in the human kidney. Aldehyde reductase is primarily localized in the cortex, while the medulla contains aldose reductase. Both enzymes displayed properties consistent with the general characteristics of aldose and aldehyde reductases obtained from either rat or dog kidney. Purified aldose reductase utilizes aldose sugars such as D-xylose, D-glucose, and D-galactose as substrates while aldehyde reductase preferentially reduces D-glucuronate and oxidizes L-gulonate to D-glucuronate. Despite the lower apparent affinity of aldehyde reductase for aldose sugars (∼20- to 100-fold less) both enzymes reduced D-xylose, D-glucose, and D-galactose to their respective sugar alcohols in in vitro incubation studies where the generated sugar alcohols were identified by gas chromatography. Both enzymes were also inhibited by aldose reductase inhibitors. Of the compounds investigated, the carboxylic acids Ponalrestat and FK 366 displayed greater selectivity for inhibiting aldose reductase, whereas the hydantoins sorbinil and Al 1576 displayed more equal inhibition of both enzymes.