Purification and properties of aldose reductase and aldehyde reductase II from human lens

Aldose reductase (EC 1.1.1.21) and aldehyde reductase II (L-hexonate dehydrogenase, EC 1.1.1.2) have been purified to homogeneity from human lens by ion exchange chromatography, chromatofocusing, affinity chromatography, and Sephadex G-100 gel filtration. Aldose reductase and aldehyde reductase II are monomers of Mr 43,000 and Mr 41,000, respectively. Aldose reductase has a pI of 6.2 and the pH optimum of 6.5. Aldehyde reductase II has a pI of 5.7 and the pH optimum of 7.0. Both enzymes have a wide overlapping substrate specificity, but aldose reductase manifests a unique ability to reduce aldo-sugars to their corresponding alcohols. Aldose reductase activity is expressed with either NADH or NADPH as cofactors, whereas aldehyde reductase II is dependent on NADPH. Aldose reductase requires 0.4 M sulfate for the expression of its full activity but aldehyde reductase II is significantly inhibited by sulfate. Both enzymes are susceptible to inhibition by various aldo-keto reductase inhibitors and sulfhydryl reagent, N-ethylmyleimide, to varying degrees. Aldose reductase and aldehyde reductase II are immunologically distinct.