Some of the diabetic complications such as cataractogenesis, retinopathy, neuropathy, and nephropathy are common among the persons suffering from diabetes for a period of time. Accumulation of sorbitol, due to its relative impermeability through biological membranes in the lens, neural tissues, and blood vessels may be the cause of these complications. The major cause of these complications could be the osmotic imbalance due to the accumulation of sorbitol, a polyol which is relatively impermeable through biological membranes, in the tissues. The osmotic imbalance will lead to increased hydration, membrane stretch, and dysfunction. Aldose reductase reduces sugars to polyols with the mediation of NADPH. This enzyme is probable involved in sugar cataracts, because administration of aldose reductase inhibitors such as sorbinil and alrestatin to experimental animals prevents or significantly delays cataractogenesis. We have found that aldose reductase from different tissues can be activated several fold by 10 Mu M each of glucose-6-phosphate, glucose, and NADPH. The unactivated enzyme is irreversibly inhibited by 15 Mu M ADP, 2,3-DPG, or 3PGA. The activated enzyme is not inhibited by these phosphorylated glycolytic intermediates and is more resistant to inhibition by sorbinil and alrestatin than is the unactivated enzyme. The kinetic properties of aldose reductase purified from the tissues of the diabetic subjects as well as from the animals with experimentally induced diabetes and galactosemia will be studied and correlated with blood sugar and polyol levels. The aldo-keto reductases will be purified to homogeneity from human tissues and their kinetic, structural, and immunological properties will be studied. We have purified aldose reductase from human erythrocytes to homogeneity and have demonstrated that in diabetic subjects the enzyme activity increases with increase in blood sugar. With blood glucose levels higher than 200 mg percent, all the enzyme remains in the activated form. Glucose and glucose-6-P probably covalently bind to aldose reductase but the mechanism of activation/inactivation of the enzyme by these metabolites is not known. The binding characteristics of all the inhibitors and the mechanism of enzyme activation will therefore be studied. Peptide mapping, N-terminal, C-terminal, amino acid sequence of the peptides at the binding centers will be determined. These studies will help in understanding the role of aldose reductase in the pathophysiology of diabetic complications and should eventually aid the investigators in devising specific inhibitors of aldose reductase.