Factors affecting the formation of sugar alcohols in ocular lens

Abstract

1.1. The enzymic reactions which influence the accumulation of sugar alcohols in lens were studied. The hexose monophosphate shunt mechanism appears closely linked to the aldose reductase (Polyol: NADP oxidoreductase, EC 1.1.1.21) reaction. It was revealed that as the concentration of glucose in the incubating medium was elevated a linear increase in the oxidation of C-1 of glucose was accompanied by an increase in the production of sorbitol. Galactose also appeared capable of stimulating the reoxidation of TPNH formed in the oxidation of C-1 of glucose. The stimulation of the hexose monophosphate shunt mechanism by the aldose reductase reaction was demonstrated in lenses from rabbits, normal rats and rats fed galactose.2.2. The formation of sorbitol is dependent upon a competition between aldose reductase and hexokinase (ATP: d-hexose 6-phosphotransferase, EC 2.7.1.1) for glucose. The unusually high Michaelis constant observed for aldose reductase indicates that normally glucose is phosphorylated and only a small fraction is converted to sorbitol. However, since the concentration of hexokinase is low in lens, a slight elevation of glucose is sufficient to stimulate sorbitol production.3.3. Unlike sorbitol, dulcitol, is not attacked by polyol dehydrogenase in lens. Other properties of polyol dehydrogenase (Polyol: NAD oxidoreductase) and aldose reductase were studied in the intact lens and in homogenates.