Hyperglycemia-induced activation of human erythrocyte aldose reductase and alterations in kinetic properties

Incubation of human erythrocytes with varying concentrations of glucose resulted in a several-fold increase in aldose reductase (alditol:NADP⁺ 1-oxidoreductase, EC 1.1.1.21) activity as determined by the rate of NADPH oxidation and the rate of sorbitol formation. As compared to aldose reductase from human erythrocytes not incubated with glucose (native enzyme), aldose reductase from 30 mM glucose-incubated erythrocytes (activated enzyme) exhibited altered kinetic and inhibition properties. Native enzyme showed biphasic kinetics with substrates (glucose and glyceraldehyde), was strongly inhibited by 15 μM ADP, 1,3-diphosphoglycerate, 2,3-diphosphoglycerate and 3-phophoglycerate, and aldose reductase inhibitors such as sorbinil and alrestatin. The acivated enzyme, on the other hand, exhibited monophasic kinetics, low K_m for substrates, was not inhibited by the phosphorylated intermediates, and was less susceptible to inhibition by aldose reductase inhibitors. In erythrocytes of the diabetic subjects, we have found an excellent correlation between aldose reductase activity and plasma glucose levels and have observed that whenever the blood glucose level was higher than 16 mM, all of the erythrocyte aldose reductase was present in the activated form and exhibited properties similar to those observed with aldose reductase obtained from 30 mM glucose-incubated erythrocytes.