High glucose concentrations partially release hexokinase from inhibition by glucose 6-phosphate.

The phosphorylation of glucose by human erythrocyte hexokinase follows classical Michaelis-Menten kinetics; hexokinase manifests maximum activity at 5 mM glucose, and no further increase in activity can be measured at higher glucose concentrations. However, the erythrocytes of diabetics and normal erythrocytes incubated with high concentrations of glucose contain increased concentrations of glucose 6-phosphate. To elucidate the mechanism of accumulation of glucose 6-phosphate when erythrocytes are exposed to high glucose concentrations, hexokinase activity was examined in the presence of naturally occurring inhibitors, such as glucose 1,6-bisphosphate, 2,3-diphosphoglycerate, ADP, and glucose 6-phosphate at physiological concentrations. Without inhibitors or in the presence of glucose 1,6-bisphosphate,2,3-diphosphoglycerate, and ADP, maximum hexokinase activity was observed at 5 mM glucose concentration. On the contrary, in the presence of glucose 6-phosphate, hexokinase activity increased at glucose concentrations greater than 5 mM; inhibition by glucose 6-phosphate was partially competitive with glucose. The relief by glucose of glucose 6-phosphate inhibition of hexokinase is a possible explanation of the increased glucose 6-phosphate level in diabetic erythrocytes.