Hexokinase and glucokinase activity in cross-linked and permeabilized pancreatic islets

The activities of hexokinase and glucokinase were measured in cross-linked and permeabilized rat pancreatic islets. After exposure to dimethyl suberimidate (20 mM) and digitonin (0.4 mM), the activity of hexokinase represented about half of that found in homogenates of freshly isolated islets. The K(m) of hexokinase for D-glucose and the Ki for its inhibition by D-glucose-6-phosphate were similar, however, in the cross-linked and permeabilized islets and in homogenates of freshly isolated islets. Glucokinase activity also was documented in the cross-linked and permeabilized islets, it being less sensitive than hexokinase activity to inhibition by D-glucose-6-phosphate. At a high concentration of D-glucose (16.7 mM), the phosphorylation of the hexose failed to be increased by D-fructose-1-phosphate, whether in the absence or presence of D-glucose-6-phosphate. These findings indicate that the intrinsic properties of hexokinase isoenzymes are preserved in cross-linked islets, but suggest that the cross-linking of proteins prevents the activation of glucokinase by its regulatory protein.