Inhibition of Rat Liver Glucokinase by Alloxan and Ninhydrin

In a recent paper, we reported that glucokinase in pancreatic islets might be a critical site which mediates the inhibition of glucose-stimulated insulin secretion by alloxan and ninhydrin. The action mechanisms of the two agents on rat liver glucokinase (instead of islet glucokinase) were studied here.Both alloxan and ninhydrin irreversibly inhibited rat liver glucokinase in concentration-dependent manners. The inhibitory effects of alloxan and ninhydrin on glucokinase were blocked by the presence of hexoses (D-glucose and D-mannose) that serve as substrates of the enzyme. The blockade provided by D-glucose showed α-anomeric preference, as was also observed in the phosphorylation of D-glucose by glucokinase. Protection against the inhibition of glucokinase by alloxan or ninhydrin was also afforded by D-mannoheptulose, a competitive inhibitor of the enzyme with respect to D-glucose. These results suggest that the inhibitory sites of alloxan and ninhydrin are at or near the substrate-binding site of glucokinase.