Pancreatic islet discrimination of hexose anomers. I. Steady-state computer simulation.

Pancreatic islets detect glucose level by phosphorylating it and converting the glycolytic rate to a signal to secrete insulin. Insulin secretion is greater from the alpha- than from the beta-anomer when the D-glucose level is below 22 mM. D-mannose behaves similarly but at nearly twofold higher concentrations. Two explanations have been proposed: 1) glucokinase, which has the same anomeric preference, is the principal hexose phosphorylating enzyme and limits glycolytic rate. 2) Phosphofructokinase limits glycolysis and hexokinase is the principal enzyme phosphorylating hexose; hexosediphosphate activators of phosphofructokinase are more readily synthesized from alpha-anomers of hexose phosphates. We have simulated both alternatives with a detailed anomerically specific model of the hexose-metabolizing glycolytic enzymes. The pathway preference for alpha-anomer of both hexoses was adequately reproduced with anomerically active limiting glucokinase. The other mechanism did not reproduce the observed pathway preference.