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Positive feedback effect of ATP on glucose phosphorylation in pancreatic islets
Ângelo Rafael Carpinelli, Abdullah Sener, Willy Malaisse
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Abstract
The affinity for ATP of hexokinase and glucokinase was measured in rat liver and pancreatic islets. In liver homogenates, the rate of D-glucose phosphorylation averaged 51 ± 6 and 232 ± 26 pmol/min per mg wet weight at hexose concentrations of 1.0 and 13.3 mM, respectively. The Km's for ATP of hexokinase and glucokinase were close to 2.1 and 1.0 mM, respectively. In islet homogenates, the maximal velocity of glucokinase represented only 25% of that of hexokinase, this being due in part to inhibition of glucokinase by excess Mg2+. The Km for ATP amounted to 1.0 and 1.6 mM in the presence of 1.0 and 10.0 mM D-glucose, respectively. These values are sufficiently high to postulate that changes in cytosolic ATP concentration, as encountered is glucose-stimulated islets, may affect the rate of phosphorylation of the hexose. The phosphorylation of D-glucose in intact islet cells may thus be the object of a sequential synarchistic regulation.