Cation-activated phosphatase activities in islet cell plasma membrane preparations

Pancreatic islets from rats or ob/ob mice were homogenized and fractionated either by a two-step or one-step sucrose gradient centrifugation. A plasma membrane enriched fraction was obtained at a sucrose density of about 1.10. The distribution of the plasma membrane probe 125I-wheat germ agglutinin was parallel to that of other plasma membrane markers. Hydrolysis of Mg-ATP-gamma-3Pp in rat islet membranes was of high specific activity, but was little affected by K+ and/or Na+. K+-activated, ouabain-sensitive phosphatase activities were, on the other hand, readily demonstrated in both rat and ob/ob mouse islet membranes. The K+-activated hydrolysis of organophosphate indicators were markedly inhibited by ATP. The binding of 45Ca2+ to mouse islet plasma membranes was increased by ATP. Cation transport and the interaction of Ca2+ with the islet cell plasma membrane may be dependent on phosphoryl-transfer reactions with ATP as the physiological substrate.