Hydrolysis of hexose pentaacetate esters in rat pancreatic islets

The pentaacetate esters of selected hexoses were recently found to stimulate insulin release. The kinetics of their hydrolysis was now investigated in both rat pancreatic islet homogenates and intact islets. In islet homogenates, the hydrolysis of alpha-d-glucose pentaacetate, as judged from the measurement of acetate production, displayed a pH optimum of 7.4 and a Km for the ester of 0.95 mM. At pH 7.4, the reaction velocity was about 5 times higher than the rate of alpha-d-glucose pentaacetate hydrolysis by intact islets, as judged from the ester-induced increase in the acetate content of both the islet and surrounding incubation medium. Comparable results were obtained in intact islets exposed to either beta-l-glucose pentaacetate or beta-d-galactose pentaacetate. The ester content of the islets after 120 min incubation was close to 0.1 nmol/islet, yielding an apparent intracellular concentration at least one order of magnitude higher than the extracellular concentration (1.7 mM). These findings indicate that hexose esters that either stimulate insulin release or fail to do so are equally well taken up and hydrolyzed by islet cells. They are compatible, therefore, with the view that the insulinotropic action of some of these esters may be favored by the catabolism of their hexose moiety, although some other mechanisms for stimulation of insulin release must be operative in the case of beta-l-glucose pentaacetate.