QUANTITATIVE STUDIES ON ISOLATED PANCREATIC ISLETS OF MAMMALS: ADENOSINE TRIPHOSPHATASE ACTIVITY IN NORMAL AND OBESE-HYPERGLYCEMIC MICE

Abstract

Microchemical and histochemical methods were used for the characterization, localization and assay of adenosine triphosphate (ATP) splitting enzymes in homogenates and sections of the endocrine pancreas from obese-hyperglycemic mice and their lean litter mates. The following observations were made: 1. Dephosphorylation of ATP was maximal at pH 9.1. It was strongly stimulated by magnesium ions at an optimal concentration of 1 mM. ATP cleavage was inhibited by adenosine diphosphate, sodium azide and p-chloromercuribenzoic acid. The addition of l-cysteine, sodium cyanide or sodium fluoride to the substrate medium had no effect on the enzyme activity. Substitution of ATP by equimolar amounts of other phosphate esters in the medium considerably reduced the substrate cleavage. These results are taken as evidence for the presence of sulfhydryl-dependent adenosine triphosphatase (ATPase) in the islet tissue. 2. Histochemical staining revealed a strong ATP splitting enzyme activity in the capillaries and walls of larger blood vessels throughout the pancreas; a rather weak and diffuse cytoplasmic reaction being found in the islet cells. 3. Microchemical assays revealed a lower cleavage of ATP in the islets as compared with the exocrine pancreas and the liver. The cleavage of ATP was more intense in the islets of the obese-hyperglycemic mice than in those of the lean litter mates. 4. Starvation for 7 days induced no significant changes in the enzyme activity of the endocrine pancreas.