Presence of ATP-Pyrophosphohydrolase in Mouse Pancreatic Islets

The presence of an enzyme that hydrolyzes ATP to AMP and PP¹ was demonstrated in a 27,000 × g particulate and supernatant fraction of mouse pancreatic islets. The enzyme was stimulated by addition of Ca²⁺, Zn²⁺, and Co²⁺. Addition of calmodulin or trifluoperazine had no effect. In the presence of Ca²⁺ and Zn²⁺, the Michaelis constant (K_m) for ATP was 0.1 mM and the maximum velocity (V_max) was close to 90 nmol · min⁻¹ · mg protein⁻¹. After preincubation of the islets for 30 min with 16.7 mM glucose or 5 mM glucose with 1 mM 3-isobutyl-1-methylxanthine (IBMX), a three- to fourfold increase in enzyme activity was seen. Direct addition of IBMX or CAMP to the enzyme assay also had a small stimulatory effect. Preincubation with the insulin secretagogues leucine and α-ketoisocaproic acid did not affect the enzyme activity. The possible function of the enzyme in pancreatic islets is discussed in relation to hypotheses given for the function of similar enzyme(s) in other tissues.