Characterization of pancreatic islet Ca2+-ATPase

Ca²⁺-dependent ATPase (Ca²⁺-dependent ATP phosphohydrolase, EC 3.6.1.3) present in a subcellular fraction derived from rat pancreatic islet homogenates was examined to determine kinetic parameters and responses to various substances with known effects upon insulin secretion. Experiments demonstrated the presence of a Ca²⁺-ATPase with a $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ ATP of 7 · 10⁻⁵ M and two $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ Ca of 1.3 · 10⁻⁷ M and 5.7 · 10⁻⁶ M. The enzyme had little activity in acidic media while retaining considerable activity in basic media. Optimal activity was obtained at pH 7.5. The enzyme was relatively temperature insensitive ($\text{Q}{}_{10}\text{= 1.49}$), since activity decreased less than 50% with a 15°C decrease in temperature. Studies on the stability of enzyme activity upon storage at −20°C indicated that for intact islets activity was stable for 3 weeks, while in homogenates activity was stable for only 1 week, after which activity rapidly declined in both cases. Certain substances known to either stimulate or inhibit insulin secretion were tested for their ability to alter enzyme activity. Potassium, glibenclamide and cyclic AMP had no effects upon activity. Mannoheptulose significantly suppressed enzyme activity while 2-deoxyglucose did not alter activity. These observations are consistent with the hypothesis that a Ca²⁺-ATPase present in pancreatic islets may act as a modulator of pancreatic islet β cell activity.