Erythrocyte Membrane Polyphosphoinositide Metabolism and the Regulation of Calcium Binding

The terminal phosphate of ATP is incorporated into erythrocyte membrane polyphosphoinositides at the rate of 1.5 nmoles per min per mg of protein in the presence of 10 mmMg2+and 0.5 mmATP. The amount of membrane polyphosphoinositide monophosphate can be doubled in 30 min. Phosphorylation proceeds until the entire supply of membrane phosphatidylinositol is exhausted. Inhibition of incorporation by Ca2+depends on the concentration of Mg2+in the medium. Incorporation is also lower in the presence of detergent, although it may be restored to normal by the addition of exogenous phosphatidylinositol. An Arrhenius plot of reaction rate had two linear portions with a transition at 17°.