Phosphatidylinositol cleavage in lymphocytes: requirement for calcium ions at a low concentration and effects of other cations

The soluble activity in lymphocytes which converts phosphatidylinositol into 1,2 diacylglycerol and inositol phosphates requires Ca²⁺ ions. At pH7 maximum activity occurs at [Ca²⁺]free 0.7 μM whereas at pH5.5 the equivalent value is approx. 50 μM. At [Ca²⁺]free μM, a concentration similar to common intracellular values, essentially all activity is confined to the peak of activity at pH7.0. Previous reports of requirements for larger amounts of Ca²⁺ may reflect the fact that the Ca²⁺ buffering capacity of phosphatidylinositol means that high substrate concentrations can effectively decrease [Ca²⁺]free. Cations which displace Ca²⁺ from association with phosphatidylinositol can, at low [Ca²⁺]free enhance enzyme activity. Phosphatidylinositol breakdown in intact cells might be controlled, at least in part, by changes in intracellular [Ca²⁺]free.