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

Abstract

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 ≈ 0.7μm whereas at pH5.5 the equivalent value is approx. 50μm. At [Ca2+]free ≈ 1μm, 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.