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The soluble activity in lymphocytes which converts phosphatidylinositol into 1,2-diacylglycerol and inositol phosphates requires Ca(2+) ions. At pH7 maximum activity occurs at Ca(2+)(free) approximately 0.7mum whereas at pH5.5 the equivalent value is approx. 50mum. At Ca(2+)(free) approximately 1mum, 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 Ca(2+) may reflect the fact that the Ca(2+)-buffering capacity of phosphatidylinositol means that high substrate concentrations can effectively decrease Ca(2+)(free). Cations which displace Ca(2+) from association with phosphatidylinositol can, at low Ca(2+)(free), enhance enzyme activity. Phosphatidylinositol breakdown in intact cells might be controlled, at least in part, by changes in intracellular Ca(2+)(free).
Allan et al. (Sun,) studied this question.