The bivalent-cation dependence of phosphatidylinositol synthesis in a cell-free system from lymphocytes

The bivalent-cation requirements of two enzymes involved in phosphatidylinositol synthesis were defined for pig lymphocyte membranes using a citric acid buffer. CTP:phosphatidic acid cytidylyltransferase (EC 2.7.7.41) is activated by free Mn2+ concentrations above 20nM and by free Mg2+ concentrations above 10 microM. When activated by Mg2+, the enzyme is weakly inhibited by Ca2+ (Ki greater than 250 microM), but Ca2+ has no effect when Mn2+ is used to stimulate CDP-diacylglycerol synthesis. The synthesis of phosphatidylinositol from phosphatidic acid is also stimulated by Mn2+ and Mg2+ concentrations similar to those above and is inhibited by free Ca2+ concentrations above 500nM, probably by its action on CDP-diacylglycerol:inositol 3-phosphatidyltransferase (EC 2.7.8.11). Taken together, these studies suggest that under physiological conditions phosphatidylinositol synthesis is activated by Mg2+ and it is possible that it is further regulated by the free concentrations of Ca2+ and/or Mn2+.