Ca(phosphatidate)2 can traverse liposomal bilayers

Phosphatidic acid can act as Ca2+ cross-membrane ionophore without the necessity of previous autoxidation. The apparent PA-CA2+ dissociation constant is 3 X 10(-3), i.e., in the range of extracellular Ca2+ concentration. There is at least 100-fold preference for Ca2+ over Mg2+. Ca2+ transfer rates are proportional to the square of phosphatidic acid concentration in the bilayer. Removal of the fatty acid ester CO groups reduces the Ca2+ ferrying rate by more than 90 percent. It appears that the cation is held in a cage formed by phosphate and carbonyl oxygens of two PA molecules. In this coordination complex both Ca2+ and the phosphatidic acid headgroups are dehydrated, and the Ca(phosphatide)2 assembly becomes lipid-soluble and can traverse the bilayer.