Consequences of the interaction of calcium with dioleoylphosphatidate-containing model membranes: calcium-membrane and membrane-membrane interactions.

Calcium binds to dioleoylphosphatidate/dioleoylphosphatidylcholine (DOPA/DOPC) (20:80, mol%) multilamellar vesicles in the presence of a calcium ionophore with stoichiometry of about 0.6 nmol calcium per nmol phosphatidate and an apparent dissociation constant of about 1.7 mM. Experiments on the behaviour of monomolecular films at an air/water interface show that calcium-phosphatidate binding results in a decrease in the area of the polar region of the phosphatidate molecule, probably caused by headgroup dehydration and partial charge neutralization. At calcium concentration higher than about 3 mM calcium neutralizes the negatively charged membrane surface of DOPA/DOPC (20:80, mol%) large unilamellar vesicles, and vesicle aggregation is observed. At 10 mM of calcium this results in a low level of vesicle fusion. These observed processes are not attended with calcium-induced phosphatidylcholine transbilayer movement in the membranes of DOPA/DOPC (20:80, mol%) large unilamellar vesicles. When these findings are compared with the results of a previous study on the permeability behaviour of large unilamellar vesicles of the same phospholipid composition under comparable conditions (Smaal, E.B., Mandersloot, J.G., De Kruijff, B. and De Gier, J. (1986) Biochim. Biophys. Acta 860, 99-108) the following conclusions can be drawn. At low millimolar calcium concentrations (less than 2.5 mM) calcium does not occupy all the binding sites of the membrane, no membrane-membrane interactions are observed and a selective translocation of calcium and calcium-chelating anions is appearing. The mechanism of this translocation may be explained by the formation of uncharged dehydrated complexes of calcium, phosphatidate and calcium chelator, which can pass the membrane via transient occurring non-bilayer structures. Between 3 and 10 mM of calcium an a selective permeability increase of the vesicular membrane is found, which is not a consequence of vesicle fusion but apparently of vesicle aggregation, possibly causing packing defects in the membrane.