Calcium binding to liposomes composed of negatively charged lipid moieties

Ca2+-binding to liposomes was investigated by means of equilibrium dialysis using 45Ca as tracer. Liposomes composed of various lipids (phospholipids: phosphatidylcholine, -serine, -inositol, -inositol-3, 4-bisphosphate; glycosphingolipids: ganglioside mixture from bovine brain; isolated ganglioside species GM1, GD1a, GT1b; cholesterol) in ratios resembling those of natural cell surface membranes were prepared by the detergent dialysis method. Ca2+ was applied in concentrations ranging from 5 microM to 0.15 mM. Pure phosphatidylcholine liposomes showed only poor Ca2+-binding affinities; however, negatively charged liposomes bound Ca2+ in a rather distinct and specific manner. Ca2+-binding to liposomes containing phosphatidylinositol-3,4-bisphosphate in their bilayers was highest. All the liposomes (except those containing phosphatidylinositol-3,4-bisphosphate) showed no simple Ca2+-binding characteristics, that is, continuous Ca2+-binding was interrupted at a concentration of about 50 microM of total Ca2+ in each Ca2+-binding plot, implying that a certain amount (10%-45%) of previously bound Ca2+ was released. These results are discussed with regard to the different possibilities on the molecular basis of physico-chemical processes as well as in relation to their possible functional role at the process of neuronal synaptic transmission.