STRUCTURAL EFFECTS OF NEUTRAL LIPIDS ON DIVALENT CATION-INDUCED INTERACTIONS OF PHOSPHATIDYLSERINE-CONTAINING BILAYERS

Ca2+ is known to induce the adhesion and collapse of phosphatidylserin e (PS) bilayers into dehydrated multilamellar structures. The aim of t his study was to examine how that interaction and the resultant struct ures might be modified by neutral lipid species. A combination of rapi d mixing, x-ray diffraction, thin-layer chromatography, density gradie nt centrifugation, and freeze-fracture electron microscopy was used in conjunction with osmotic stress techniques to characterize the struct ures formed by the Ca2+-induced interaction of multilamellar liposomes and of large unilamellar vesicles. The results showed that dioleoylph osphatidylcholine and dioleoylphosphatidylethanolamine at concentratio ns of up to similar to 30 mol % are accommodated in a single dehydrate d multilamellar structure. Similar results were obtained using mixed P S species isolated from bovine brain. Principally, the data indicate t hat neutral lipid is both dehydrated during the rapid collapse process of Ca(PS)(2) formation and accommodated within this dehydrated struct ure. The large energies available on formation of the Ca(PS)(2) bilaye rs contribute to the dehydration of neighboring neutral lipids that li kely form continuous bilayers with them. Higher concentrations of thes e neutral lipids modify Ca2+-induced bilayer interactions, leading to progressively weaker interactions, larger bilayer separations, and in some cases separation into two structures; phosphatidylethanolamine sp ecies favoring nonbilayer structures tended to promote such separation at lower concentrations than bilayer lipids.