Effects of platelet-activating factor (PAF), lyso-PAF and lysophosphatidylcholine on phosphatidylcholine bilayers, an...

The effect of asymmetric phospholipids, such as platelet-activating factor (PAF), lyso-PAF and lysophosphatidylcholine, on phosphatidylcholine bilayers has been examined using ESR, 31P-NMR and X-ray diffraction methods. ESR and 31P-NMR experiments have been performed on oriented multibilayers. ESR measurements of 5-doxyl stearic acid, as a spin probe, show that PAF disorients phosphatidylcholine bilayers when present at molar ratios greater than 40%. This is manifest as a broadening of the local director orientation distribution, a parameter required to simulate the lineshape. Despite the marked change of the regular in-plane orientation of the films, there are only slight changes in the order parameter of the acyl chains. Cholesterol orients films containing asymmetric phospholipids, in a way consistent with the formation of 1/1 stoichiometric complexes between cholesterol and the asymmetric phospholipid. Such complexes can be detected with 3-doxyl cholestane, a spin-labelled sterol analogue interacting with lyso-PAF and PAF. Simulations of 31P-NMR resonance linewidth of oriented multibilayers of PAF/egg lecithin mixtures indicate a rippled structure which accounts for the perturbed distribution of the local director observed by ESR spin probe measurements. Micellisation of the film can be discounted on the basis of the 31P-NMR linewideth for the concentration range investigated. X-ray diffraction studies of liposomes of dimyristoyl- and dioleoyl-phosphatidylcholine containing PAF relate the disorientation of the film with the emergence of a lamellar interdigitated phase of reduced (4.2 nm) repeat distance. This interdigitated phase coexists with the lecithin lamellar phase (repeat spacing 6.0 nm) at temperatures below and above the gel-to-liquid crystal transition temperature of the lecithin. Cholesterol/PAF mixtures give X-ray diffraction patterns indexing a lamellar repeat distance of 6.7 nm. Cholesterol also prevents formation of interdigiatated lamellae with the various asymmetric phospholipids. A model is proposed where the asymmetric phospholipid segregates from lecithin to form 'blister-like' structures within the film consisting of thin interdigitated lamellae. Formation of complexes between cholesterol and the asymmetric lipid prevents the creation of these structures.