A slight asymmetry in the transbilayer distribution of lysophosphatidylcholine alters the surface properties and poly(ethylene glycol)-mediated fusion of dipalmitoylphosphatidylcholine large unilamellar vesicles.

Abstract

Large, unilamellar vesicles (LUV) composed of dipalmitoylphosphatidylcholine (DPPC) were made asymmetric in L-alpha-lysopalmitoylphosphatidylcholine (LPC) either by adding a small amount (total mole fraction = 0.003) of LPC to their outer leaflets (LUV-LPCout) or by extracting a small amount from outer leaflets which already contained 0.0015 mol fraction LPC (LUV-LPCin). The slow rate of the transbilayer redistribution of LPC allowed the asymmetric vesicles to be characterized with regard both to their physical properties and to their ability to fuse in the presence of poly(ethylene glycol) (PEG). The fraction of LPC extractable with bovine serum albumin was taken as a measure of LPC transbilayer asymmetry. The ratio of LPC available to that unavailable to BSA extraction was 6 for LUV-LPCout and 0.3 for LUV-LPCin. These asymmetries could not be enhanced significantly by increasing the vesicle LPC content. Measurements of the mixing and leakage of vesicle contents showed that LUV-LPCin fused in the presence of 15% (w/w) PEG without loss of contents but that LUV-LPCout did not fuse in the presence of up to 35% PEG. Vesicles prepared from palmitoyloleoylphosphatidylcholine could also be made asymmetric in LPC, but did not fuse even in the presence of 30% PEG. Quasi-elastic light scattering revealed that LUV-LPCin aggregated at 25 degrees C except when swollen by an osmotic gradient while LUV-LPCout were much less likely to aggregate. Trapped volume determinations suggested that neither type of vesicle was perfectly spherical in shape, but no correlation was found between fusogenicity and vesicle shape. Measurements of the fluorescence properties of TMA-DPH and of C6-NBD-PC suggested that the interface region of the outer leaflet of LUV-LPCin was slightly less ordered and less well packed than that of LUV-LPCout. This slight perturbation of the external vesicle surface correlated with the ability of juxtaposed vesicle bilayers to fuse.