No abstract

Nascent HDL is known to be formed by the interaction of apolipoprotein A-I (apoA-I) with transmembrane ABCA1, but the molecular mechanism by which nascent HDL forms is less well understood. Here, we studied how reconstituted high density lipoprotein (rHDL) forms spontaneously on the interaction of apoA-I with model membranes. The formation of rHDL from pure phosphatidylcholine (PC) large unilamellar vesicles (LUVs) proceeded very slowly at 37.0jC, but sphingomyelin (SM) -rich PC/SM LUVs, which are in a gel/liquid-disordered phase (L d phase) at this temperature, were rapidly microsolubilized to form rHDL by apoA-I. The addition of cholesterol decreased the rate at which rHDL formed and induced the selective extraction of lipids by apoA-I, which preferably extracted lipids of L d phase rather than lipids of liquid-ordered phase. In addition, apoA-I extracted lipids from the outer and inner leaflets of LUVs simultaneously. These results suggest that the heterogeneous interface of the mixed membranes facilitates the insertion of apoA-I and induces L d phase-selective but leaflet-nonselective lipid extraction to form rHDL; they are compatible with recent cell works on apoA-I-dependent HDL generation.-Fukuda, M., M. Nakano, S. Sriwongsitanont, M. Ueno, Y. Kuroda, and T. Handa. Spontaneous reconstitution of discoidal HDL from sphingomyelin-containing model membranes by apolipoprotein A-I. J. Lipid Res. 2007. 48: 882-889.

Egg phosphatidylcholine (EggPC) or dimyristoylphosphatidylcholine (DMPC) liposomes containing polyethylene glycol (PEG)-lipids covering a range of 0-30 mol% have been prepared by Extrusion method. The physicochemical properties including size evolution and calcein permeation were evaluated to investigate the effect of PEG-lipids on bilayer structure. The results from quasielasetic light scattering (QELS), freeze-fracture microscopy, and gel exclusion chromatography revealed that presence of low concentration of PEG-lipid results in decreasing of vesicle size and further increase in the PEG-lipid concentrations lead to a transition from the lamellar membranes to micelles. The permeability for calcein increased with increase in concentration of distearoylphosphatidylethanolamine (DSPE)-PEG. On the other hand, the permeability decreased with low amount of cholesterol-PEG (blow 20% cholesterol-PEG) and increased with high amount of it. The maximum concentration of PEG-lipid that may be incorporated without alteration of the liposome structure depends on the composition of the bilayer. The concentration of DSPE-PEG2000 incorporated into vesicles without damaging vesicle structures were <20 mol% for EggPC and <10% for DMPC.

No abstract

The influence of the freeze-thawing process on the size and lamellarity of multilamellar PEG-lipid liposomes prepared from a mixture of egg yolk phosphatidilcholine (EggPC) and distearoyl phosphatidylethanolamine polyethyleneglycol (DSPE-PEG) 2000 was investigated. Trapped volume measurement, quasielastic light scattering (QELS) and freeze-fracture electron microscopy were used to estimate the morphology and lamellarity of liposomes. During the freeze-thawing process, the lamellarity of multilamellar vesicles (MLVs) depended strongly on both PEG-lipid concentration and the number of freeze-thaw cycles. The decrease in the number of lamellae was a function of the number of freeze-thaw cycles. The increase in trapped volume coincided with the decrease in the number of lamellae observed in electron micrographs. After comparing the results obtained from EggPC/DSPE-PEG2000 MLV and from pure EggPC MLV, it was concluded that during the freeze-thawing the liposomes with DSPE-PEG2000 achieved a unilamellar structure more readily than the pure EggPC liposomes.