title: Phase behaviour of mixtures of lipid X with phosphatidylcholine and phosphatidylethanolamine.
abstract: The effect of increasing concentrations of lipid X (2,3-bis(3-hydroxymyristoyl)-alpha-D-glucosamine 1-phosphate) on the phase behaviour of EPC (egg phosphatidylcholine) and EPE (egg phosphatidylethanolamine) is studied at a pH greater than or equal to 7 where lipid X carries one to two negative charges. Small amounts of lipid X (molar ratio approximately 0.01) induce continuous swelling of EPC and EPE bilayers and consequently the formation of large unilamellar vesicles in excess water. In many respects, the effect of lipid X on EPC and EPE bilayers is similar to that of phosphatidic acid. However, lipid X/EPC mixtures form micelles in excess lipid X whereas mixtures of phosphatidic acid/EPC vesiculate at all ratios. The same is true for lipid X/EPE mixtures. Small unilamellar vesicles of an average diameter of 40 nm form spontaneously upon dispersion of a dry lipid X/EPE film (molar ratio = 10). Unsonicated dispersions of lipid X/EPC (molar ratio = 1) are subjected to pH-jump treatment which involves raising of the pH to 11-12 and subsequent lowering of the pH to between 7.5 and 8.5. Such a treatment has little effect on the vesicle size and size distribution as compared to a control dispersion at pH 8.2. The mean size is determined to be 92 +/- 60 nm. Electron micrographs of freeze-fractured samples of lipid X/EPC (molar ratio = 1) reveal the presence of mainly micelles at pH 12. Upon lowering the pH to neutrality these micelles become unstable and aggregate/fuse rapidly to unilamellar vesicles (average diameter 95 +/- 40 nm). Sonication of equimolar mixtures of lipid X and EPC at pH 7 yields small unilamellar vesicles of a diameter of 20-25 nm as well as mixed micelles of a size between 15 and 17 nm. This behaviour is again different from that of mixed EPC/phosphatidic acid dispersions which form small unilamellar vesicles. The presence of lipid X in such mixtures does not prevent the aggregation/fusion to larger vesicles during freezing of the dispersion. As with pure EPC bilayers, stabilization is, however, achieved in the presence of 10% sucrose. This indicates that the covalently bonded glucosamine group of lipid X cannot substitute water of hydration in neighbouring EPC molecules.
authors.affiliation: Laboratorium für Biochemie, ETH Zürich, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Lipka
authors.forename: H
authors.initials: H
authors.lastname: Hauser
mesh_terms: D002850:Chromatography, Gel; D005614:Freeze Fracturing; D005615:Freezing; D006017:Glycolipids; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D008050:Lipid A; D008051:Lipid Bilayers; D008081:Liposomes; D009682:Magnetic Resonance Spectroscopy; D008854:Microscopy, Electron; D010316:Particle Size; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D013010:Sonication; D013053:Spectrophotometry
chemical_list: D006017:Glycolipids; D008050:Lipid A; D008051:Lipid Bilayers; D008081:Liposomes; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; C044283:lipid X