Bilayer distribution of phosphatidylserine and phosphatidylethanolamine in lipid vesicles.

The distribution of phosphatidylethanolamine (PE) and phosphatydilserine (PS) in liposomes was studied as a function of aminophospholipid concentration using fluorescamine as labeling reagent. The method is suitable for such determination since, in the assay conditions, fluorescamine does not penetrate the vesicles nor does it disrupt them. The liposomes were obtained by sonication, extrusion, or mechanical dispersion (MLV). For any kind of vesicle, the percentage of PS in the external monolayer is higher than that obtained for PE in the corresponding vesicles. In extruded PS liposomes, this aminophospholipid is located preferentially in the outer layer, while for PE liposomes the localization depends on the size of vesicle. Sonicated liposomes present an asymmetrical distribution of both aminophospholipids, and the external location of PS or PE always predominates. In contrast, in MLV, aminophospholipids are mainly found in the inner layers of the vesicles, except for liposomes formed by the lowest PS proportion. A remarkable feature of PS liposomes is the reduction of vesicle size, especially in MLV liposomes, in comparison with neutral liposomes.