Transverse Membrane Asymmetry in Model Phospholipid Bilayers:

摘要

Coliposomes of 1:7 head group labeled-7-nitro- 2-oxa-1,3-diazol-4-yl (NBD)phosphatidylethanolamine (PE) and dipalmitoylphosphatidylcholine (DPPC) were created by extrusion at pH 7.4. The coliposomes were 345 A in average diameter (light scattering) and had a gel to liquid crystal phase transition (Tc) at 40 deg C (differential scanning calorimetry). The distribution of NBD-PE lipids in the outer (exo) and inner (endo) leaflets of the liposomal bilayers was 56:44, as determined by exo-specific NBD reduction by added dithionite, monitored by following the decay of NBD absorbance at 450 nm. Exo-specific dithionite reductionalso surface differentiated the coliposomes, leaving intact NBD-PE probelipids only in the endo leaflet. The endo-labeled coliposomes were freedof excess dithionite by ultrafiltration. Coliposomes with NBD-PE only intheir exo leaflets were made by adding ethanolic NBD-PE to DPPC liposomes. The dynamics of NBD-PE exo or endo translocation (flop) or endo or exo migration (flip) were