Creation and Relaxation of Phospholipid Compositional Asymmetry in Lipid Bilayers Examined by Sum-Frequency Vibrational Spectroscopy

摘要

Eukaryotic cells contain an asymmetric distribution of phospholipids in the two leaflets of the lipid bilayer which is known to contribute to cellular function.'-'2 In the plasma membrane of eukaryotic cells, the aminophospholipids with phosphatidylserine (PS) and phosphatidylethanolamine (PE) headgroups are predominately located on the cytosolic leaflet while sphingolipids with phosphatidylcholine (PC) headgroups and sphingomeylin are on the extra-cellular leaflet." There have been a number of theories about the mechanism of transbilayer movement of lipids in cellular systems and the physical process by which lipid compositional asymmetry in the plasma membrane of eukaryotic cells is maintained. It is generally accepted that a significant barrier to native lipid translocation (movement between leaflets of the bilayer) exists which is related to the energetic penalty of moving the hydrophilic headgroup of a phospholipid through the hydrophobic core of the membrane.13'14 Overcoming this energetic barrier represents the rate limiting step in the spontaneous flip-flop of phospholipids in biological membranes, yet, while numerous kinetic studies of phospholipid flip-flop have been conducted15-22, few researchers have reported thermodynamic parameters for the process.