Lipid Transfer Reactions: Fluorescence Studies

Abstract

In cells the transport of lipid molecules from sources of biosynthesis or ingestion to the various membranes and organelles represents an important component of metabolism (Pagano and Sleight, 1985). The analogous problems of protein synthesis, membrane assembly, turnover, and secretion are also being actively studied by cell biologists. The important sites of lipid trafficking include the plasma compartment of blood and the cells of the vascular and extravascular compartment. The lipid composition of these sites includes cholesterol, cholesteryl esters, triglycerides, fatty acids, and phospholipids. Figure 1 schematically illustrates several pathways of lipid transfer in a membrane; these include lateral diffusion of the lipid in the membrane plane, spontaneous desorption into the surrounding aqueous compartment with subsequent uptake by an adjacent membrane surface, and bilayer translocation, which is commonly referred to as flip-flop. This latter mechanism is often accelerated by enzymes termed “translocases”. Other lipid transfer mechanisms not pictured include transient collisional complexes of lipid surfaces with lipid exchange occurring at the contact point, protein mediated transfer between lipid surfaces involving specific lipid-carrier proteins (e. g., phospholipid exchange protein), enzymatic degradation of lipid molecules and subsequent release of soluble lipid fragments from the membrane, and finally, transport of large lipid aggregates, such as lipoproteins, across the cell membrane via endocytosis.