Surface-to-core and interparticle equilibrium distributions of triglyceride-rich lipoprotein lipids.

The phase equilibria of human very low density lipoprotein (VLDL) and monkey chylomicron lipids was examined. Triglyceride (TG), cholesterol ester (CE), and cholesterol (C) partitioned into both surface monolayer and oil "core" phases of emulsions of lipoprotein lipids, whereas phospholipid was found exclusively in the surface. In addition to phospholipid, the surface lipids consisted of 2-4% (by weight) TG, less than 1% CE, and 22% C (VLDL), 5-8% C (chylomicrons). The oil lipids consisted mainly of TG, but also 13-16% CE (VLDL), 3% CE (chylomicrons), and 1-2% C (VLDL), 0.3-0.4% C (chylomicrons). The equilibrium state of lipids within size subfractions of native lipoproteins was defined using phase diagram analysis. Subfractions were in equilibrium with respect to surface-to-core and interparticle distributions of C molecules. In contrast, subfracitions were not in a state of interparticle TG and CE equilibrium. By using the phase diagrams, the percentages of the total particle lipids carried in the phases of lipoproteins of varying size were calculated. For Sf greater than 400 particles, greater than or equal to 30% of the total particle C molecules are carried in the core, and less than 3% of the TG and CE molecules are located in the surface. Nascent, plasma, remnant, and beta-migrating TG-rich lipoprotein compositions taken from the literature were compared using phase diagrams. Although the total compositions of nascent liver VLDL and lymph chylomicrons vary substantially, they have identical concentrations of C in their respective phases. Upon equilibration with plasma, the surface and core of nascent TG-rich lipoproteins become enriched (2-4-fold) with C. Remnants and beta-VLDL have the most C-rich phases of the TG-rich lipoproteins examined. The analysis indicates that the C concentrations of the phases of a lipoprotein are related to its metabolic status.