Structure and Metabolism of Plasma Lipoproteins as Biological Emulsions

Abstract

Plasma triglyceride-rich lipoproteins, chylomicrons and VLDL, vary in lipid composition during their metabolism. Using lipid emulsions of defined composition as model particles, we investigated the effects of the lipid composition on the surface and core structures and the metabolism of the emulsion particles. Surface cholesterol significantly facilitated the removal of triglyceride-phosphatidylcholine emulsions from plasma after intravenous injection to rats, presumably due to the change in the ratio of bound apolipoprotein E (apoE) to Cs at the emulsion surface. In contrast, sphingomyelin, the second most abundant phospholipid in lipoproteins, delayed the emulsion clearance and showed the strong inhibition of the apoE binding and lipolysis by lipoprotein lipase. These changes in the emulsion metabolism caused by cholesterol and sphingomyelin are thought to result from the modification of the emulsion surface structures. On the other hand, cholestery oleate, major component of core lipids in LDL, was shown to retard the emulsion clearance by means of the decrease in the bound apoE content, which was closely correlated with the mobility of core lipids rather than the surface structure. These results indicate that not only the surface structure but also the core mobility modulate the emulsion metabolism through the alteration in the binding behavior of apolipoproteins.