Apolipoprotein C-III displacement of apolipoprotein E from VLDL: effect of particle size.
ApoC-III and apoE are important determinants of intravascular lipolysis and clearance of triglyceride-rich chylomicrons and VLDL from the blood plasma. Interactions of these two apolipoproteins were studied by adding purified human apoC-III to human plasma at levels observed in hypertriglyceridemic subjects and incubating under specific conditions (2 h, 37 degrees C). As plasma concentrations of apoC-III protein were increased, the contents in both VLDL and HDL were also increased. Addition of apoC-III at concentrations up to four times the intrinsic concentration resulted in the decreasing incremental binding of apoC-III to VLDL while HDL bound increasing amounts without evidence of saturation. No changes were found in lipid content or in particle size of any lipoprotein in these experiments. However, distribution of the intrinsic apoE in different lipoprotein particles changed markedly with displacement of apoE from VLDL to HDL. The fraction of VLDL apoE that was displaced from VLDL to HDL at these high apoC-III concentrations varied among individuals from 20% to 100% its intrinsic level. The proportion of VLDL apoE that was tightly bound (0% to 80%) was found to be reproducible and to correlate with several indices of VLDL particle size. In the group of subjects studied, strongly adherent apoE was essentially absent from VLDL particles having an average content of less than 50,000 molecules of triglyceride. Addition of apoC-III to plasma almost completely displaces apoE from small VLDL particles. Larger VLDL contain tightly bound apoE which are not displaced by increasing concentration of apoC-III.