Effect of lipid particle size on association of apolipoproteins with lipid

Triolein particles stabilized with egg yolk phosphatidylcholine monolayer were prepared with two different diameters: 26.7 +/- 3.9 and 229 +/- 80 nm. All the phosphatidylcholine molecules in those particles were readily digested by phospholipase A2 while only the molecules in the outer leaflet of phosphatidylcholine unilamellar vesicles were hydrolyzed under the same conditions. Binding of human plasma apolipoproteins A-I, A-II, C-II, and C-III2 to the particles was studied by two independent techniques: (i) rapid gel permeation chromatography and (ii) ultracentrifugation. All four apolipoproteins bound to the small and large particles in a saturable manner without altering their gross structure, and were displaced by equivalent molecules. The dissociation constant of apolipoprotein A-I for the large particle was 3.17 X 10(-6) M and 4.24 X 10(-6) M by methods (i) and (ii), respectively. These values were more than 10-fold greater than those for the small particles (2.0 X 10(-7) and 1.6 X 10(-7) M, respectively). In contrast, apolipoproteins A-II, C-II, and C-III2 bound to the large particles as strongly as to the small particles with dissociation constants of 2.4-6.8 X 10(-7), 4.5-10.7 X 10(-7), and 5.3-10.7 X 10(-7) M, respectively. The maximum binding level was of a similar order for each of the four apolipoproteins with both lipid particles when they were compared on the basis of amino acids per phospholipid. These results suggest that the apolipoproteins share common binding sites on the lipid particles, and are consistent with the characteristic distribution of apolipoproteins A-I and C among various classes of lipoproteins in plasma.