Association of apolipoprotein A-II with egg phosphatidylcholine unilamellar vesicles

Interaction of human apolipoprotein A-II (apoA-II) with egg yolk phosphatidylcholine unilamellar vesicles (diameter, 20-26 nm) was studied. ApoA-II bound to the vesicles in a saturable manner and was displaced by apolipoprotein C-III1 (apoC-III1) while bound apoC-III1 was displaced by apoA-II, with apparent deformation of the vesicles, as observed in electronmicrographs. Free apoA-II that had been displaced from the lipid surface was readsorbed to vesicles successively added to the medium. Disruption of the vesicular structure occurred in less than 3% of the vesicles under the experimental conditions. Isothermal binding data were analyzed according to a reversible equilibrium binding model. The dissociation constant, Kd, was 5.8 X 10(-7) M, and the maximum binding level, N, was 1.46 amino acid residues per phospholipid for apoA-II assuming that 70% of the phospholipid is in the outer leaflet of the vesicles, while for apoC-III1 the Kd and N values were 7.8 X 10(-7) M and 1.34 amino acid residues per phospholipid in the outer leaflet, respectively. All these parameters show good agreement with those for triglyceride-phospholipid particles of approximately the same diameter as that of the vesicles (J. Biol. Chem. (1983) 258, 10073-10082). Thus the parameters represent the equilibrium binding of apoA-II to the surface of phosphatidylcholine unilamellar vesicles.