Interaction of apoliprotein C-III with phosphatidylcholine vesicles. Dependence of aproprotein-phospholipid complex formation on vesicle structure

We have studied the interaction of an apolipoprotein from human very low density lipoproteins (apoC-III) with egg yolk phosphatidylcholine in the form of single- and multi-bilayer vesicles. The reactivity of single-bilayer vesicles with apoC-III₁ appears to be greater than that of the multi-bilayer vesicles according to several thermodynamic and spectroscopic criteria. In the complexes formed by the association of apoC-III with single-bilayer vesicles, the α-helical content of the peptide backbone and the apolarity of the environment around the tryptophan residues are greater than that observed in the complexes formed with the multibilayer vesicles. A higher yield and more homogeneous density distribution of lipid-apoprotein complexes results from the interaction of apoC-with the single-bilayer vesicles relative to those obtained with the multibilayer vesicles. The enthalpy of association of apoC-III₁ with phospholipid was greater for the single-shelled vesicles (25 kcal/mol apoC-III₁) than for the multishelled ones (18 kcal/mol apoC-III₁). The difference in reactivity of these two types of liposomes is not due to a difference in their fluidities since their fatty acid compositions are identical, but may be due to a difference in their areas of sterically accessible phospholipid, their permeabilities to the apoprotein, their radii of curvation, or a combination of these factors.