Structure of an apolipoprotein-phospholipid complex: ApoC-III induced changes in the physical properties of dimyristoylphosphatidylcholine

The effect of ApoC-III, a major apoprotein constituent of human very low density lipoproteins, on the physical properties of dimyristoylphosphatidylcholine (DMPC) vesicles has been studied by magnetic resonance and fluorescence techniques. The sharp gel-liquid crystalline transition usually observed at 23 C in DMPC is both broadened and elevated when ApoC-III is bound as determined (a) from measurements of microscopic viscosity by pyrene excimer fluorescence, (b) from the distribution of di-tert-butyl nitroxide between the bulk aqueous phase and the fluid lipid phase, and (c) from the motion of fatty acyl chains of spin-labeled phosphatdylcholine. Experiments involving the translocation of ascorbate and charged nitroxide ions and the movement of paramagnetic Eu 3+ ions indicate that when ApoC-III binds to DMPC vesicles, it increases their permeability or destroys their original bilayer structure. These two possibilities were distinguishable by gel filtration of the DMPC-ApoC-III complex (approximately 34 mol mol) that indicated that the product particles were significantly smaller than the original vesicles. Taken together, the data indicate that ApoC-III binding to DMPC not only decreases the acyl chain motion of individual lipid molecules, but also induces break-down of bilamellar vesicular structure to give significantly smaller complexes.