Reassembly of human apoproteins A-I and A-II with unilamellar phosphatidylcholine-cholesterol liposomes. Association kinetics and characterization of the complexes.

The kinetics of association between the human apoprotein A-I and apoprotein A-II and cholesterol dimyristoyl phosphatidylcholine (DMPC) vesicles are compared in this study and the lipid-apoprotein complexes are characterized. The association kinetics are followed by turbidity measurements monitoring the decrease of the vesicular size and by fluorescence polarization measurements monitoring the decrease in the mobility of the phospholipid acyl chains during complex formation. The influence of the incubation temperature and of the cholesterol/DMPC ratio has been studied by both techniques. Under all incubation conditions the apoprotein A-II associates more readily with cholesterol-DMPC vesicles than apoprotein A-I, as the kinetics are faster and the complex yield larger. With both apoproteins optimal complex formation takes place around the phospholipid transition temperature and around 10 mol% cholesterol. The apoprotein A-I/lipid association seems restricted to this narrow range for the temperature and the cholesterol/DMPC ratio, while the apoprotein A-II still associates with vesicles containing 20 mol% cholesterol and at temperatures up to 32 degrees C. The lipid-apoprotein complexes were isolated by gradient ultracentrifugation and by gel chromatography. According to these data the apoprotein A-II associates more readily than apoprotein A-I with cholesterol-DMPC vesicles to form protein-rich complexes, whilst the optimal apoprotein A-I-lipid association requires a more disordered lipid structure.