The influence of cholesterol on the assembly and structure of model high-density lipoproteins (HDL) has been investigated. Model HDL composed of apolipoprotein A-I (apoA-I) and 1,2-dimyristoylphosphatidylcholine (DMPC) formed spontaneously at the transition temperature (Tc) of the lipid. Those composed of apoA-I and 1-palmitoyl-2-oleoylphosphatidylcholine were formed by a cholate dialysis method. At low cholesterol/phospholipid ratios both lipids and assembly methods yielded a model HDL whose composition was identical with that of the initial mixture; as the cholesterol/phospholipid ratio of the initial mixture was increased, the fraction of cholesterol appearing in the model HDL decreased, and a negative correlation between the cholesterol and protein contents of the model HDL was observed. At high cholesterol/phospholipid ratios the association of apoA-I and phospholipids appeared to be thermo-dynamically unfavorable. The effects of cholesterol content on the thermal properties of a model HDL composed of DMPC and apoA-I were further investigated by differential scanning calorimetry, fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, fluorescence energy transfer, and excimer fluorescence of pyrenyl derivatives of phosphatidylcholine (PC) and cholesterol. The addition of cholesterol decreased the transition enthalpy of DMPC, raised the midpoint of the transition, and modulated motional freedom in the phospholipid matrix. The amount of cholesterol required to produce these effects was lower in the model HDL than in multilamellar liposomes. In a model HDL composed of DMPC and apoA-I, the lateral diffusion of a pyrene-labeled cholesterol was dramatically changed at the Tc whereas little change was observed in that of a pyrene-labeled PC. The relative locations of a fluorescent PC and cholesterol derivative in a model HDL were measured by fluorescence energy transfer from the tryptophan residues of the protein to the probe molecules. On the average, the fraction of cholesterol adjacent to the protein, though measurable, was less than that of the PC; therefore, the cholesterol is partially excluded from the phospholipid surrounding the protein but has the same modulating effect on the remainder of the phospholipid in the recombinant as that seen in multilayer liposomes. The effect of cholesterol on the reassembly of model lipoproteins appears to be due to the competition between cholesterol and apoA-I for "hydrophobic solvation" by the phospholipid. The results suggest that PC is preferentially included and cholesterol is partially excluded from the region adjacent to apoA-I.
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