Apolipoprotein A-II (apoA-II) from human plasma high-density lipoproteins associates with dimyristoylphosphatidylcholine (DMPC) to give complexes whose structure is determined by the temperature at which the reaction is conducted. The temperature dependence is related to the gel → liquid crystalline transition temperature, Tc, of DMPC which occurs at 23.9°C. At T < Tc (20°C), T = Tc, and T > Tc (30°C), three different complexes can be isolated. At 20°C, a 75:1 (molar ratio of lipid to protein) complex is formed. This complex has a molecular weight (Mr) of 343 000, a Stokes radius, Rs, of 65 Å, and a partial specific volume (v̄) of 0.914 mL/g. At 24°C, two different complexes may be formed. One is similar to the one formed at 20°C and the other is a complex with a DMPC:apoA-II ratio of 240:1; the corresponding physical constants for the latter complex are Mr = 1 580 000, Rs = 120 Å, and v̄ = 0.948 mL/g. This complex is asymmetric, having a frictional coefficient f/f0 = 1.20. At 30°C, a 45:1 complex was formed; for this complex, Mr = 229 000, Rs = 57 Å, and v̄ = 0.892 mL/g. Electron microscopy reveals that the negatively stained complexes are arranged in rouleaux having subunits with average dimensions of 175 × 60, 250 × 62, and 500 × 55 Å for the 45:1, 75:1, and 240:1 complexes, respectively. The multiple lipid-protein species formed by apoA-II and DMPC suggest the possible existence of more than one macromolecular species of lipid and apoA-II in the plasma.
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