Physical properties of lipid-protein complexes formed by the interaction of dimyristoylphosphatidylcholine and human high-density apolipoprotein A-II

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Abstract
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. Temperature dependence is related to the gel .fwdarw. liquid crystalline transition temperature, Tc, of DMPC which occurs at 23.9.degree. C. At T < Tc (20.degree. C), T = Tc, and T > Tc (30.degree. C), 3 different complexes can be isolated. At 20.degree. C, a 75:1 (molar ratio of lipid to protein) complex is formed. This complex has a MW of 343,000, a Stokes radius, Rs, of 65 .ANG., and a partial specific volume (.hivin..nu.) of 0.914 mL/g. At 24.degree. C, 2 different complexes may be formed. One is similar to the 1 formed at 20.degree. 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 MW = 1,580,000, Rs = 120 .ANG., and .hivin..nu. = 0.948 mL/g. This complex is asymmetric, having a frictional coefficient f/fo = 1.20. At 30.degree. C, a 45:1 complex was formed; for this complex, MW = 229,000, Rs = 57 .ANG. and .hivin..nu. = 0.892 mL/g. Electron microscopy reveals that the negatively stained complexes are arranged in rouleaux having subunits with average dimensions of 175 .times. 60, 250 .times. 62, and 500 .times. 55 .ANG. 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 > 1 macromolecular species of lipid and apoA-II in the plasma.

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