Interactions of dipalmitoyl- and dimyristoylphosphatidylcholines and their mixtures with apolipoprotein A-I.

Human and bovine A-I apolipoproteins were incubated with multibilayer liposomes of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) and several mixtures of these two lipids. The reactions were carried out at temperatures around the transition temperature of the lipids, and the formation of small, micellar complexes of protein with lipid was followed as a function of time. Micellar complexes were isolated by ultracentrifugation and were characterized in terms of stoichiometry, lipid composition by gas chromatography, approximate size by gel filtration, and phase transition behavior by fluorescence polarization measurements. The results indicate a decrease in reaction rates with increasing DPPC contents of the mixtures, consistent with the higher stability of DPPC bilayers. Reactions have optimal rates at the transition temperature and are limited to the temperature range where gel and liquid-crystalline phases coexist. The isolated complexes with DMPC and DPPC have similar molecular weights in the range from 2 X 10(5) to 2.5 X 10(5), but lipid/protein mole ratios differ by about 40%. The lower lipid/protein ratio of DPPC complexes (100:1 mol/mol) is compensated by the longer acyl chains of this lipid, such that the acyl chain area of both complexes stabilized by apolipoprotein is essentially identical.