Thermal dependence of apolipoprotein A-I/phospholipid recombination

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Abstract
Studies of the recombination of apolipoprotein A-I (apo a-I), the major protein constituent of human high-density lipoprotein, and various synthetic phospholipids, both alone and in mixtures, were performed. Pure diacyl phospholipids containing homologous fatty acids of the C12, C13, C14 and C15 series, and the 2 positional isomers containing C14 and C16 fatty acids in positions 1 and 2, undergo reaction with the apo A-I protein only near their gel-liquid-crystalline transition temperatures; the degree of reactivity of these phospholipids toward recombination was observed to decrease as the transition temperature increased. The presence of lysolecithin in the incubation mixtures, at proportions of 5 mol/mol of protein or lower, did not have a significant effect on the rate of recombination. Binary mixtures of dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine at various proportions react maximally with apo A-I at the onset of the phase transition, as judged by comparison with published phase diagrams; in this case also, the rate of recombination was observed to decline for mixtures with higher phase transition temperatures. These results are interpreted in terms of protein insertion at lattice defects which arise from the presence of phospholipid clusters undergoing the phase transition; these clusters are derived from the cooperative and simultaneous melting of a number of molecules, the cooperativity being dependent upon the nature of the phospholipid. Phospholipids which melt in a more highly cooperative fashion are apparently more capable of interacting with the apolipoproteins since these phospholipids will possess larger lattice defects during the phase transition.

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