Reassembled model lipoproteins. Lipid dynamics in recombinants of human apolipoprotein A-II and dimyristoylphosphatidylcholine.

Dimyristoylphosphatidylcholine (DMPC) and apolipoprotein A-II (apoA-II) combine to form three isolatable complexes of molar stoichiometry of 240:1 75:1, and 45:1, respectively. Steady state fluorescence depolarization studies with diphenylhexatriene (DPH) and parinaric acid incorporated into the DMPC/apoA-II complexes reveal that increasing the protein content reduces fluidity and increases the apparent lipid phase transition (Tc). Time-resolved studies with DPH reveal hindered rotational motion and a loss of fluidity with increasing protein content, arising from an increase in the order of the lipid assay. The parinaric acid probes could detect no lateral phase separation of lipid ensembles in the DMPC/apoA-II complexes, suggesting that the DMPC adjacent to the protein is as mobile as bulk lipid. Measurements of the excimer-forming properties of a pyrene lecithin analog show that the increasing protein content in the DMPC/apoA-II complexes reduces lateral diffusion of the lipid.