The Interaction of Apoprotein from Porcine High‐Density Lipoprotein with Dimyristoyl Phosphatidylcholine

The morphology and structure of lipoprotein complexes reconstituted from apoprotein from porcine high‐density lipoprotein and dimyristoyl lecithin have been studied by a range of physical techniques. At pH 8.6 and ionic strength I ≳ 0.1, disc‐shaped particles are observed with a radius of 4.0–4.5 nm and a thickness of 4.36 ± 0.07 nm, the latter value being in good agreement with that of the average lipid bilayer thickness. The values for the number, weight and Z‐average particle weights are M n = 227000 daltons < M W = 325000 daltons < M Z = 410000 daltons, respectively, and indicate the polydispersity of these complexes. The number‐average particle weight is in good agreement with that of 236000 daltons determined from sedimentation and diffusion experiments. The latter particle weight is also consistent with the particle volume, determined from the dimensions of the disc, and the experimental partial specific volume of the complex, ū 25 = 0.917 ml/g. At the point where all the apoprotein is complexed the average lipid/protein molar ratio is 100 ± 6. Particle size and weight analysis indicate that each lipoprotein disc of radius 4.0–4.5 nm consists of 200 lipid and 2 apoprotein molecules. If lecithin is present in excess of the complex described above, all the lipid may interact with apoprotein up to molar lipid/protein ratios of 200:1 depending on the sonication conditions used. The lipoprotein discs undergo an ionic‐strength‐dependent monomer‐dimer equilibrium with low ionic strength leading to the side‐by‐side aggregation of discs. At an ionic strength I ≥ 0.01 the disc radius is about twice that observed at I ≥ 0.1. The results with dimyristoyl lecithin — apoprotein complexes are compared with those obtained with dialauroyl and dipalmitoyl lecithin—apoprotein complexes. Lecithin and apoprotein interact to form a loose, disc‐shaped complex in which the apoprotein is oriented at the lipid‐water (hydrophobic‐hydrophilic) interface such that all the polar groups of both lipid and protein are readily accessible to the aqueous medium. Most of the lipid is present as a bilayer, however the cooperative motion of the lipid is significantly reduced due to the interaction with apoprotein. The latter shows a significant increase in α helical content from 50% to about 60–65% when complexed with lecithin. The dispersing (solubilizing) effect which the apoprotein has on unsonicated multilayers of lecithin is explained in terms of its surface activity and conformational properties which may allow for the regional separation (accumulation) of opposite charges.