Interaction of tryptic peptides of apolipoprotein B-100 with dimyristoylphosphatidylcholine

Apolipoprotein B-100, the major protein constituent of human plasma low-density lipoproteins (LDL), was carboxyamidomethylated, digested with trypsin and the water-soluble tryptic peptides were coincubated with liposomes of dimyristoylphosphatidylcholine (DMPC). At 24.3 degrees C the peptides induced lipid solubilization as evidenced by optical clearing of the lipid-peptide mixture. Lipid-peptide complexes were isolated by density-gradient ultracentrifugation in KBr and had the following properties: DMPC/peptide ratio of 5.6 (w/w); buoyant density of 1.07-1.09 g/ml; discoidal morphology (51 +/- 4 X 260 +/- 28 A) as determined by electron microscopy; and molecular weight of 1.5 X 10(6) as determined by nondenaturing polyacrylamide gel electrophoresis. Compared to liposomes and sonicated vesicles of DMPC, the lipid-peptide complexes had a more rigid structure as assessed by fluorescence polarization. Whereas intact LDL had 42% alpha-helix and 15% beta-pleated sheet, the lipid-peptide complexes contained 70% alpha-helix and less than 5% beta-pleated sheet. The lipid-peptide complexes did not bind to the fibroblast high-affinity LDL receptor. These results show that specific regions in apolipoprotein B-100 which interact with phospholipid have an amphipathic character and may represent primary sites for lipid-protein interaction in LDL.