Interaction of tryptic peptides of apolipoprotein B-100 with dimyristoylphosphatidylcholine

Abstract

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° 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 · 260 ± 28 A ̊ ) as determined by electron microscopy; and molecular weight of 1.5 · 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% α-helix and 15% β-pleated sheet, the lipid-peptide complexes contained 70% α-helix and less than 5% β-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.