We have used thrombin to cleave apolipoprotein C-III-1 into two fragments constituting residues 1-40 (apoLP-C-III-A) and 41-79 (apoLP-C-III-B). The lipid binding properties of these fragments with dimyristoyl- and 1-palmitoyl-2-oleoylphosphatidylcholines have been determined using circular dichroic and intrinsic tryptophan fluorescence spectroscopy. The peptide-phospholipid mixtures were fractionated by density gradients of cesium chloride. ApoLP-C-III-A showed disordered structure in the absence and presence of DMPC and no significant amount of peptide-phospholipid complex was isolated. ApoLP-C-III-B showed conformational changes in the circular dichroic spectrum and a shift in the intrinsic tryptophan fluorescence spectrum. Ultracentrifugation in cesium chloride gradients yielded peptide-phospholipid complexes isolated between density 1.10 and 1.18. The molar ratio of lipid to protein was 12:1. The results of these studies and the examination of space filling models of apoLP-C-III provide evidence that an amphipathic a helix which contains a nonpolar face and a polar face is the basic structural unit for binding of phospholipid by the plasma apolipoproteins. These results also provide direct evidence that the hydrophobicity of the nonpolar face is important in lipid binding since the nonpolar face of residues 1-40 is considerably less hydrophobic than the nonpolar face of residues 41-79.
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