Interaction of L-alpha-palmitoyl lysophosphatidylcholine with the AI polypeptide of high density lipoprotein.

The AI polypeptide chain from human high density serum lipoprotein has two accessible conformational states in aqueous solution. L-alpha-Palmitoyl lysophosphatidylcholine induces the transition between these two states at an equilibrium concentration of ligand of 2 X 10(-5)M, and the protein has a maximum binding capacity of 95 to 100 mol of lipid/mol of protein. The present study, together with previous investigations in this laboratory, suggests that the conformational state of AI in the presence of high levels of bound amphiphiles is similar to the in vivo state, and further, that this complex does not result from the insertion of AI into amphiphilic micelles. The mode of interaction of AI with amphiphilic ligands is shown to be significantly different from that of membrane proteins thus far investigated.