Interaction of human and bovine A-1 apolipoproteins with L-alpha-dimyristoyl phosphadicylcholine and L-alpha-myristoyl lysophosphatidylcholine.

The major protein components from human and bovine high density serum lipoproteins (apo-A-I proteins) were investigated in their interactions with L-alpha-myristoyl lysophosphatidylcholine and L-alpha-dimyristoyl phosphatidylcholine. Complex formation was followed by 25 degrees by observing changes in fluorescence polarization, rotational relaxation times (ph), and CD spectra of the proteins, covalently labeled with fluorescent dimethylaminonaphthalene sulfonyl groups. Monomeric human apo-A-I and initially oligomeric bovine apo-A-I interact with similar efficiency with the same amphiphiles. During binding of L-alpha-myristoyl lysophosphatidylcholine and L-alpha-dimyristoyl phosphatidylcholine, the structure of both proteins changes drastically, exhibiting about 35% increases in secondary structure with the phospholipid and about 20% increases with the lysophospholipid. Simultaneously, regions of the proteins adjacent to the fluorescent probes become more mobile. With the bovine protein, binding of both amphiphiles results in changes in the oligomeric structure: dissociation with L-alpha-myristoyl lysophosphatidylcholine and dissociation or rearrangement with L-alpha-dimyristoyl phosphatidylcholine. The complexes formed in the presence of excess L-alpha-myristoyl lysophosphatidylcholine are flexible structures with considerable rotational freedom. The largest rotational unit in these complexes has ph = 60 ns for both proteins; by comparison, the monometic human apo-A-I has a ph = 73.5 ns. Interactions of both proteins with the lysophospholipid take place well above its critical micelle concentration, but probably involve binding of monomeric lipid. With L-alpha-dimyristoyl phosphatidylcholine the complexes are different from those formed with the lysophospholipid. They have limiting ph values of 250 +/- 50 and 280 +/- 60 ns with the human and bovine proteins, respectively. These ph values are consistent with particles of the general size of human high density lipoprotein rather than of liposomes and indicate the formation of distinct, relatively small structures upon interaction of the self-associated lipid with the apo-A-I-proteins.