Kinetics and Mechanism of the Spontaneous Transfer of Fluorescent Phosphatidylcholines between Apolipoprotein-Phospholipid Recombinants

The kinetics and mechanism of the spontaneous transfer of fluorescent phosphatidylcholines (PCs) between model lipoproteins were studied with PCs that contained 9-(1-pyrenyl)nonanoic acid in the sn-2 position and myristic, palmitic, stearic, oleic, or linoleic acid in the sn-1 position. The model lipoproteins were recombinants of apolipoproteins A-I (apoA-I) and A-II (apoA-II) from human high-density lipoproteins and the synthetic phospholipids dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-palmitoleoylphosphatidylcholine (PPOPC). The kinetics of transfer of the fluorescent PCs were first order and independent of acceptor concentration and structure when recombinants of different apolipoprotein-phospholipid stoichiometries were used. The addition of neutral salts changed the rate of transfer by almost 3 orders of magnitude. The rates of transfer were also influenced by properties of the lipid-protein surface of the recombinant donor; transfer half-times were 5 times slower from a 240/1 than from a 45/1 DMPC/apoA-II recombinant. The rate of transfer of a pyrene-labeled PC was dramatically affected by the fatty acyl chain composition in the sn-1 position. From a 45/1 DMPC/apoA-II recombinant, the half-times of transfer were 9.6, 114, 630, 6.9, and 74.5 min for the myristoyl-, palmitoyl-, stearoyl-, linoleoyl-, and oleoylphosphatidylcholine derivatives, respectively. Thus, the addition of two methylene units to the acyl chains decreased the rate by a factor of 8-10 whereas the addition of a double bond increased the rate by a factor of 10. When a PPOPC matrix was used, the change was a factor of 5-7 per two methylene units and about 3-4 per double bond. Increases in the transfer rate with fatty acyl composition correlated with changes in the aqueous solubility of monomeric phospholipids that could be predicted from the critical micelle concentration of homologous lipids. The dramatic effects of salts, acyl chain length, and unsaturation on the rates of transfer are substantial proof that these phospholipids transfer as monomers via the aqueous phase. The transfer of the pyrenyl-PCs was highly temperature dependent. Activation energies for PC transfer with the host matrix in the liquid-crystalline state was 23-26 kcal/mol. By contrast, the activation energy decreased when the lipid matrix was in the gel state, and the magnitude of change in the enthalpy of transition from the gel to liquid-crystalline state correlated with the differences in the activation energy. Thus, the physical state of the lipid in the donor complex affects the magnitude of the activation energy. Because the spontaneous transfer of phospholipids can produce a net flux that is a function of the structure of both the lipid and the lipid-protein surface from which the lipid dissociates, this mechanism of equilibration can be quantitatively important for unsaturated or short-chain PCs.