Kinetics and mechanism of the spontaneous transfer of fluorescent phospholipids between apolipoprotein-phospholipid recombinants. Effect of the polar headgroup.

Fluorescent derivatives of a phosphatidylglycerol, phosphatidylserine, phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine, and diacylglycerol have been studied to establish the effect of different polar headgroups on the mechanism and kinetics of spontaneous phospholipid transfer between recombinants of human plasma apolipoprotein A-II and dimyristoylphosphatidylcholine. The fluorescent lipids are all 1-myristoyl-2-[9-(1-pyrenyl)nonanoyl] glycerides. The transfer of the lipids is a first order process where the rate is independent of the concentration over a 50 fold range of the acceptor recombinants. These results are consistent with the lipids transferring as monomers being a water-soluble intermediate. The rate of transfer of the different phospholipids are slightly slower than phosphatidylcholine, with that of phosphatidylethanolamine being about 4 times slower. The transfer of phospholipids with a titratable headgroup is pH-dependent. The difference in the rates and pH dependence may be a function of the interactions (hydrogen bonding) between polar headgroups. The rate of transfer of the diacylglycerol is 20 times slower than phosphatidylcholine, but its activation energy (21 kcal/mol) is only 2 to 3 kcal less than most of the phospholipids (23 kcal/mol). These results suggest that the rate and activation energy for the spontaneous transfer of phospholipids can be predicted to a first approximation on the basis of its hydrophobic content, irrespective of the pH or identity of the polar headgroup.