Kinetics of phosphatidylcholine and lysophosphatidylcholine exchange between unilamellar vesicles

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Abstract
The rates of exchange of phosphatidylcholine and lysophosphatidylcholine from unilamellar donor vesicles to acceptor vesicles of similar composition were followed in a protein-free system to establish the relationship between the rate of exchange and the aqueous-phase solubility of the lipid. Further, the rate of exchange of dimyristoylphosphatidylcholine (DMPC) between vesicles was examined over a range of temperatures to determine the effect of the lipid phase transition on the rate of lipid exchange. Intervesicular exchange of DMPC is faster than transbilayer exchange; lipid molecules in the outer monolayer of the bilayer exchange with t1/2 [half-life] = 2.0 h at 37.degree. C. A discontinuity is observed in Arrhenius plots of DMPC exchange; the activation energy over the temperature range 27.degree.-45.degree. C is 70 kJ mol-1. The t1/2 for DMPC exchange extrapolated to 24.5.degree. C (the phase transition temperature of the donor bilayer) is 6.5 h and from temperatures below 24.degree. C is 82.6 h. The differences in the thermodynamic parameters of activation for DMPC exchange above and below 24.5.degree. C are 25 kJ mol-1 for the activation enthalpy and 197 J/mol per K for the activation entropy. These differences are similar to the enthalpy and entropy changes associated with the gel to liquid-crystalline phase transition of DMPC. The rate of exchange of lysopalmitoylphosphatidylcholine (LPPC) was difficult to measure since LPPC transfers rapidly to the columns used for separating donor and acceptor vesicles; the t1/2 for transfer is < 2 min. LPPC at 5 mol% in cholesterol-egg PC vesicles does not affect the rate of cholesterol exchange. The rates of exchange of cholesterol, LPPC, DMPC, dipalmitoylphosphatidylcholine, and 1-palmitoyl-2-oleoylphosphatidylcholine were used to calculate activate free energies for exchange of each molecule. The activation free energies and free energies of transfer from self-micelles to water increase by 2.2 and 2.1 kJ/mol per methylene group, respectively. Thus, the free energy of transfer is a good predictor of the relative exchange rates of lipid molecules. However, the activation free energies are 30 .+-. 1 kJ/mol > the free energies of transfer. This excess free energy is proposed to be associated with restriction of the lipid molecule to the surface of the vesicle in the transition-state complex.