Kinetics of phosphatidylcholine and lysophosphatidylcholine exchange between unilamellar vesicles

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 = 2.0 h at 37 degrees C. A discontinuity is observed in Arrhenius plots of DMPC exchange; the activation energy over the temperature range 27-45 degrees C is 70 kJ mol-1. The t1/2 for DMPC exchange extrapolated to 24.5 degrees C (the phase transition temperature of the donor bilayer) is 6.5 h and from temperatures below 24 degrees C is 82.6 h. The differences in the thermodynamic parameters of activation for DMPC exchange above and below 24.5 degrees C are 25 kJ mol-1 for the activation enthalpy and 197 J mol-1 K-1 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 lysopalmitoyl-phosphatidylcholine (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 less than 2 min. LPPC at 5 mol % in cholesterol-egg PC vesicles does not affect the rate of cholesterol exchange.(ABSTRACT TRUNCATED AT 250 WORDS)