Kinetics of phosphatidylcholine and lysophosphatidylcholine exchange between unilamellar vesicles.

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 = 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)