Mechanism of the phospholipid transfer protein-mediated transfer of phospholipids from model lipid vesicles to high density lipoproteins

To study the effects of the phospholipid transfer protein (PLTP) on the thermodynamic parameters governing the transfer of phospholipids (PL) from single bilayer vesicles (SBV) to high density lipoprotein (HDL), we performed transfer measurements at various temperatures between 4 and 65°C, using a pyrenylphosphatidylcholine (Pyr-PC) as probe. The proportion of excimer (E) to monomer (M) fluorescence of a pyrenyl moiety constitutes a direct measure of its local concentration. The transfers of Pyr-PC were monitored by following the decrease of E/M. The data were used to calculate the rate constants K+1 for the transfer from SBV to HDL and to generate the corresponding Arrhenius plots. The equilibrium constants, Keq, for the same reactions were also determined and used to generate Van’t Hoff plots. From these data, we calculated the thermodynamic parameters for both the whole transfer reaction and the transition state. Both K+1 and Keq values clearly varied with temperature. PLTP induced very similar decreases in the free energy for the whole reaction (ΔG) and in that for the transition state (ΔG#). At 37°C, the decreases were of 0.37 and 0.29 kcal/mol, respectively. We studied the thermal denaturation of PLTP between 37 and 65°C, and the effects of denatured PLTP samples on the PL transfer reaction were then determined. In all cases, the changes of ΔG remained comparable to those of ΔG#. Thus the essential action of PLTP is to facilitate the first step of the reaction, which can be considered as the desorption of PL molecules from the surface of donor particles.