Translocation of phospholipids and dithionite permeability in liquid-ordered and liquid-disordered membranes

We present a detailed study of the translocation rate of two headgroup-labeled phospholipid derivatives, one with two acyl chains, NBD-DMPE, and the other with a single acyl chain, NBD-lysoMPE, in lipid bilayer membranes in the liquid-disordered state (POPC) and in the liquid-ordered states (POPC/Cholesterol (Chol), molar ratio 1:1, and sphingomyelin (SpM)/Chol, molar ratio 6:4). The study was performed as a function of temperature and the thermodynamic parameters of the translocation process have been obtained. The most important findings are 1), the translocation of NBD-DMPE is significantly faster than the translocation of NBD-lysoMPE for all bilayer compositions and temperatures tested; and 2), for both phospholipid derivatives, the translocation in POPC bilayers is approximately 1 order of magnitude faster than in POPC/Chol (1:1) bilayers and approximately 2-3 orders of magnitude faster than in SpM/Chol (6:4) bilayers. The permeability of the lipid bilayers to dithionite has also been measured. In liquid disordered membranes, the permeability rate constant obtained is comparable to the translocation rate constant of NBD-DMPE. However, in liquid-ordered bilayers, the permeability of dithionite is significantly faster then the translocation of NBD-DMPE. The change in enthalpy and entropy associated with the formation of the activated state in the translocation and permeation processes has also been obtained.