Cerebrosides alter the lyotropic and thermotropic phase transitions of DOPE:DOPC and DOPE:DOPC:sterol mixtures

摘要

Freezing injury in rye and oat is a consequence of the formation of the inverted hexagonal (HII) phase in regions where the plasma membrane is brought into close proximity with cytoplasmic membranes during freeze-induced dehydration. Susceptibility to plasma membrane destabilization and HII phase formation during freezing is associated with alterations in plasma membrane lipid composition. This paper examines the influence of lipid composition and hydration on the propensity of lipid mixtures of DOPE:DOPC and DOPE:DOPC:sterols with added cerebrosides (CER) to form the HII phase during dehydration. The addition of CER to DOPE:DOPC:β-sitosterol mixtures decreased the water content of the dispersions in a manner suggesting that most or all of the water in the dehydrated mixtures was associated with the phospholipids. The addition of CER significantly decreased the osmotic pressure at which the Lα→HII phase transition occurred from an osmotic pressure of 76.1 MPa for DOPE:DOPC (50:50) to 20 MPa in DOPE:DOPC:β-sitosterol:CER (22.5:22.5:50:5) and 8 MPa in DOPE:DOPC:β-sitosterol:CER (15:15:50:20). Experiments examining the effects of CER on the thermally-induced formation of the HII phase in fully hydrated mixtures and examining the influence of CER on the formation of the HII phase in DOPE:DOPC mixtures lacking β-sitosterol suggested that CER facilitated the Lα→HII phase transition by effecting a decrease in bilayer hydration and by increased lateral packing pressures within the acyl domain of the bilayer. Taken in sum, these data indicate that the differential propensity of the rye and oat plasma membranes to undergo freeze-induced formation of the Lα→HII phase cannot be attributed to one lipid species. Rather, the propensity towards freeze-induced membrane destabilization is a consequence of the summation of physical characteristics of the membrane lipid components that included bilayer hydration, packing pressures within the hydrophobic domain of the membrane, the propensity of the lipid components to demix, and the relative proportions of the various lipid components.