CEREBROSIDES ALTER THE LYOTROPIC AND THERMOTROPIC PHASE-TRANSITIONS OF DOPE-DOPC AND DOPE-DOPC-STEROL MIXTURES

Freezing injury in rye and oat is a consequence of the formation of th e inverted hexagonal (H-parallel to) phase in regions where the plasma membrane is brought into close proximity with cytoplasmic membranes d uring freeze-induced dehydration. Susceptibility to plasma membrane de stabilization and H-parallel to, phase formation during freezing is as sociated with alterations in plasma membrane lipid composition. This p aper examines the influence of lipid composition and hydration on the propensity of lipid mixtures of DOPE:DOPC and DOPE:DOPC:sterols with a dded cerebrosides (CER) to form the H-parallel to phase during dehydra tion. The addition of CER to DOPE:DOPC:beta-sitosterol mixtures decrea sed the water content of the dispersions in a manner suggesting that m ost or all of the water in the dehydrated mixtures was associated with the phospholipids. The addition of CER significantly decreased the os motic pressure at which the L-alpha --> H-parallel to phase transition occurred from an osmotic pressure of 76.1 MPa for DOPE:DOPC (50:50) t o 20 MPa in DOPE:DOPC: beta-sitosterol:CER (22.5:22.5:50:5) and 8 MPa in DOPE:DOPC: beta-sitosterol:CER (15:15:50:20). Experiments examining the effects of CER on the thermally-induced formation of the H-parall el to phase in fully hydrated mixtures and examining the influence of CER on the formation of the H-parallel to phase in DOPE:DOPC mixtures lacking beta-sitosterol suggested that CER facilitated the L-alpha --> H-parallel to phase transition by effecting a decrease in bilayer hyd ration and by increased lateral packing pressures within the acyl doma in of the bilayer. Taken in sum, these data indicate that the differen tial propensity of the rye and oat plasma membranes to undergo freeze- induced formation of the L-alpha --> H-parallel to phase cannot be att ributed to one lipid species. Rather, the propensity towards freeze-in duced membrane destabilization is a consequence of the summation of ph ysical 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.