ANTIFREEZE GLYCOPROTEINS INHIBIT LEAKAGE FROM LIPOSOMES DURING THERMOTROPIC PHASE-TRANSITIONS

Antifreeze glycoproteins (AFGPs), found in the blood of polar fish at concentrations as high as 35 g/liter, are known to prevent ice crystal growth and depress the freezing temperature of the blood. Previously, Rubinsky et al. [Rubinsky, B., Mattioli, M., Arav, A., Barboni, B. & Fletcher, G. L. (1992) Am. J. Physiol. 262, R542-R545] provided eviden ce that AFGPs block ion fluxes across membranes during cooling, an eff ect that they ascribed to interactions with ion channels, We investiga ted the effects of AFGPs on the leakage of a trapped marker from lipos omes during chilling, As these liposomes are cooled through the transi tion temperature, they leak approximate to 50% of their contents, Addi tion of less than 1 mg/ml of AFGP prevents up to 100% of this leakage, both during chilling and warming through the phase transition, This i s a general effect that we show here applies to liposomes composed of phospholipids with transition temperatures ranging from 12 degrees C t o 41 degrees C, Because these results were obtained with liposomes com posed of phospholipids alone, we conclude that the stabilizing effects of AFGPs on intact cells during chilling reported by Rubinsky ct al. may be due to a nonspecific effect on the lipid components of native m embranes, There are other proteins that prevent leakage, but only unde r specialized conditions, For instance, antifreeze proteins, bovine se rum albumin, and ovomucoid all either have no effect or actually induc e leakage, Following precipitation with acetone, all three proteins in hibited leakage, although not to the extent seen with AFGPs, Alternati vely, there are proteins such as ovotransferrin that have no effect on leakage, either before or after acetone precipitation.