EFFECT OF LIPID PHYSICAL STATE ON THE RATE OF PEROXIDATION OF LIPOSOMES

The effect of cholesterol on the rate of peroxidation of arachidonic acid and 1-palmitoyl-2-arachidonoyl phosphatidylcholine (PAPC) in dimyristoylphosphatidylcholine (DMPC) liposomes was examined above and below the phase transition temperature (T(m)) of the lipid. The rate of peroxidation of arachidonic acid was more rapid below the phase transition temperature of the host lipid. At a temperature below the T(m) (4-degrees-C), increasing concentrations of cholesterol reduced the rate of peroxidation of arachidonic acid as judged by the production of thiobarbituric acid reactive substances. Above T(m) (37-degrees-C), cholesterol increased the rate of peroxidation of the fatty acid. Similarly, PAPC was peroxidized more rapidly at 4-degrees-C than at 37-degrees-C. However, cholesterol had little effect on the rate of peroxidation of PAPC at 4-degrees-C. The rate of peroxidation of arachidonic acid was related to the lipid bilayer fluidity as judged by fluorescence anisotropy measurements of diphenylhexatriene. The rate of peroxidation increased slowly with increasing rigidity of the probe environment when the bilayer was relatively fluid and more rapidly as the environment became more rigid. The increase in the rate of peroxidation of arachidonic acid in the less fluid host lipid was unrelated to differences in iron binding or to transfer of arachidonic acid to the aqueous phase. Decreasing the concentration of arachidonic acid in DMPC to < 2 mole% dramatically decreased the rate of peroxidation at 4-degrees-C, suggesting that formation of clusters of fatty acids at 4-degrees-C is required for rapid peroxidation. These data support the hypothesis that an increase in the packing density of the acyl chains of peroxidizable lipids promotes lipid peroxidation in less fluid environments which favor phase separation.