Effect of Lipid Composition of Liposomes on Their Sensitivity to Peroxidation1

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Abstract
The effect of lipid composition of liposomes on peroxidation induced by ferrous ion and ascorbate was examined. Temperature affects the sensitivity of liposomes; the peroxidation rate was increased with increase of the incubation temperature. With liposomes consisting of 1-palmitoyl-2-arachidonyl phosphatidylcholine (substrate) and a peroxidation-insensitive lipid, 1-palmitoyl-2-oleoyl phosphatidylcholine, peroxidation was dependent on the density of the substrate. No appreciable peroxidation was observed with liposomes containing less than 10 mol% of the substrate at 37°C. When 1 mol substrate was mixed with 9 mol dimyristoyl phosphatidylcholine, peroxidation occurred below 10°C, but not above 20°C. Above 20°C, the substrates should be located homogeneously on the membranes, whereas they should be clustered below 10°C, since the gel-liquid crystalline phase transition temperature of matrix membrane of dimyristoylphosphatidylcholine was 17–21°C. Peroxidation of liposomes consisting of 1-palmitoyl-2-arachidonyl phosphatidylcholine was also suppressed by cholesterol. These findings indicate that the lateral distribution as well as the density of the substrate on membranes affects the sensitivity of the substrate to peroxidation. It was also found that a-tocopherol is preferentially located in the 1-palmitoyl-2-arachidonyl phosphatidylcholine-rich regions of membranes consisting of mixed phospholipids, and efficiently suppresses peroxidation of liposomal lipids.