Effect of cholesterol on egg yolk phosphatidylcholine peroxidation in multilamellar liposomes

Lipid peroxidation of aerated multilamellar liposomes composed of egg yolk phosphatidylcholine (EYPC) and cholesterol (CHOL) at molar ratios CHOL:EYPC = 0, 0.1, 0.2, 0.3, 0.4, 0.6 and 1.0 was studied during autooxidation and during Fe2+/H2O2-induced peroxidation by following the formation of conjugated diene and thiobarbituric acid reactive substances. The presence of cholesterol in the fluid lipid bilayers has an inhibiting effect on the EYPC peroxidation in the propagation phase of both the autooxidation and Fe2+-induced peroxidation free radical chain reactions. This inhibiting effect increases with the increase in CHOL: EYPC molar ratio. The inhibition of EYPC peroxidation by cholesterol probably originates a) from the increased lateral separation of polyunsaturated EYPC acyl chains caused by insertion of cholesterol between EYPC molecules, b) from the increased molecular packing of both the bilayer polar and hydrophobic regions due to the reduced bilayer hydration, and c) from the antioxidant properties of cholesterol.