Fe2+ and ascorbic acid induced oxidation of cholesterol in phosphatidylcholine liposomes and its inhibition by alpha-tocopherol.

Fe2+ and L-ascorbic acid induced oxidation of cholesterol was investigated in liposomes of egg yolk phosphatidylcholine (PC). Cholesterol 5,6-epoxide, 7-hydroxycholesterol, 7-oxocholesterol, and cholestane-3,5,6-triol were detected as the oxidation products by gas chromatography, indicating that epoxidation and hydroperoxidation took place independently in cholesterol. Oxidation of cholesterol was accompanied with oxidative loss of unsaturated fatty acid moiety of PC. When egg yolk PC was replaced by dimyristoyl-PC, little oxidation was observed in the cholesterol fraction. These results indicate that cholesterol oxidation depends on peroxidation of polyunsaturated fatty acids in liposomal phospholipids. alpha-Tocopherol incorporated into liposomes at a level of 1.0 mol% to egg yolk completely suppressed not only the loss of unsaturated fatty acid moiety, but also oxidation of cholesterol. alpha-Tocopherol may inhibit cholesterol oxidation by suppressing phospholipid peroxidation. tert-Butyl hydroxytoluene, a synthetic antioxidant, was less effective against the oxidation of both PC and cholesterol.