Antioxidant protection of phospholipid bilayers by alpha-tocopherol. Control of alpha-tocopherol status and lipid peroxidation by ascorbic acid and glutathione.
Factors affecting the balance between pro- and antioxidant effects of ascorbic acid and glutathione were studied in soybean phosphatidylcholine liposomes challenged with Fe2+/H2O2. Effective antioxidant protection by alpha-tocopherol appeared to be due to efficient reaction with lipid oxy-radicals in the bilayer rather than to interception of initiating oxygen radicals. At concentrations above a threshold level of approximately 0.2 mol % (based on phospholipid content), alpha-tocopherol completely suppressed lipid oxy-radical propagation, which was measured as malondialdehyde production. Both ascorbic acid and glutathione, alone or in combination, enhanced lipid oxy-radical propagation. Alpha-Tocopherol, incorporated into liposomes at concentrations above its threshold protective level, reversed the pro-oxidant effects of 0.1-1.0 mM ascorbic acid but not those of glutathione. Ascorbic acid also prevented alpha-tocopherol depletion. The combination of ascorbic acid and subthreshold levels of alpha-tocopherol only temporarily suppressed lipid oxy-radical propagation and did not maintain the alpha-tocopherol level. Glutathione antagonized the antioxidant action of the alpha-tocopherol/ascorbic acid combination regardless of alpha-tocopherol concentration. These observations indicate that membrane alpha-tocopherol status can control the balance between pro- and antioxidant effects of ascorbic acid. The data also provide the most direct evidence to date that ascorbic acid interacts directly with components of the phospholipid bilayer.
D000975:Antioxidants / Q000378:metabolism; D001205:Ascorbic Acid / Q000494:pharmacology*; D005978:Glutathione / Q000494:pharmacology*; D008054:Lipid Peroxides / Q000378:metabolism*; D008081:Liposomes; D008315:Malondialdehyde / Q000378:metabolism; D010101:Oxygen Consumption; D014810:Vitamin E / Q000494:pharmacology*
D000975:Antioxidants; D008054:Lipid Peroxides; D008081:Liposomes; D014810:Vitamin E; D008315:Malondialdehyde; D005978:Glutathione; D001205:Ascorbic Acid