2 RELATED PHENOLIC ANTIOXIDANTS WITH OPPOSITE EFFECTS ON VITAMIN-E CONTENT IN LOW-DENSITY LIPOPROTEINS OXIDIZED BY FERRYLMYOGLOBIN - CONSUMPTION VS REGENERATION

Endogenous alpha-tocopherol of low density lipoprotein (LDL) particles exposed to ferrylmyoglobin (iron in the form of Fe-IV=O) vanishes as a function of myoglobin concentration. After alpha-tocopherol depletio n, subsequent heavy lipid peroxidation is prevented by caffeic and p-c oumaric acids, i.e., phenolic acids present in foods and beverages, by a mechanism involving the one-electron transfer reaction between the phenols and the ferrylmyoglobin, with formation of metmyoglobin and th e corresponding phenoxyl radicals from caffeic and p-coumaric acids, a s previously discussed, Caffeic acid delays alpha-tocopherol consumpti on when present before oxidation challenging and restores alpha-tocoph erol when added halfway during the reaction, Conversely, p-coumaric ac id accelerates the rate of alpha-tocopherol consumption when added eit her before or during the oxidation reaction, In LDL enriched with alph a-tocopherol, caffeic acid induces an inhibition period of oxidation l onger than that expected from the sum of discrete periods characterist ic of the phenolic acid and alpha-tocopherol, Surprisingly, p-coumaric acid decreases the peroxidation chain rate, Similar effects of these phenolic acids on alpha-tocopherol consumption were observed in a Trit on X-100 micellar system, i.e., in the absence of a peroxidation chain reaction. Results suggest that caffeic acid acts synergistically with alpha-tocopherol, extending the antioxidant capacity of LDL by recycl ing alpha-tocopherol from the alpha-tocopherol radical (i.e., alpha-to copheroxyl radical). By contrast, the phenoxyl radical from p-coumaric acid (produced by electron-transfer reaction between phenolic acid an d ferrylmyoglobin) oxidizes alpha-tocopherol. However, in spite of alp ha-tocopherol consumption, the exchange reaction recycling p-coumaric acid can still afford an antioxidant protection to LDL on basis of the chain-breaking activity of p-coumaric acid, These results emphasize t he biological relevance of small structural modifications of phenols o n the interaction with cu-tocopherol in LDL, The significance of these results in the context of atherosclerosis is discussed. (C) 1995 Acad emic Press, Inc.