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Such in vitro systems must, however, be interpreted carefully. In some circumstances, phenolics (including flavonoids) can act to promote oxidative damage, giving rise to cytotoxicity and genotoxicity in in vitro assay systems. For example, in the presence of transition metal ions like iron (Fe3+) and copper (Cu2+), caffeic acid can induce cellular and DNA damage (45). This was ascribed to autoxidation, which can occur to phenolics in the presence of such metal ions, resulting in the formation of H2O2 which mediates the damage. It would appear that where such ions are protein-bound, the capacity for damage is much less. There are a number of other factors which also need to be taken into account when evaluating the potential bioactivity of dietary phenolics (46).

Phenolics as Protectants against LDL Oxidation

The antioxidant potential of ferulic acid was investigated in lipid peroxidizing systems by Castelluccio et al. (47), alongside, among others, chlorogenic acid, caffeic acid and p-coumaric acid, each of which has been identified in cocoa. These workers investigated the relative effectiveness of these phenolic compounds in preventing peroxidation of LDL, protecting LDL cholesterol from oxidation, and preventing the oxidative modification of apoprotein B100. The most effective were caffeic acid and chlorogenic acid, followed by ferulic acid and then p-coumaric acid. Vinson et al. (31) also noted that chlorogenic acid had the capacity to protect LDL from oxidation in vitro.

In another in vitro study of LDL oxidation, Laranjinha et al. (48) confirmed that chlorogenic acid and caffeic acid rapidly scavenge peroxyl (ROO) radicals in the aqueous phase, thereby preventing the chain initiation of LDL peroxidation. In contrast, the behaviour of protocatechuic acid in their system was different, suggesting that this compound was not acting as an antioxidant but as a retardant; that is, it reacts with ROO - to slow down the initiation and propagation steps of lipid peroxidation to some extent. The same group (49) have further elucidated some of the reactions occurring within LDL and illustrated possible mechanisms by which phenolic antioxidants could protect against lipid peroxidation.

The investigators employed a test system using ferrylmyoglobin as an oxidant, and examined a-tocopherol consumption in LDL. a-Tocopherol is considered to be the major antioxidant present in LDL. In the test system, a-tocopherol disappeared as a function of ferrylmyoglobin concentration. After depletion of a-tocopherol, caffeic and p-coumaric acids were able to prevent lipid peroxidation, by a mechanism that converts ferrylmyoglobin back to metmyoglobin. Caffeic acid also delayed a-tocopherol consumption when present before the oxidation challenge or added during it, whereas p-coumaric acid accelerated the rate of a-tocopherol consumption. Thus these two phenolics may play different roles, the results suggesting that caffeic acid acts synergistically with a-tocopherol, by

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