Phenolic content of various beverages determines the extent of inhibition of human serum and low-density lipoprotein oxidation in vitro: identification and mechanism of action of some cinnamic acid derivatives from red wine.

1. An antioxidant effect of phenolic substances in red wine to reduce oxidizability of low-density lipoprotein has been proposed as the basis for a relatively lower incidence of coronary disease in populations with high red wine intake. We have now investigated the possible antioxidant effects of various beverages, including red wines, white wines, beers and red grape juices (diluted 1:500), on metal ion-dependent (copper) and -independent (aqueous peroxyl radicals) oxidation of isolated human low-density lipoprotein. We also tested the effects of these beverages on copper-initiated oxidation of lipoproteins in serum. 2. The higher the polyphenolic content of a beverage, the greater was its antioxidative effect measured as change in lag time in the different oxidation systems. Upon stripping the polyphenolics from the drinks, the lag times returned to control levels in isolated low-density lipoprotein; however, the low concentrations of phenolics remaining after stripping had a lesser but still significant effect on oxidation of lipoproteins in serum. The inhibitory effect of these phenolics appeared to be more pronounced for metal ion (copper)-induced oxidation than for those induced by aqueous peroxyl radicals, suggesting that both copper-binding and free radical-trapping activities may be involved. A mixture of the carboxylic acids representative of those present in red wine exhibited no significant effect on lag time of metal ion-dependent and -independent low-density lipoprotein oxidations. Ethanol, at concentrations of 0.1-0.5%, had no effect on either copper-induced or aqueous peroxyl radical oxidations. 3. Extracts of acid-hydrolysed red wine were separated by thin-layer chromatography and the most active antioxidant fractions identified. GC-MS and HPLC analysis of these fractions resulted in the identification of several cinnamic acid derivatives, such as coumaric acid, caffeic acid and protocatechuic acid. Dose-response studies using the pure compounds indicated that caffeic acid was the most active antioxidant with an IC50 < 1 mumol/l for copper-initiated low-density lipoprotein oxidation. Caffeic acid (1 mumol/l) significantly inhibited lipid hydroperoxide formation while sparing alpha-tocopherol consumption. Caffeic acid at the same concentration also inhibits aqueous peroxyl radical-induced oxidation of low-density lipoprotein, sparing alpha-tocopherol. There was no evidence of caffeic acid preventing the binding of copper to low-density lipoprotein. 4. We conclude that phenolics in both alcoholic and non-alcoholic beverages can give dose-dependent protection against oxidation of low-density lipoprotein. Caffeic acid and protocatechuic acid are two compounds likely to contribute to these effects. These findings may be relevant to the putative cardiovascular-protective effects of high phenolic content alcoholic beverages such as red wine; however, the widespread occurrence of antioxidants such as caffeic acid in fruits and vegetables suggests that these protective principles are not limited to red wine.