Antioxidant interactions of catechin, cyanidin, caffeic acid, quercetin, and ellagic acid on human LDL oxidation

Flavonoids and phenolic acids are currently believed to exert cardioprotective effects in humans via their ability to inhibit oxidation of low-density lipoprotein (LDL). The influence of chemical structure on antioxidant activity of catechin, quercetin, cyanidin, caffeic acid, and ellagic acid was evaluated by measuring inhibition of copper-catalysed human LDL oxidation in vitro. The five plant phenols investigated all possess a similar o-dihydroxy moiety. The order of antioxidant activity was catechin > cyanidin ≈ caffeic acid > quercetin > ellagic acid. The observed differences in activities are discussed in terms of structural dissimilarities of the compounds. Potential synergistic or antagonistic effects between catechin, cyanidin, caffeic acid, quercetin, and ellagic acid were investigated by measuring the antioxidant activities on LDL of 20 different combinations of two/three of these phenols. All the antioxidant effects were additive except for combinations including ellagic acid with catechin, where ellagic acid exerted a significant antagonistic effect. It is proposed that the mechanism behind this antagonistic interaction is due to hydrogen bonding between carbonyls in ellagic acid and o-dihydroxyl groups in catechin.