Tart cherry produces various kinds of polyphenolics in its fruits that include cyanidin derivatives (mostly cyanidin 3-glucosylrutinoside, cyanidin 3-rutinoside, cyanidin sophoroside), peonidin 3-glucoside; kaempferol, quercetin, and isorhamnetin and their derivatives, as well as the alkaloid, melatonin. The antioxidant capacities of these constituents were analysed using the TEAC antioxidant assay. Kaempferol, quercetin, isorhamnetin 3-rutinoside, cyanidin 3-rutinoside, and melatonin showed significant antioxidant capacities; of these, kaempferol proved to be the most active. In order to determine how these constituents interact in terms of expression of their antioxidant action, we performed an isobolographic analysis. Using different dose ratios (e.g., 1:1, 1:2, 1:4) for the selected polyphenol constituents, we found evidence that three types of interactions may occur: synergistic, additive, and negative. The most important and new finding here was that pairs of compounds with the highest antioxidant capacity (e.g., kaempferol and melatonin at a dose ratio of 2:1, respectively, and cyanidin 3-rutinoside and isorhamnetin 3-rutinoside at a dose ratio of 1:4, respectively) showed strong synergistic types of interactions. Additive or negative types of interactions occurred for pairs of compounds that had lower antioxidant capacities. Thus, not all polyphenols in tart cherry fruits are equally effective in alleviating oxidative stress. Those which are most effective are likely to be acting synergistically.
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