Abstract
Flavonoids containing phenol B rings, e.g. naringenin, naringin, hesperetin and apigenin, formed prooxidant metabolites that oxidised NADH upon oxidation by peroxidase/H 2O 2. Extensive oxygen uptake occurred which was proportional to the NADH oxidised and was increased up to twofold by superoxide dismutase. Only catalytic amounts of flavonoids and H 2O 2 were required indicating a redox cycling mechanism that activates oxygen and generates H 2O 2. NADH also prevented the oxidative destruction of flavonoids by peroxidase/H 2O 2 until the NADH was depleted. These results suggest that prooxidant phenoxyl radicals formed by these flavonoids cooxidise NADH to form NAD · radicals which then activated oxygen. Similar oxygen activation mechanisms by other phenoxyl radicals have been implicated in the initiation of atherosclerosis and carcinogenesis by xenobiotic phenolic metabolites. This is the first time that a group of flavonoids have been identified as prooxidants independent of transition metal catalysed autoxidation reactions.
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