It has been assumed that ROS-induced oxidation of flavonoids, which is common in several food processing, necessarily involves the loss of its antioxidant properties. However, it has been established that, after inducing quercetin-oxidation, a mixture of metabolites is obtained whose intracellular-antioxidant-capacity is 20-times higher than quercetin. It was demonstrated that the antioxidant-effect of this mixture lay exclusively in one metabolite, identified as 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. This metabolite protected Caco-2 cells exposed to ROS at a concentration 200-times lower than quercetin. This work seeks to identify the formation of benzofuranones from other quercetin-like flavonoids, and to test their antioxidant-capacities.
HPLC-DAD was used to investigate the generation of benzofuranones in the oxidation-mixtures of flavonoids: myricetin, fisetin and morine. The structural identity of the benzofuranones was corroborated by HPLC-ESI-MS/MS. The antioxidant-capacity of each oxidative-mixture and its corresponding benzofuranone was tested in Caco-2 cells exposed to ROS-generator, indomethacin. Additionally, it was evaluated the degree to which the antioxidant-capacity of each oxidative-mixtures was affected by the chemical subtraction of their benzofuranones.
The formation of benzofuranones was confirmed in mixtures obtained after the oxidation of fisetin, morine or myricetin. The oxidative-mixtures of these flavonoids protected against the oxidative-stress at equivalent-dilutions 10, 15 and 50-times lower than those required by their respective flavonoids. When each benzofuranones were tested, degrees of antioxidant protections were seen at equivalent-dilutions 50, 500 and 750-times lower than that of the original flavonoid. Finally, when the benzofuranone was removed from each oxidative mixture, a total loss of the antioxidant-capacity was observed.
The oxidation of flavonoids studied here translate into a considerable increase of their original antioxidant-capacity. It was shown that such increased capacity lies exclusively in the presence of benzofuranones in the mixtures. Although benzofuranones exhibited an antioxidant-capacity substantially higher than that of their precursors, significant differences in their “apparent antioxidant potencies” appear to exists.