Formation of Hydroxylated-Benzoyl-Benzofuranones Following the Exposure of Quercetin and Kaempferol to aNitrite-Containing Acidic Medium

Previous studies indicate that the alkali-induced oxidation of quercetin (Que) or kaempferol (Kae) converts them into hydroxylated-benzoyl-benzofuranones (BZFs), a type of metabolite whose antioxidant potency is substantially higher. To explore whether the former conversion can take place under a more biologically relevant oxidizing setting, these flavonols were incubated in a nitrite-containing acidic medium. Under such condition, the concentrations of Que or Kae rapidly decayed, simultaneously generating their corresponding BZFs. When Que and Kae were co-incubated, the depletion of the parent flavonols accelerated without changes in BZF formation. Similar results were observed when either Que or Kae were incubated with some of their corresponding 3-O-glycosides. In turn, when Que was co-incubated with certain flavanols or phenolic acids, the latter compounds nearly doubled not only the oxidative disappearance of Que but also its BZF formation. Interestingly, the concentrations of Que-BZF and Kae-BZF reached under all former incubation scenarios are greater than those previously reported to be needed to protect intestinal epithelial cells against the oxidative and lytic damage induced by ROS. We suggest that, if these conversions also took place in vivo, the gastric fluid could be potentially seen as a chemical milieu where the antioxidant properties of Que and Kae could be enormously amplified.