RESUMEN
Flavonoids display a broad range of health-promoting bioactivities. Among these, their capacity to act as antioxidants has remained most prominent. The canonical reactive oxygen species (ROS)-scavenging mode of the antioxidant action of flavonoids relies on the high susceptibility of their phenolic moieties to undergo oxidation. As a consequence, upon reaction with ROS, the antioxidant capacity of flavonoids is severely compromised. Other phenol-compromising reactions, such as those involved in the biotransformation of flavonoids, can also markedly affect their antioxidant properties. In recent years, however, increasing evidence has indicated that, at least for some flavonoids, the oxidation of such residues can in fact markedly enhance their original antioxidant properties. In such apparent paradoxical cases, the antioxidant activity arises from the pro-oxidant and/or electrophilic character of some of their oxidation-derived metabolites and is exerted by activating the Nrf2-Keap1 pathway, which upregulates the cell's endogenous antioxidant capacity, and/or, by preventing the activation of the pro-oxidant and pro-inflammatory NF-κB pathway. This review focuses on the effects that the oxidative and/or non-oxidative modification of the phenolic groups of flavonoids may have on the ability of the resulting metabolites to promote direct and/or indirect antioxidant actions. Considering the case of a metabolite resulting from the oxidation of quercetin, we offer a comprehensive description of the evidence that increasingly supports the concept that, in the case of certain flavonoids, the oxidation of phenolics emerges as a mechanism that markedly amplifies their original antioxidant properties. An overlooked topic of great phytomedicine potential is thus unraveled.
RESUMEN
Phenolic compounds in oilseeds occur in the free, esterified and insoluble-bound forms and serve as natural antioxidants by preventing food oxidation as well as oxidative stress and various disorders in the body. In this work, free, esterified and insoluble-bound phenolics were extracted from defatted camelina and sophia seeds. Samples were evaluated for their total contents of phenolics (TPC), flavonoids (TFC) and proanthocyanidins (PC) as well as the antioxidant activity of their various phenolic fractions. Several in vitro methods, namely Trolox equivalent antioxidant capacity (TEAC), reducing power (RP) and metal chelation activity were used for all fractions. High performance liquid chromatography-diode array detection- tandem mass spectrometry (HPLC-DAD-MS/MS) identified 29 phenolics belonging to phenolic acids and their derivatives, flavonoids and procyanidins in different fractions of camelina and sophia extracts. Esterified fraction was the predominant form of phenolics in both seeds. Thus, camelina and sophia seeds may serve as viable functional food ingredients with protective antioxidant potential.