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Specific role of oxidized species in the bioactivity of two antioxidants: apigenin and 20-hydroxyecdysone
Antioxidants are generally considered as „double-edged swords”: various health benefits are attributed to them since they can decrease oxidative stress caused by reactive oxygen species (ROS), but they can also increase it by directly or indirectly contributing to the formation of ROS . On the other hand, a necessarily existing third face of antioxidants is generally overlooked, namely the specific bioactivities of their oxidized metabolites that are formed after scavenging ROS, and, as such, that must represent amounts proportional to the oxidative stress.
In the present work, a phenolic (apigenin; Ap) and a non-phenolic (20-hydroxyecdysone; 20E) antioxidant was studied for the biological importance of their oxidized metabolites. Protoapigenone (Pa), a p-quinol B-ring containing protoflavone known for its strong anticancer properties  was obtained from Ap upon oxidation with PIFA. In silico studies revealed the favorable formation of Pa from Ap when scavenging OH radicals, which was confirmed by HPLC-DAD when subjecting Ap to Fenton-reaction. Moreover, incubation with GSH yielded Ap from Pa, revealing an Ap-Pa redox cycle of outmost biomedical interest. 20E was subjected to base-catalyzed auto-oxidation, yielding B-ring modified derivatives including calonysterone and its desmotrope pair. In vitro activity of the compounds was tested on the phosphorylation of Akt (playing an important role in cell survival/apoptosis), and much stronger activities than that of 20E were observed. All major metabolites were detected from the Fenton-reaction of 20E.
Based on these examples, our results demonstrate that oxidized metabolites, forming when antioxidants scavenge ROS, can play specific role in the bioactivity of these compounds, and that such metabolites worth the attention concerning related drug discovery initiatives.
Acknowledgement: COST Action CM1407.
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