Unraveling the Antioxidant Activity of ,-dihydroquercetin.

It has been reported that in an oxidative environment, the flavonoid ,-dihydroquercetin (,-DHQ) oxidizes into a product that rearranges to form quercetin. As quercetin is a very potent antioxidant, much better than ,-DHQ, this would be an intriguing form of targeting the antioxidant quercetin. The aim of the present study is to further elaborate on this targeting. We can confirm the previous observation that ,-DHQ is oxidized by horseradish peroxidase (HRP), with HO as the oxidant. However, HPLC analysis revealed that no quercetin was formed, but instead an unstable oxidation product. The inclusion of glutathione (GSH) during the oxidation process resulted in the formation of a ,-DHQ-GSH adduct, as was identified using HPLC with IT-TOF/MS detection. GSH adducts appeared on the B-ring of the ,-DHQ quinone, indicating that during oxidation, the B-ring is oxidized from a catechol to form a quinone group. Ascorbate could reduce the quinone back to ,-DHQ. No ,-DHQ was detected after the reduction by ascorbate, indicating that a possible epimerization of ,-DHQ quinone to ,-DHQ quinone does not occur. The fact that no epimerization of the oxidized product of ,-DHQ is observed, and that GSH adducts the oxidized product of ,-DHQ on the B-ring, led us to conclude that the redox-modulating activity of ,-DHQ quinone resides in its B-ring. This could be confirmed by chemical calculation. Apparently, the administration of ,-DHQ in an oxidative environment does not result in 'biotargeting' quercetin.