Figure 1.

Quercetin applies either antioxidant or pro-oxidant effects depending on the level of intracellular reduced glutathione (GSH). Quercetin and H2O2 react to form a semiquinone radical which is oxidized to Quercetin-Quinone (Qu-Quinone). In greater levels of GSH, QQ reacts with GSH to form the stable glutathionyl quercetin (GSQ) forms of 8-GSQ and 6-GSQ. However, when there are lower levels of GSH, Qu-Quinone reacts with protein thiols, resulting in cell damage leading to apoptosis. Adapted from ⁷⁵.