Quercetin can act either as an inhibitor or an inducer of the mitochondrial permeability transition pore: A demonstration of the ambivalent redox character of polyphenols.

"In swelling experiments with suspensions of mitochondria, high (20-50 microM) concentrations of quercetin, the most efficient inhibitor, promoted instead the onset of the MPT. Chelators of Fe(2+/3+) and Cu(+/2+) ions counteracted this effect. Fluorescent indicators of superoxide production confirmed that quercetin potentiates O(2)(*-) generation by isolated mitochondria and cultured cells. Since this was not affected by chelating Fe and Cu ions, the MPT-inducing effect can be ascribed to a "secondary", metal ion-catalyzed production of ROS."

De Marchi, U., Biasutto, L., Garbisa, S., Toninello, A., and Zoratti, M. (2009). Quercetin can act either as an inhibitor or an inducer of the mitochondrial permeability transition pore: A demonstration of the ambivalent redox character of polyphenols. Biochimica et biophysica acta, 1787(12):1425-1432. http://www.citeulike.org/user/HEIRS/article/5369424