A study by the University of Padova, Italy, investigated the protective effect of quercetin on oxidative damage in mitochondria. Calcium and oxidative stress induce mitochondrial permeability transition, which can lead to mitochondrial swelling and cell death and plays an important role in some types of apoptosis. This damage is caused by the formation on of a large pore in the inner mitochondrial membrane. Normally polyphenols induced closure of the mitochondrial permeability transition channel but De Marchi and his team found that high concentrations of quercetin promoted instead the onset of the mitochondrial permeability transitions. Fluorescent indicators of superoxide production confirmed that quercetin potentiates superoxide generation.
A study conducted by German scientists at Ludwig Maximilians University found evidence of a strong antioxidant effect of quercetin, and also coenzyme Q10, on mucosal cells of the nasal turbinates . Macroscopically healthy tissue of nasal mucosa was harvested from patients undergoing surgery of the nasal turbinates and incubated with quercetin. DNA damage induced by H2O2 was significantly reduced by both antioxidants.
Brazilian researchers showed in 2009 that the flavonoid quercetin decreased the oxidative stress induced by tert-butyl hydroperoxide in human erythrocyte . They found that quercetin partially prevented the depletion of gluthatione but did not prevent the formation methemoglobin and Heinz bodies (inclusions within red blood cells composed of denatured hemoglobin). Rutin was more effective and also prevented the formation of Heinz bodies. A Canadian study also found that quercetin reduced apoptotic cell death of neurons by acting on the expression of pro- and anti-apoptotic genes . The aim of this study was to evaluate the effect of the flavonoids resveratrol and quercetin on the apoptotic cascade induced by the administration of 1-methyl-4-phenylpyridinium ion (MMP+), which causes parkinsonism in primates by killing certain dopamine-producing neurons in the substantia nigra. Pre-treatment of neurons with quercetin for 3 hours before MMP+ administration reduced apoptotic death induced by MPP+.
Fahlman and Krol of the College of Pharmacy and Nutrition, University of Saskatchewan, Canada, investigated the potential for quercetin to protect skin against UV-induced damage . They found that the protective effect of quercetin against UV-radiation was the result of its antioxidant capacity. Quercetin was an inhibitor of lipid peroxidation and scavenged radical species produced both by UVA and UVB radiation.
 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. Biochim Biophys Acta. 2009 Jun 11.
 Effect of vitamin C, deferoxamine, quercetin and rutin against tert-butyl hydroperoxide oxidative damage in human erythrocytes. Hematology. 2009 Jun;14(3):168-72.
 Inhibition of UVA and UVB radiation-induced lipid oxidation by quercetin. J Agric Food Chem. 2009 Jun 24;57(12):5301-5.
 Protective Effects of Resveratrol and Quercetin Against MPP(+) -Induced Oxidative Stress Act by Modulating Markers of Apoptotic Death in Dopaminergic Neurons. Cell Mol Neurobiol. 2009 May 23.
 Antioxidant effects of quercetin and coenzyme Q10 in mini organ cultures of human nasal mucosa cells.Anticancer Res. 2009 Jan;29(1):33-9.