Quercetin is a naturally occurring flavonoid compound found in most plant tissues. It is being studied for apparent benefical effects including cardiovascular protection, anti-cancer activity, anti-ulcer effects, anti-allergy activity, cataract prevention, antiviral activity, and anti-inflammatory effects (Flavonoids in health and disease. Eds C.A. Rice-Evans and L. Packer. New York: Marcel Dekker, 1998). Many of these effects have been attributed to quercetin`s antioxidant activity. These apparent effects have lead to the sale of plant-derived quercetin as a natural remedy and nutritional supplement. The degradation of quercetin is of particular interest since the enzyme quercetin 2,3-dioxygenase is one of the few copper-based dioxygenase enzymes. This enzyme has been structurally characterized [http://www.ncbi.nlm.nih.gov/pubmed/11839311|(Fusetti et al., 2002)].