Inhibitory action of quercetin on xanthine oxidase and xanthine dehydrogenase activity.

Quercetin is an equally good inhibitor of xanthine oxidase (type O, oxygen-reducing enzyme) and xanthine dehydrogenase (type D, NAD+-reducing enzyme) activity of a preparation of the xanthine-oxidizing enzyme partially purified from rat liver. The inhibition seems competitive with the oxidase form and non-competitive (mixed-type) with the dehydrogenase form of the enzyme. These inhibitory properties should be referred to the flavonoid structure of quercetin rather than to its antioxidant power. The antioxidant properties of quercetin and its inhibitory effect on the xanthine-oxidizing enzyme are discussed with reference to hyperuricemic and ischemic states.