Xanthine oxidase (XO, sometimes 'XAO') is a form of xanthine oxidoreductase, an enzyme that generates reactive oxygen species. It catalyzes the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid. This enzyme plays an important role in the catabolism of purines in some species, including humans.
Xanthine oxidase is defined as an enzyme activity (EC 22.214.171.124). The same protein, which in humans has the HGNC approved gene symbol XDH, can also have xanthine dehydrogenase activity (EC 126.96.36.199). Most of the protein in the liver exists in a form with xanthine dehydrogenase activity, but it can be converted to xanthine oxidase by reversible sulfhydryl oxidation or by irreversible proteolytic modification.