Xanthine oxidase XO, sometimes XAO, is a form of xanthine oxidoreductase, a type of enzyme that generates reactive oxygen species.These enzymes catalyze the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid. These enzymes play an important role in the catabolism of purines in some species, including humans. The image show the two chains of the molecule in blue and orange.