Xanthine oxidase is an enzyme that generates reactive oxygen species by catalyzing the oxidation of hypoxanthine to xanthine and further catalyzing the oxidation of xanthine to uric acid. It plays an important role in purine catabolism in humans. The protein is a homodimer with three domains - an N-terminal FeS cluster domain, an FAD-binding domain, and a molybdenum cofactor domain. It can exist as xanthine dehydrogenase or be converted to xanthine oxidase via oxidation or proteolysis.