- hypoxanthine + O2 <-> xanthine + H2O2
- xanthine + O2 + 2H2O <-> uric acid + 2H2O2
In humans, xanthine oxidase is normally found in the liver and not free in the blood. During severe liver damage, xanthine oxidase is released into the blood, so a blood assay for XO is a way to determine if liver damage has happened. As well, because xanthine oxidase is a metabolic pathway for uric acid formation, the xanthine oxidase inhibitor allopurinol is used in the treatment of gout.