The enzyme xanthine oxidase, or XO, (bovine milk enzyme is PDB 1FIQ, EC 1.1.3.32) catalyzes the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid:
- hypoxanthine + O2 <-> xanthine + H2O2
- xanthine + O2 + 2H2O <-> uric acid + 2H2O2
The protein is large, having a molecular weight of 270,000, and has 2 flavin molecules (bound as FAD), 2 molybdenum atoms, and 8 iron atoms bound per enzymatic unit.
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.