Xanthine oxidase is a molybdenum-containing enzyme that catalyzes the reaction that follows:

Hypoxanthine + O2 <=> Xanthine + H2O2

Xanthine oxidase can oxidize xanthine further to uric acid, as well (Figure 22.7)

Xanthine oxidase is a target of anti-gout drugs. Allopurinol, which is similar to hypoxanthine (see here), is used to treat gout because it inhibits xanthine oxidase, leading to accumulation of hypoxanthine and xanthine, both of which are more soluble and more readily excreted than uric acid (the causative agent of gout).