Figure 1.

Uric acid formation through xanthine oxidase activity. (A) Under ischemic or inflammatory conditions, xanthine dehydrogenase (XDH) is converted to xanthine oxidase (XO) via the oxidation of sulfhydryl residues or proteolysis of XDH. In the presence of oxygen, XO catalyses the oxidation of hypoxanthine to xanthine and then to uric acid (UA), with consequent production of the superoxide anion (O₂⁻) and hydrogen peroxide (H₂O₂). The competitive antagonist allopurinol is converted in the active form, oxypurinol, via XO activity, acting as an XO inhibitor; (B) During hypoxanthine conversion to xanthine and then UA, high levels of H₂O₂ and O₂⁻ are produced and converted to O₂ and H₂O₂, spontaneously or in a reaction catalyzed by the enzyme superoxide dismutase (SOD).