Key enzyme in purine degradation. Catalyzes the oxidation of hypoxanthine to xanthine. Catalyzes the oxidation of xanthine to uric acid. Contributes to the generation of reactive oxygen species. Has also low oxidase activity towards aldehydes (in vitro).Involvement in disease;Defects in XDH are the cause of xanthinuria type 1 (XU1) . Xanthinuria is characterized by excretion of very large amounts of xanthine in the urine and a tendency to form xanthine stones. Uric acid is strikingly diminished in serum and urine. XU1 is due to isolated xanthine dehydrogenase. XU1 patients can metabolize allopurinol.