Key enzyme in purine degradation. Catalyzes the oxidation of hypoxanthine to xanthine. Catalyzes the oxidation of xanthine to uric acid. Oxidizes xanthine, hypoxanthine and pterine at high rates. Can also act on purine and guanine. H2O + hypoxanthine + NAD(+) = H(+) + NADH + xanthine H2O + NAD(+) + xanthine = H(+) + NADH + urate Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Binds 2 [2Fe-2S] clusters. Completely inhibited by allopurinol and significantly inhibited by adenine. Inhibited by Fe(2+), Cd(2+) and Zn(2+) and strongly inhibited by Cu(2+). Mg(2+) and Mo(2+) have no effect on activity. Optimum pH is 8.5 in glycine-NaOH. Optimum pH is between 7.5 and 9.5 in potassium phosphate, Tris-HCl and Tris-acetate. Optimum temperature is 43 degrees Celsius. Quite stable as loses only 5% of activity in 1 h at 30 degrees Celsius, 12% at 40 degrees Celsius and 25% at 50 degrees Celsius. Purine metabolism. Homodimer. Most efficiently by hypoxanthine. Twice the level of the transcription increase compared with the level due to induction by adenine, which exhibits 10-fold increase from non-induced (PubMed:23773263, PubMed:25513995). Low levels induced by ammonium and nitrate. 43 mmol/l ammonium or nitrate and 2.5 mmol/l adenine together induce only 2- to 3-fold higher levels compared to non-induced level (PubMed:23773263). Exhibits less than 1% of the wild-type activity towards xanthine and hypoxanthine under inducible conditions (2.5 mmol/l adenine). No activity after cultivation without inducer. This enzyme can be used as a potential additive in the production of low purine content food, which is recommended to people suffering from hyperuricemia that can at worst lead to gout. When used in conjunction with other enzymes of the purine degradation pathway, uric acid concentration is significantly reduced. Belongs to the xanthine dehydrogenase family.