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. H2O + NAD(+) + xanthine = H(+) + NADH + urate H2O + hypoxanthine + NAD(+) = H(+) + NADH + xanthine H2O + O2 + xanthine = H2O2 + urate Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Binds 2 [2Fe-2S] clusters per subunit. Homodimer. Detected in liver (at protein level). Belongs to the xanthine dehydrogenase family.