Key enzyme involved in purine catabolism. Catalyzes the oxidation of hypoxanthine to xanthine and the oxidation of xanthine to urate. Regulates the level of ureides and plays a role during plant growth and development and senescence. H2O + NAD(+) + xanthine = H(+) + NADH + urate H2O + hypoxanthine + NAD(+) = H(+) + NADH + xanthine Binds 2 [2Fe-2S] clusters. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Homodimer. Expressed in roots, leaves, stems, flowers and siliques. Plants silencing simultaneously XDH1 and XDH2 show reduced growth, impaired silique development, increased seed sterility, precocious senescence of mature leaves and overaccumulation of xanthine. Belongs to the xanthine dehydrogenase family.