Cytosolic xanthine dehydrogenase (XDH) catalyzes the reaction of hypoxanthine, H2O, and NAD+ to form xanthine and NADH + H+. Studies of partially purified enzyme from chicken liver established its dehydrogenase activity (and lack of oxidase activity, in contrast to homologous mammalian enzymes such as the one from bovine milk) (Richert and Westerfeld 1951; Morell 1955; Remy et al. 1955). While the gene encoding chicken XDH has been cloned and shown to encode active enzyme (Sato et al. 1995), the pure enzyme has not been extensively characterized and is therefore annotated here simply as a monomer with no cofactors.