Oxidase with broad substrate specificity, oxidizing aromatic azaheterocycles, such as N1-methylnicotinamide and phthalazine, as well as aldehydes, such as benzaldehyde, retinal and pyridoxal. Plays a key role in the metabolism of xenobiotics and drugs containing aromatic azaheterocyclic substituents. Is probably involved in the regulation of reactive oxygen species homeostasis. Is a prominent source of superoxide generation via the one-electron reduction of molecular oxygen. Also catalyzes nitric oxide (NO) production; under anaerobic conditions, reduces nitrite to NO with NADH or aldehyde as electron donor, but under aerobic conditions, NADH is the preferred substrate. These reactions may be catalyzed by several isozymes. an aldehyde + H2O + O2 = a carboxylate + H(+) + H2O2 Binds 2 [2Fe-2S] clusters per subunit. Binds 1 FAD per subunit. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Homodimer. AOX genes evolved from a xanthine oxidoreductase ancestral precursor via a series of gene duplication and suppression/deletion events. Different animal species contain a different complement of AOX genes encoding an equivalent number of AOX isoenzymes. In mammals, the two extremes are represented by certain rodents such as mice and rats, which are endowed with 4 AOX genes, and by humans, whose genome is characterized by a single active gene (PubMed:23263164). Belongs to the xanthine dehydrogenase family. The experimental design does not allow to distinguish AOX1 from AOX3 in rat liver as the effector of the superoxide and nitric oxide production (PubMed:17353002 and 19801639).