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. May be a prominent source of superoxide generation via the one-electron reduction of molecular oxygen. May also catalyze nitric oxide (NO) production via the reduction of nitrite to NO with NADH or aldehyde as electron donor. 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. Inhibited by potassium cyanide, menadione, benzamidine, raloxifene and norharmane. kcat is 4 sec(-1) for phthalazine oxidation (PubMed:11562361). kcat is 130 min(-1) for benzaldehyde oxidation, 384 min(-1) for butyraldehyde oxidation and 1279 min(-1) for 2-OH-pyrimidine oxidation (PubMed:21705476). kcat is 41.1 min(-1) for phthalazine oxidation, 41.9 min(-1) for benzaldehyde oxidation, 14.7 min(-1) for N1-methylnicotinamide oxidation and 51.7 min(-1) for phenanthridine oxidation (PubMed:23019336). Homodimer. Highly expressed in liver (at protein level). In liver, the expression is greater in males than females. Induced by testosterone. 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.