Oxidase with broad substrate specificity, oxidizing aromatic azaheterocycles, such as N1-methylnicotinamide, N-methylphthalazinium and phthalazine, as well as aldehydes, such as benzaldehyde, retinal, pyridoxal, and vanillin. Plays a role in the metabolism of xenobiotics and drugs containing aromatic azaheterocyclic substituents. Participates in the bioactivation of prodrugs such as famciclovir, catalyzing the oxidation step from 6-deoxypenciclovir to penciclovir, which is a potent antiviral agent. Also plays a role in the reductive metabolism of the xenobiotic imidacloprid (IMI) via its nitroreduction to nitrosoguanidine (IMI-NNO) and aminoguanidine (IMI-NNH(2)). 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. May play a role in adipogenesis. Cannot use xanthine and hypoxanthine as substrate. an aldehyde + H2O + O2 = a carboxylate + H(+) + H2O2 H2O + O2 + retinal = H(+) + H2O2 + retinoate Binds 2 [2Fe-2S] clusters per subunit. Binds 1 FAD per subunit. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. kcat is 317.6 min(-1) for benzaldehyde oxidation, 128.1 min(-1) for phthalazine oxidation, 49.5 min(-1) for retinal oxidation, and 519.9 min(-1) for acetaldehyde oxidation (at 30 degrees Celsius and pH 7.5) (PubMed:19401776). However in this article, the measures are done with the recombinant protein, which is only 20% active, due to an incomplete saturation of the molybdenum cofactor with the sulfido ligand (PubMed:19401776). Homodimer. Highest expression in esophagus. Moderately low expression in lung, liver, heart, Harderian gland, olfactory mucosa, skin and testis. In brain, expression is very high in choroid plexus, high in hind brain and low in hippocampus and cerebellum. In spinal cord expression is strongest in anterior horns. Low expression detected in spleen and eye. AOX1 expression in the livers of mice is approximately seven times greater in males than females. Expressed in adult liver but not in neonatal or embryonic liver. Not detected in preadipocytes but strongly induced in mature adipocytes. Repressed by pioglitazone, fenofibrate and PPARA agonists. 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.