Oxidase with broad substrate specificity, oxidizing aromatic azaheterocycles, such as phthalazine, as well as aldehydes, such as benzaldehyde and retinal. 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.