Aldehyde oxidase able to catalyze the oxidation of retinaldehyde into retinoate. Is responsible for the major all-trans-retinaldehyde-metabolizing activity in the Harderian gland, and contributes a significant amount of the same activity in the skin. Is devoid of pyridoxal-oxidizing activity, in contrast to the other aldehyde oxidases. Acts as a negative modulator of the epidermal trophism. May be able to oxidize a wide variety of aldehydes into their corresponding carboxylates and to hydroxylate azaheterocycles. an aldehyde + H2O + O2 = a carboxylate + H(+) + H2O2 H2O + O2 + retinal = H(+) + H2O2 + retinoate all-trans-retinal + H2O + O2 = all-trans-retinoate + H(+) + H2O2 Binds 2 [2Fe-2S] clusters per subunit. Binds 1 FAD per subunit. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Homodimer. Highly expressed in Harderian glands and sebaceous glands with detectable levels in the epidermis and other keratinized epithelia (at protein level). Detected in testis. The expression is 3 times greater in females than in males. Repressed by testosterone in Harderian glands. In skin, induced by UVB light. Mice are viable, fertile and born at the expected Mendelian rate. However, they show a deficiency of retinoic acid synthesis in both the Harderian gland and skin. The Harderian gland's transcriptome of knockout mice demonstrates overall down-regulation of direct retinoid-dependent genes as well as perturbations in pathways controlling lipid homeostasis and cellular secretion, particularly in sexually immature animals. The skin is characterized by thickening of the epidermis in basal conditions and after UVB light exposure. 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.