In higher plants aldehyde oxidases (AO) appear to be homo- and heterodimeric assemblies of AO subunits with probably different physiological functions. AO-alpha may be involved in the biosynthesis of auxin, and in biosynthesis of abscisic acid (ABA) in seeds. In vitro, AO-alpha uses heptaldehyde, protocatechualdehyde, benzaldehyde, indole-3-aldehyde (IAld), indole-3-acetaldehyde (IAAld), cinnamaldehyde and citral as substrates; AO-beta uses IAAld, IAld and naphtaldehyde as substrates. H2O + indole-3-acetaldehyde + O2 = (indol-3-yl)acetate + H(+) + H2O2 Binds 2 [2Fe-2S] clusters. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Strongly inhibited by iodoacetate and potassium cyanide (KCN). Weakly inhibited by 2-mercaptoethanol, dithiothreitol (DTT), menadione, estradiol, 4'-(9-acridinylamino)methanesulfon-m-anisidine (mAMSA), allopurinol and tritonX-100. Not affected by p-chloromercuribenzoate. Kinetic values were obtained with the AO-alpha dimer. Optimum pH is 8. Optimum temperature is 65 degrees Celsius. Aldehyde oxidases (AO) are homodimers and heterodimers of AO subunits. AO-alpha is an AAO1 homodimer; AO-beta is an AAO1-AAO2 heterodimer. Predominantly expressed in roots, seedlings, mature siliques and seeds, and to lower extent in stems and rosettes. In seedlings, mostly expressed in lower part of hypocotyls and roots. Belongs to the xanthine dehydrogenase family.