<p>Aldehyde oxidase (<db_xref db="EC" dbkey="1.2.3.1"/>) catalyses the conversion of an aldehyde in the presence of oxygen and water to an acid and hydrogen peroxide. The enzyme is a homodimer, and requires FAD, molybdenum and two 2FE-2S clusters as cofactors. Xanthine dehydrogenase (<db_xref db="EC" dbkey="1.1.1.204"/>) catalyses the hydrogenation of xanthine to urate, and also requires FAD, molybdenum and two 2FE-2S clusters as cofactors. This activity is often found in a bifunctional enzyme with xanthine oxidase (<db_xref db="EC" dbkey="1.1.3.22"/>) activity too. The enzyme can be converted from the dehydrogenase form to the oxidase form irreversibly by proteolysis or reversibly through oxidation of sulphydryl groups.</p>