Aldehyde Oxidases and Xanthine Oxidases

Aldehyde oxidases (AOX, EC 1.2.3.1) and xanthine oxidases (XOR, EC 1.17.3.2) are molybdo-flavoproteins, with complex structural and mechanistic features.60 The enzymes principally carry out oxidation reactions on nitrogen heterocycles, ultimately inserting one atom of oxygen from water without the need for an external source of reducing equivalents. In the main, AOX and XOR target sp2-hybridized carbon atoms rendered electron-deficient by a nitrogen atom to which they are linked by a double bond (CH = N), with the resulting formation of lactam metabolites.61 Historically, the majority of studies with these proteins have been carried out with the rabbit liver form of AOX, and XOR isolated from cow milk, because these are rich and readily available sources of the two enzymes, and because of difficulties in expressing recombinant forms of these complex enzymes in high yield.62

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