Xanthine Oxidase (XAO)

The 6-oxopurine, xanthine (Xan, 2,6-dioxopurine), and its nucleosides and nucleotides are involved in a variety of intracellular metabolic pathways as substrates and/or intermediates of numerous enzymes or enzyme systems, e.g. Xan, a substrate of both xanthine oxidase (XO) and xanthine dehydrogenase, is an intermediate in the formation, from Hyp, of urate, the end product of purine nucleotide catabolism. This enzyme and xanthine dehydrogenase, interconvertible (by proteolysis or disulfide formation) forms of the same gene product known as xanthine oxidoreductase, catalyzes the oxidation of many purines and related analogues. In mammalian systems it catalyzes the last two steps of purine catabolism, the oxidation of Hyp to xanthine and of Xan to uric acid. The initial methyl acceptor for caffeine biosynthesis in tea and coffee plants is  is xanthosine (Xao) and/or xanthosine-5 -phosphate (XMP). XMP is an intermediate in the formation of GMP from IMP by IMP dehydrogenase. XTP is as efficient a phosphate donor as ATP for human deoxycytidine kinase. In purine salvage pathways, some purine phosphoribosyltransferases (PRTase) will accept xanthine, in several instances with high selectivity relative to other purines.


1.Kulikowska E,et al. Acta Biochim Pol. 2004;51(2):493–531.