[4] The same protein, which in humans has the HGNC approved gene symbol XDH, can also have xanthine dehydrogenase activity (EC 1.17.1.4).

[10] Another reaction catalyzed by xanthine oxidase is the decomposition of S-nitrosothiols (RSNO), a class of reactive nitrogen species, to nitric oxide (NO), which reacts with a superoxide anion to form peroxynitrite under aerobic conditions.

In the reaction with xanthine to form uric acid, the S=MoVIO-H group ionizes and the resulting MoVI-O− attacks carbon concomitant with transfer of H− to Mo=S.

[citation needed] Xanthine oxidase is a superoxide-producing enzyme found normally in serum and the lungs, and its activity is increased during influenza A infection.

Type I xanthinuria has been traced directly to mutations of the XDH gene which mediates xanthine oxidase activity.

[18] Increased concentration of serum uric acid has been under research as an indicator for cardiovascular health factors, and has been used to strongly predict mortality, heart transplant, and more in patients.

[16] But it is not clear whether this could be a direct or casual association or link between serum uric acid concentration (and by proxy, xanthine oxidase activity) and cardiovascular health.

[18] Reactive nitrogen species, such as peroxynitrite that xanthine oxidase can form, have been found to react with DNA, proteins, and cells, causing cellular damage or even toxicity.