Xanthine oxidase (xanthine dehydrogenase) is composed of two identical subunits of approximately 150,000 daltons. Each subunit contains four oxdation-reduction active cofactors/monomers. In vivo, the enzyme exists mostly as the dehydrogenase type (the NAD-dependent type). The cDNA has been cloned from human liver, and the amino acid sequence has been determined. As xanthine oxidase seems to produce superoxide in postischemic reperfusion, the relation between the superoxide and postischemic tissue injury has been discussed. It has also been reported that inhibition of xanthine oxidase by allopurinol may cause severe 6-mercaptopurine toxicity.