Lower Xanthine Oxidoreductase Activity in Isolated Perfused Hearts if Xanthine Replaces Hypoxanthine as Substrate

In normal cardiac tissue xanthine oxidoreductase is present in the dehydrogenase form. During ischemia xanthine dehydrogenase is converted to xanthine oxidase, which generates free radicals during reperfusion. Both forms catalyze the breakdown of hypoxanthine to xanthine and xanthine to urate. Its activity can be measured with hypoxanthine or xanthine as substrate. Most widely xanthine is used as substrate because only one product is formed and therefore the enzyme activity can easily be determined. With hypoxanthine as substrate the difficult two-step reaction can raise problems if one wants to calculate the activity. Since either substrate is used in studies of xanthine oxidoreductase, this can explain the controversial data reported for various species.1,2,3