Oxypurinol, an inhibitor of xanthine oxidase (XO), is being studied to block XO-catalyzed superoxide radical formation and thereby treat and protect failing heart tissue. Allopurinol, a prodrug that is converted to oxypurinol by xanthine oxidase, is also being studied for similar purposes. Because allopurinol, itself, may be generating superoxide radicals, we currently studied the reaction of allopurinol with xanthine oxidase and confirmed that allopurinol does produce superoxide radicals during its conversion to oxypurinol. At pH 6.8 and 25 degrees C in the presence of 0.02 U/ml of XO, 10 and 20 microM allopurinol both produced 10 microM oxypurinol and 2.8 microM superoxide radical (determined by cytochrome C reduction). The 10 microM allopurinol was completely converted to oxypurinol, while the 20 microM allopurinol required a second addition of xanthine oxidase to complete the conversion. Fourteen percent of the reducing equivalents donated from allopurinol or xanthine reacted with oxygen to form superoxide radicals. Superoxide dismutase prevented the reduction of cytochrome C by these substrates. At higher xanthine oxidase concentrations, or at lower temperatures, more of the 20 microM allopurinol was converted to oxypurinol during the initial reaction. At lower xanthine oxidase concentrations, or higher temperatures, less conversion occurred. At pH 7.8, the amount of superoxide radicals produced from allopurinol and xanthine was nearly doubled. These results indicate that allopurinol is a conventional substrate that generates superoxide radicals during its oxidation by xanthine oxidase. Oxypurinol did not produce superoxide radicals.