Luminol chemiluminescence using xanthine and hypoxanthine as xanthine oxidase substrates

Luminol chemiluminescence induced by the xanthine or hypoxanthine-O 2-xanthine oxidase system is analyzed and compared. Characteristics of the light emission curves were examined considering the conventional reaction scheme for the oxidation of both substrates in the presence of xanthine oxidase. The ratio of the areas of the light intensity versus time curves for hypoxanthine compared with xanthine is greater than that expected from the rate of superoxide production during substrate oxidation to uric acid. The O 2 ⨪ to uric acid ratio for each substrate can account for differences in xanthine and hypoxanthine-supported light emission, since uric acid is a strong inhibitor of O 2 ⨪-dependent luminol chemiluminescence. These results are consistent with a free radical scavenging role for uric acid. A similar but weaker scavenging effect of xanthine may also contribute to the observed differences in chemiluminescent yields between both substrates.