Substrate Activation with a Xanthine Oxidase Reaction

Handler and coworkers [1,2] have studied the non‐linearity of xanthine oxidase reciprocal plots for the chicken liver and the Micrococcus lactilyticus systems. In the latter case it was suggested that the non‐linearity was due to a dismutation reaction observed at high xanthine concentrations. The studies reported here were undertaken in an effort to demonstrate the presence of such a reaction in the chicken liver system. On the basis of ultraviolet spectra taken during the course of xanthine and hypoxanthine oxidations, it was found that xanthine accumulates in the reaction mixture with hypoxanthine as the substrate and that hypoxanthine does not accumulate in detectable quantities during xanthine oxidation. Conditions were used such that hypoxanthine should have been detectable as a transient if the dismutation reaction occurred. Hypoxanthine which has not been reported to undergo a dismutation reaction shows the same type of non‐linearity in reciprocal plots as does xanthine. Relatively high rates of xanthine conversion to uric acid under anaerobic conditions (due to vigorous sweeping with nitrogen) were explained on the basis of residual oxygen. Similar results were obtained for hypoxanthine oxidation. On the basis of these results it seems highly unlikely that a dismutation reaction occurs in this system. Random and multi‐site mechanisms can be used to explain the non‐linearity.