Competitive inhibition of xanthine oxidase by guanidinium: dependence upon monovalent anions and effects on production of superoxide.

Guanidinium chloride inhibits xanthine oxidase competitively with respect to xanthine. Although previously attributed solely to the guanidinium cation, it is now apparent that this inhibition owes much to the counter anion. Thus KCl or KBr, which were not themselves inhibitory, markedly increased the inhibitory potency of guanidinium sulfate. Weak binding of the guanidinium cation evidently creates a binding site for a monovalent anion, whose subsequent binding then stabilizes the binding of the guanidinium. In effect the ion pair is bound to the catalytic center. The proportion of univalent reduction of dioxygen by xanthine oxidase, at fixed concentrations of xanthine and dioxygen and at fixed pH, can be markedly increased by addition of a competitive inhibitor such as guanidinium bromide.