Mechanism of the inhibition of milk xanthine oxidase activity by metal ions: a transient kinetic study

The nature and mechanism of the inhibition of the oxidoreductase activity o f milk xanthine oxidase (XO) by Cu2+, Hg2+ and Ag+ ions has been studied by steady state and stopped flow transient kinetic measurements. The results show that the nature of the inhibition is noncompetitive. The inhibition co nstants for Cu2+ and Hg2+ are in the micromolar and that for Ag+ is in the nanomolar range. This suggests that the metal ions have strong affinity tow ards XO. pH dependence studies of the inhibition indicate that at least two ionisable groups of XO are involved in the binding of these metal ions. Th e effect of the interaction of the metal ions on the reductive and oxidativ e half reactions of XO has been investigated, and it is observed that the k inetic parameters of the reductive half reaction are not affected by these metal ions. However, the interaction of these metal ions with XO significan tly affects the kinetic parameters of the oxidative half reaction. It is su ggested that this may be the main cause for the inhibition of XO activity b y the metal ions. (C) 2000 Elsevier Science B.V. All rights reserved.