High-affinity iron binding by xanthine oxidase

Equilibrium dialysis studies on competitive binding of ⁵⁹FeCl₃ to xanthine oxidase and citrate or ATP have been carried out. Iron binding to the enzyme was observed in the presence of 0.1 mM of either chelator, suggesting that xanthine oxidase is likely to have iron bound in many in vitro experimental systems and raising the possibility that it may be able to compete for intracellular chelatable iron. One high-affinity-binding site per monomer was found, with an affinity constant of 5 × 10¹² M⁻¹. The significance of this iron as a Fenton reaction catalyst is discussed.