The mechanism of action of xanthine oxidase. The relationship between the rapid and very rapid molybdenum electron-paramagnetic-resonance signals

On the basis of the work of Gutteridge, Tanner & Bray [Biochem. J. (1978) 175, 887-897] and of other data in the literature, a mechanism for the reaction of xanthine oxidase with reducing substrates is proposed. In the Michaelis complex, xanthine is bound to molybdenum via the N-9 nitrogen atom. Coupled transfer of two electrons to molybdenum and the C-8 proton to the enzyme yields (Enzyme)-Mo-SH. Concerted with this process, reaction of the xanthine residue with a nucleophile in the active centre yields a covalent intermediate that breaks down to give the product by alternative pathways at high and at low pH values.