Alloxanthine, complex with xanthine

We have not so far mentioned the Phase III increase in the Rapid signal (Fig. 5). It seems (67) that Phase II represents over reduction of molybdenum to Mo(IV), possibly by substrate radicals (see Section V H). The system then comes towards thermodynamic equilibrium by interaction between reduced active enzyme molecules and oxidized inactive ones (67, cf. 64). As Mo(IV) of the former is oxidized to Mo(V), during Phase III, so iron or flavin of the inactive enzyme is reduced. Later, in Phase IV, molybdenum of the inactive enzyme is reduced also to give the Slow signed. Alloxanthine, which as noted above, forms a stable complex with Mo(IV), seems to abolish both the slow phase in the 450 nm bleaching of the enzyme by xanthine and the Phase III increase in Rapid signal (91). 

Finally, in the case of inhibitory substrate analogues such as allo-xanthine, strong evidence has recently been presented that these bind to molybdenum in reduced xanthine oxidase (33). If the enzyme is reduced with xanthine, then treated anaerobically with alloxanthine and finally exposed to air, catalytic activity is lost. Though flavin and iron in the final product are in the oxidized state, there are significant spectral differences between it and the native enzyme. These are believed (33) due to reduction of molybdenum from Mo(VI) to Mo(IV) and complexing of... 

Hille, R., George, G. N., Eidsness, M. K., and Cramer, S. P., 1989, EXAES of xanthine oxidase complexes with alloxanthine, violapterin, and 6-pteridylaldehyde, Inorg. Chem. 28 4018n4022. 

Fig. 8.19. Allopurinol is a suicide inhibitor of xanthine oxidase. A. Xanthine oxidase catalyzes the oxidation of hyrpoxanthine to xanthine, and xanthine to uric acid (urate) in the pathway for degradation of purine nucleotides. B. The oxidations are performed by a molybdenum-oxo-sul-fide coordination complex in the active site that complexes with the group being oxidized. Oxygen is donated from water. The enzyme can work either as an oxidase (O2 accepts the 2e and is reduced to H2O2) or as a dehydrogenase (NAD accepts the 2e and is reduced to NADH.) C. Xanthine oxidase is able to perform the first oxidation step and convert allopurinol to alloxanthine (oxypurinol). As a result, the enzyme has committed suicide the oxypurinol remains bound in the molybdenum coordination sphere, where it prevents the next step of the reaction.

There is, however, more direct evidence for the presence of Mo (IV) in the cycle of xanthine oxidase. This evidence comes from the experiments of Massey and co-workers (24) who used alloxanthine (l) to trap the enzyme in its reduced state. A strong complex is formed between the reduced enzyme and alloxanthine, and excess alloxanthine and reductant can be removed. The enzyme is then reoxidized with Fe(CN)63", and two electrons per molybdenum center are found after the electrons required for the reoxidation of the iron-sulfur and flavin groupings are... 

Fig 7. Structure of the complex formed between alloxanthine and the iwlyb-denum(IV) center of xanthine oxidase consistent with EXAFS and EPR results (S S = pterin ligation) (110, 111). 

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