Bovine xanthine oxidase electrocatalysis: substrate oxidation and its role in nitrite reduction

Xanthine oxidase plays a central role in purine catabolism in animals, plants and bacteria where it oxidises hypoxanthine to xanthine and xanthine to uric acid. In this work we have immobilised bovine xanthine oxidase on a glassy carbon electrode within a glutaraldehyde film and demonstrated hypoxanthine and xanthine oxidase electrocatalysis using mediator methylene blue as a mediator. The role of xanthine oxidase in the reduction of nitrite to nitric oxide has been probed in two ways. Using the electrochemically reduced mediator methyl viologen, nitrite reduction by xanthine oxidase is complicated by non-enzymatic reduction of nitrite by the mediator. Using hypoxanthine as reductant, nitrite reduction occurs but the product nitric oxide rapidly inactivates the enzyme. Xanthine oxidase can be restored to its functional form by redox cycling on an electrode in the presence of sulfide showing that nitric oxide leads to desulfuration of the molybdenum active site.