An electrochemical perspective on human sulfite oxidase as a potential nitrite reductase

The Mo-dependent enzyme human sulfite oxidase (HSO) oxidises highly neurotoxic sulfite to benign sulfate in the final step of cysteine catabolism. Although sulfite is its only known physiological substrate, HSO has been suggested to play a role in the generation of nitric oxide (NO) from nitrite under ischemic conditions. In this work we have investigated the electrochemically driven nitrite reductase activity of HSO mediated by the benzyl viologen radical cation. We show that HSO can act as an effective nitrite reductase with a K_M value of 3.5 mM at pH 7. A heme-free variant of HSO behaves similarly. We also demonstrate electrochemically driven tandem sulfite oxidation and nitrite reduction with HSO using a known Fe^III coordination compound as mediator. Significant pH-dependence of catalytic activity is found.