This work reports on the direct electrochemistry of the xanthine oxidase (XO) from buttermilk, a mononuclear molybdenum enzyme that comprises four redox active cofactors: a five-coordinate mononuclear Mo ion, two [2Fe-2S] clusters, and a flavin adenine dinucleotide (FAD) group. The Mo, [2Fe-2S] and FAD redox responses are obtained from the enzyme immobilized on an activated single-wall carbon nanotubes (SWNTs) modified glassy carbon electrode using protein film voltammetry. The formal potentials of which are -0.61 V, -0.47 V and -0.37 V (vs. SCE) at pH 5.0, respectively. Upon addition of nitrate to the electrochemical cell, a steady-state voltammogram and i-t amprometric response were observed, indicating XO can catalyze the reduction of nitrate.