The chemistry of xanthine oxidase. 8. Electronspin-resonance measurements during the enzymic reaction

Abstract

Electron-spin-resonance signals obtained from xanthine oxidase during the enzymic oxidation of low concentrations of xanthine by molecular oxygen have been studied with a fast-reaction technique. Two preparations of xanthine oxidase were examined. For one, the kinetics of the appearance and disappearance of the Mo(v) signal correlated well with the known kinetics of the overall reaction, thus providing the first direct evidence for the participation of the metal in the electron-transport processes of the enzyme. Reoxidation of the 2d sample at the high enzyme concentrations needed for electron-spin-resonance measurements was slow; this may have been due to the presence of an inhibitor. The detection of signals possibly arising from the reduction of iron may indicate a redox function for this metal in addition to flavin and molybdenum in xanthine-oxidase action.