The mechanism of the activity-dependent luminescence of xanthine oxidase.

The weak luminescence that accompanies the aerobic xanthine oxidase reaction is inhibited by superoxide dismutase, by catalase, and by scavengers of hydroxyl radicals. It is also entirely dependent upon the presence of carbonate. It thus appears that the O₂ and H₂O₂ produced during the aerobic action of xanthine oxidase interact to generate OH which, in turn, reacts with carbonate to yield the carbonate radical (CO₃^?). The species that is directly responsible for light emission appears to be produced by a dimerization of carbonate radicals, since the light intensity was a function of the square of the carbonate concentration. The data provide no reason to suppose that the light-emitting species is singlet oxygen.