an enzyme (usually a metalloflavoenzyme) catalyzing the transfer of hydrogen from some metabolite to oxygen, forming hydrogen peroxide in the process; usually a metalloflavoenzyme; e.g., xanthine oxidase and others in several sub-subclasses (e.g., EC 1.1.3, 1.2.3, 1.7.3, 1.8.3, 1.10.3).