ABSTRACT
The univalent reduction of oxygen results in the production of superoxide radicals. Superoxide radicals are generated in many biological reactions that can reduce molecular oxygen. Autooxidation of leukoflavins, 1 hydroquinones, 2 catecholamines, 3 thiols, 4 tetrahydropterines, 5 hemoproteins, 6 , 7 and reduced ferrodoxins 8 has been shown to generate superoxide radicals. Subcellular organelles such as chloroplasts 9 and mitochondria 10 also produce superoxide radicals. These radicals are also formed by several oxidative enzymes — xanthine oxidase, aldehyde oxidase, dihydroorotic acid dehydrogenase, and several flavin dehydrogenases. 11 The radical also occurs as an intermediate in the reactions catalyzed by galactose oxidase, 12 indoleamine dioxygenase, 13 and 2-nitropropane dioxygenase. 14 Finally, superoxide is produced by neutrophils 15 and macrophages. 16
