In the presence of transition metals, such as iron, superoxide can catalyze the generation of hydroxyl radicals by a sequence of reactions called the Haber-Weiss reaction. Because white rot fungi also produce hydrogen peroxide, it may not be necessary to dismutate superoxide to produce hydrogen peroxide. The production of oxygen radicals by white rot fungi had been suggested earlier by several investigators. However, the source of oxygen radicals remained unknown. Now the involvement in the biodegradation of environmental pollutants must be elucidated.
Polyaromatic hydrocarbons present a case in point.
Some of these chemicals, such as in coal tar and creosote, have oxidation potentials too high to be oxidized by the lignin peroxidases. However, polyaromatic hydrocarbons are oxidized by the fungi, and there is evidence that the lignin peroxidases are indeed involved in their oxidation. Perhaps polyaromatic hydrocarbons are being oxidized by the hydroxyl radicals. These mechanisms may also explain why the peroxidases catalyze the depolymerization of lignin rather than polymerization, as well as dehalogenation rather than halogenation. The presence of reductants would help form reduced, not polymerized, products of radicals.