Superoxide radical: controversies, contradictions, and paradoxes.

The study of free radical biology has engendered a great deal of controversy and apparently conflicting observations, particularly with regard to the use of the antioxidant enzyme superoxide dismutase as a protective or therapeutic agent. Slowly, the reasons behind the confusion are beginning to emerge. The superoxide radical, O2.-, has a number of paradoxical physiological and pathophysiological roles. Several examples of the radical's schizophrenic behavior include its roles in bactericidal action versus inflammation, as a modulator of cell division versus malignant transformation and apoptosis, and as both an initiator and a terminator of lipid peroxidation.