Oxygen-derived radicals: a link between reperfusion injury and inflammation.

Oxygen-derived free radicals (superoxide and hydroxyl) and related species (hydrogen peroxide and hypohalous acids) have well-defined roles in the inflammatory process. Their actions include the killing of microorganisms as well as participation in cell-to-cell communication among phagocytes via the activation of a superoxide-dependent chemoattractant. The active oxygen species also have roles in postischemic injury brought about by the conversion during ischemia of the enzyme xanthine dehydrogenase (EC 1.1.1.204) to the radical-producing xanthine oxidase (EC 1.1.3.22). Although the enzymes responsible for producing superoxide in inflammation and ischemia are quite distinct, and are triggered by very different events, there are points of interplay in the two mechanisms whereby an ischemia/reperfusion-induced injury would lead to inflammation, and conversely whereby inflammation could lead to impairment of the circulation and hence to ischemic injury.