Free Radical Signaling in Reactions Catalyzed by Enzymes—Producers of Reactive Oxygen and Nitrogen Species

There is some justice in the general opinion that contemporary free radical studies in biology and medicine began after the discovery by McCord and Fridovich [1-3] of superoxide production by xanthine oxidase (XO). These authors showed that XO catalyzed cytochrome c reduction, which was inhibited by the enzyme superoxide dismutase (SOD). (Actually, McCord and Fridovich applied another enzyme, bovine erythrocyte carbonic anhydrase containing SOD as an admixture). In 1968-1970, superoxide (O2·−) was positively identied in biological systems by biological, physicochemical, and spectral analytical methods [4-6].