Free radicals in medicine. I. Chemical nature and biologic reactions

Free radicals are reactive chemical species that differ from other compounds in that they have unpaired electrons in their outer orbitals. They are capable of damaging cellular components, and accumulating evidence suggests they may contribute to various disease entities. Biologic systems are exposed to free radicals that have been formed endogenously or that result from external influences such as ionizing radiation. Oxygen free radicals are continuously being produced intracellularly by oxidation-reduction reactions. The sequential univalent reduction of molecular oxygen initially forms the superoxide anion radical, which in turn is converted, in the presence of transition metal ions, into the highly reactive hydroxyl radical. Free radicals are detected by electron spin resonance spectroscopy, but often this procedure is difficult to use for study of free radical involvement in biologic systems, and investigators have resorted to inferring their presence by identifying the products of free radical reactions. All aerobic organisms possess substances that help prevent free radical-mediated injury. These include antioxidants such as vitamin E and the enzymes superoxide dismutase and glutathione peroxidase. A second part of this review will describe the role of free radicals in specific disease entities.