Oxy-Radicals and Related Species: Their Formation, Lifetimes, and Reactions

In this brief overview I will review some of the ways in which free radicals and related highly reactive species are produced in biological systems. I will discuss the reactivities and lifetimes of these species, and also will briefly list some pathological conditions and chronic diseases in which these species may be involved. The rich variety of the species of interest in free radical biology is notewor­ thy. Table 1 lists species that have been implicated in various physiological and pathological processes; some of these are neutral radicals, some are radical ions, and some are molecules that contain an even number of electrons and, therefore, are not free radicals. As we shall see, radicals differ greatly in their stabilities; one of the radicals listed in Table 1, nitrogen dioxide, is sufficiently stable to reach ppm levels in polluted urban air. What name can be used to encompass the species listed in Table I? A phrase that is finding increased use in free radical biology is partially reduced oxygen products. However, even this broad title is unsatisfactory since some of the species in Table 1 contain oxygen atoms at the same oxidation level as elemental oxygen (e.g. ozone and singlet oxygen). The property that these species share is that they have the potential to cause radical reactions to occur in biological systems. Some of these species are radicals and therefore react by radical processes; others react to produce molecules that can decompose to form radicals (e.g. singlet oxygen leads to hydroperoxides). However, these species need not always react via free radical pathways. For example, ozone, hydrogen