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The use of electron-spin-resonance techniques to detect free-radical formation and tissue damage

Free radicals are commonly defined as molecules or molecular fragments containing a single unpaired electron; the presence of the unpaired electron usually confers a considerable degree of chemical reactivity on such species, though this may vary dramatically with the radical under consideration. Although a number of stable free radicals are known (for example, diphenylpicrylhydrazyl and Fremy's salt (potassium nitrosodisulphonate)), most of the radicals of biological interest have considerable chemical reactivity and corresponding short half-lives; for example, the calculated half-lives of the hydroxyl radical (HO') or trichloromethylperoxyl radical (CCbO;?') in the liquid phase in a biological milieu are of the order of 1 ns and 1 ps respectively (Slater, 1984). Such radicals can be produced in tissues by a wide variety of methods (see Table 1; Pryor, 1966; Slater, 1972) and it is this wide range of possible generating systems which underlies the considerable and widespread current interest in free-radical-mediated tissue injury (for reviews, see Slater, 1972, 1978, 1984; Pryor, 1976-86; Mason, 1982; Halliwell & Gutteridge, 1984, 1985). An abbreviated list of cellular injuries and diseases which have been shown to involve free-radical production as either a primary or secondary feature is given in Table 2. Nutritional factors can play an important part in the aetiology of many free-radical disturbances in two ways: (1) by altering the levels of naturally occurring protective systems (see Table 3) or (2) by increasing tissue levels of substances (such as some transition metal ions) which can stimulate radical production in some way (Slater et af . 1987). Direct proof of radical production in a particular system is, however, very difficult to achieve due to the high reactivity and short half-lives of many of the species involved. In many cases the only evidence for the involvement of radicals arises from studies on the relatively long-term effects of such species, i.e. the detection of products which arise from radical reactions (malondialdehyde, conjugated dienes, lipid hydroperoxides etc., for methodology, see Packer, 1984). This is often unsatisfactory due to the possibilities of misinterpretation and the lack of information as to the time-course of events; radical