FREE-RADICALS - FUNDAMENTAL NOTIONS AND M ETHODS OF EXPLORATION

Oxygen has invaded progressively, and through the ages, an initially a naerobic world. Living organisms hail to invent, in the course of evol ution, diverse and ingenious defence systems, to survive the toxicity of this element, which was new for them. Strengthened by this experien ce over billions of years, the present superior organisms, and particu larly human species, are thoroughly adapted to 21 per cent of atmosphe ric oxygen. Nevertheless, the equilibrium is fragile and the menace of oxygen hovers continually. This deleterious potential of oxygen is at tributed to the formation, in vivo, of free radicals, a free radical b eing, by definition, any chemical species possessing one or several mi smatched electrons. These free radicals are, in general, very active. They trigger chain reactions able to damage the different constituents of the living organism. Basic oxygen, must be pre-activated to manife st its toxicity. Such an activation can be achieved in two ways: it ca n be photodynamic, ending mainly in singlet oxygen, it can be reducing , followed by the formation of the anion hydrogen peroxide and of radi cal hydroxyl; the latter is the most reactive chemical species in the biological world. The reductive process is accelerated in the presence of transition metals, such as iron and copper, and/or specific enzyme s (monoxygenase and certain oxydases). This activation takes place in different cellular compartments: mitochondria, microsomes, peroxysomes , cytoplasmic membrane. To this potential toxicity of oxygen, the orga nism opposes different anti-oxidant defence systems. A first group wor ks up the radical drain, inhibiting activation mechanisms. Such a grou p, as a consequence, warns of the initiation of radical reactions. The second group neutralizes the free radicals already formed and thus st ops the chain of propagation. In this group can be found detoxifying e nzymes, notably superoxide dismutase and catalase, producing jointly p eroxidases, particularly peroxidase glutathions. Such enzymes for the most part have trace elements as cofactors. In this secund group can a lso be found various molecules which act like 'substrate suicide', or as an anti-oxidant shield. Among these molecules, some can act in the lipidic phase, such as tocopherols. carotenoids and ubiquinones. Other molecules which are lipophobic, mainly ascorbic acid and uric acid, a re active in a hydrated environment. In the case of a weakening of suc h an antioxidant defence or excess production of radicals, a state of oxidative str ess occurs. Uncontrolled, these radicals will damage dif ferent biological targets: lipids, DNA, proteins. Disturbances of cell ular metabolism will occur, unless corrective defences intervene. The identification of these radical phenomena is an obligatory stage. But because of the very short life span of free radicals, identification p oses a real analytical problem. However, three approaches are possible : identification of free radicals, either directly by means of paramag netic electron resonance, or indirectly by identifying some more stabl e intermediates, evaluation of the traces of radical attack on biologi cal molecules, for example by high performance liquid chromatography, gas-liquid chromatography, colorimetric tests, estimation of the antio xidant status, for example by colorimetric tests, immunoenzymatic meth ods, high performance liquid chromatography.