Can dietary micronutrients influence tissue antioxidant capacity?

Numerous recent studies have indicated possible roles for free-radical-mediated processes in human pathology. This has led to a great interest in the substances thought to prevent the deleterious effects of these substances in vivo, the antioxidants. The biochemistry of free-radical reactions, the manner in which they damage cells and the antioxidants inhibiting their reaction with cellular constituents, have been the subject of a number of recent reviews (see Halliwell & Gutteridge, 1989). Some of the antioxidants are micronutrients derived directly from the diet, while others require an adequate supply of specific micronutrients to ensure they remain at appropriate concentrations within the body. Examples of the former are vitamin E and the carotenoids, while the latter are exemplified by the absolute requirement of glutathione peroxidase (EC 1.11.1.9) for Se or cytosolic superoxide dismutase (EC 1.15.1.1) for Zn and Cu. The aim of the present short review is to examine aspects of some of these antioxidant micronutrients with particular reference to the impact which dietary manipulation can have on the ‘antioxidant capacity’ of tissues. Antioxidant capacity is a concept which has arisen to describe the overall ability of tissues to inhibit free-radical-mediated processes; it is dependent on the concentrations of individual antioxidants and activity of protective enzymes, but also reflects interactive effects of the different antioxidants. A question of particular current interest is whether supplementation of adequately nourished subjects with antioxidants will lead to an enhanced ability to reduce possible deleterious effects of free-radical activity and the incidence of certain common pathologies. The possibility that reported protective effects of some antioxidant micronutrients may be due to properties other than their ability to inhibit or scavenge free-radical reactions will also be briefly examined. The commonest and probably most important of the antioxidant micronutrients are shown in Table 1. Vitamin E, the major lipid-soluble antioxidant, is capable of breaking the chain of free-radical-mediated lipid peroxidation in cell membranes, thus preventing formation of lipid hydroperoxides. Carotenoids appear to have antioxidant properties as do some quinones, such as ubiquinone. These also can react directly with free radicals. Vitamin C can directly scavenge radicals in the cytosolic fraction of cells, and may also play a role in regeneration of vitamin E. The trace elements Zn, Cu, Se and Mn are essential components of the antioxidant enzymes superoxide dismutase (Cu, Zn or Mn)