Glutathione could well be involved in protecting against cancer

The best known functions of selenium at nutritionally adequate, but not at excessive, levels are its role as a part of the enzyme Glutathione peroxidase and its interaction with heavy metals. Glutathione peroxidase destroys hydroperoxides and lipoperoxides, thereby protecting the constituents of the cells against free radical damage. Ip and Sinha (1981) have shown that selenium, through its function in Glutathione peroxidase, could well be involved in protecting against cancer induced by high intakes of fat, especially polyunsaturated fatty acids. Glutathione peroxidase activity in human blood increases with increasing selenium intakes, but reaches a plateau at intakes well below those customary in the United States (Thomson and Robinson, 1980). Thus, if the antitumorigenic effect of selenium is mediated through its function in Glutathione peroxidase, attempts to increase the enzyme activity by selenium supplementation, superimposed on an adequate diet in the United States, would not be successful. The second function of selenium is to protect against acute and chronic toxicity of certain heavy metals. Although selenium is known to interact with cadmium and mercury, the mechanism of action is not known. Selenium does not cause an increased elimination of the toxic elements, but, rather, an increased accumulation in some nontoxic form (National Academy of Sciences, 1971). It is conceivable that carcinogenic effects of these, and perhaps other heavy metals, could be counteracted by selenium, in a manner similar to its protection against their general toxicity. Diet Nutrition Cancer by National Research Council, page 168