Interactions of selenium and antioxidants with mercury, cadmium and silver

Abstract

Until 1957, the only physiological significance of selenium was thought to be its potential toxicity. The discovery by Schwarz and Foltz (1957) that this element would prevent dietary liver necrosis in rats resulted in a reassessment of selenium. Subsequently, extensive investigations into the metabolic functions of selenium culminated in its identificat ion as an integral part of the mammalian enzyme, glutathione peroxidase (Rotruck et a/., 1 973). Another milestone in selenium research was reached when it was discovered to counteract the toxicity of some heavy metals. Thus, this created a situation in which an element known to be very toxic itself would counteract the toxicity of other elements such as the heavy metals. This rather unique behavior of selenium has resulted in intensive research on the relationship of this element to the metabolic effects of heavy metals. This protective effect of Se against metal toxicity is further complicated by the well established metabolic interaction between selenium and vitamin E and other antioxidants. The purpose of this presentat ion is to discuss some of these relationships and to present a few possible mechanisms for these interactions. The interactions between Se, metals, and antioxidants is very complex and subject to additional influences as illustrated by the fol lowing. High levels of dietary vitamin E (500 I.U./kg) do not decrease mortality in Japanese quails fed diets containing 0.6 ppm selenium and 30 ppm mercuryas methylmercury, whereas high levels of vitamin E do decrease mortality in these birds when lower levels of selenium (0.01 ppm) are included in this diet (Welsh and Soares, 1976). Other interacting factors include cystine levels, level of protein, and the kind of protein. The combination of selenium and cystine produces a greater additive effect against methylmercury toxicity in rats than either one of these alone (Still ings et al., 1974). Toxicity signs were reduced when fish protein replaced casein in the basal diet and a 20% protein level from either source reduced toxicity symptoms as compared to a 1()% protein level. A summary of the relative effectiveness of selenium and vitamin E against heavy metals is shown in Table 1. Selenium does not counteract the toxicity of all heavy metals, and vita rain E is more effective against some than selenium. As far as effectiveness of selenium and vitamin E, there appear to be three classes of heavy metals. One class consists of metals like cadmium, inorganic mercury, and possibly methylmercury in which selenium is highly effective in altering their toxicities but where vitamin E has little or no influence. A second group which includes silver shows vitamin E to be highly effective against its toxicity but selenium is effective only at excessively high levels. A third group of metals, of which lead is an example, is counteracted by vitamin E but selenium has little or no effect. Thus, the effectiveness of selenium and vitamin E is dependent upon the metal, and the mechanisms for counteracting metals by selenium and vitamin E would appear to be