The discovery of selenoproteins in 1973 was the starting point for today''s flourishing selenium field [1,2]. It provided evidence that selenium had biochemical functions that could account for its nutritional effects [3,4]. Further, it opened the selenium field to investigation by the methods of biochemistry, which led to the identification of several more selenoproteins and showed that selenocysteine was the form of the element in animal selenoproteins and in most bacterial ones. Although noteworthy efforts were made to uncover the mechanism of selenocysteine and selenoprotein synthesis using biochemical methods, the problem yielded only when attacked with the methods of molecular biology [5,6]. The bacterial mechanism was characterized first; characterization of the animal mechanism is a work in progress. It is interesting to note that the only genes that are devoted to selenium metabolism are those that support selenoprotein synthesis and selenocysteine catabolism. Consequently, it seems likely that competition for selenium between selenoprotein synthesis and the production of selenium excretory metabolites [7] controls who- body selenium homeostasis. The physiological functions of selenium derive fi-om the catalytic and physical properties of selenoproteins. Selenoproteins such as the glutathione peroxidases and the thioredoxin reductases have redox activities that allow them to serve in oxidant defense. The deiodinases use their redox activities to activate and inactivate thyroid hormones. From these two examples, it can be seen that selenoprotein functions are diverse while having in common a redox mechanism.
