Biochemical features of selenium have emerged only in recent years, and investigations into selenoproteome gene expression and the selenocysteine synthetic pathway have provided evidence that underpins its biochemical roles in health and disease.
National Cancer Institute/Purdue University Center for Cancer Research; Ji-Xin Cheng.jpg
Some of the main biochemical features of selenium have emerged only recently, although it has been known to be an essential element for nearly 50 years. The investigations into selenoproteome gene expression and a better understanding of the selenocysteine synthetic pathway have provided the evidence that underpins the biochemical roles of the element. The roles of the selenium-containing enyzmes (glutathione peroxidases, thioredoxin reductases and iodothyronine deiodinases) are well established, the first two being linked with antioxidant activity, and the latter involved with thyroid hormone metabolism. Recently, the interaction between sulphur, in the same periodic group and therefore chemically similar, and selenium has been investigated in a bid to understand the role of both elements in disease. There is also renewed interest in the anticancer properties of selenium-containing compounds as evidence of their effectiveness, as demonstrated in animal models.
Biological properties
Selenium (Se) is a naturally occurring metalloid that was not identified as an essential element until 1973, having previously been considered to be toxic. It belongs to the same group of the periodic table as oxygen, sulphur, tellurium and polonium. Whereas the essential biological properties of oxygen and sulphur are well known, those for Se, which appears just below sulphur in the periodic group, have emerged only over the past decade or so. Sulphur and Se share similar chemical properties. Furthermore, their metabolic pathways are intricately linked, and interestingly there are Se analogues for every sulphur compound. However, in contrast to sulphur, which is present at approximately 140 g, the amount of Se in a 70 kg man is about 10 mg. Diet is the main source of the sulphur-containing amino acids cysteine and methionine and their Se analogues selenocysteine (SeCys) and selenomethionine (SeMet), respectively. Water-soluble inorganic forms (selenate and selenite) are also present in food and drinking water.
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