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Selenium /563 SUPPLEMENT MONOGRAPHS SELENIUM /563 Schottner M, Spiteller G. Lignans interfering with 5alpha- Selenium is a metalloid element with atomic number 34 and dihydrotestosterone binding to human sex hormone-binding an atomic weight of 78.96 daltons. It belongs to the sulfur globulin. J Nat Prod. 1998;61:119-121. group of elements, which also includes oxygen, tellurium Thompson LV, Seidl MM, Rickard SE, et al. Antitumorigenic and polonium. Its atomic symbol is Se. Selenium was effect of a mammalian lignan precursor from flaxseed. Nutr discovered in 1817 by Berzelius who named it after Selene, Cancer. 1996;26: 159-165. the Greek goddess of the moon. The essentiality of selenium for animals was first reported in 1957. It was found that selenium administered to vitamin E- deficient rats prevented liver necrosis. Subsequently, it was Selenium found that selenium could prevent a number of disorders of DESCRIPTION farm animals. Isolated selenium deficiency in humans has Selenium is an essential trace element in human and animal not been described. Selenium deficiency appears to cause an nutrition. It is involved in the defense against the toxicity of illness or disorder in combination with a co-factor. In the reactive oxygen species, in the regulation of thyroid hormone case of Keshan disease, the co-factor appears to be the metabolism and the regulation of the redox state of cells. Coxsackievirus. It has been shown that infection of mice on Recognition of the vital importance of selenium in human a selenium-deficient diet with a nonvirulent Coxsackievirus and animal nutrition was long impeded by its very real toxic selects a stable cardiovirulent strain. In the case of Kashin- potential and by fears that selenium might be carcinogenic, Beck osteoarthropathy, the co-factor appears to be iodine fears that have now been largely displaced by some evidence deficiency. suggesting just the opposite-that selenium may provide protection against some cancers. Selenium is found in human and animal tissues as L- selenomethionine or L-selenocysteine. L-selenomethionine is The amount of selenium in food is a function of the selenium incorporated randomly in proteins in place of L-methionine. content of the soil. Selenium enters the food chain through These proteins are called selenium-containing proteins. Only incorporation into plant proteins as the amino acids L- a small fraction of L-methionine in proteins is present as L- selenocysteine and L-selenomethionine. Selenium, like most selenomethionine. On the other hand, the incorporation of L- trace elements and minerals, is not evenly distributed in the cysteine into proteins known as selenoproteins is not world's soil. Because of the uneven global distribution of random. That is, in contrast to L-selenomethionine, which selenium, disorders of both selenium deficiency and seleni- randomly substitutes for L-methionine, L-selenocysteine um excess are known. China has regions with both the does not randomly substitute for L-cysteine. In fact, L- lowest and highest selenium-containing soils in the world. selenocysteine has its own triplet code and is considered to be the 21st genetically coded amino acid. Marco Polo gave the first account of selenium toxicity, which he observed during his travels in western China in the The selenoproteins are comprised of four selenium-depen- 13th century. He linked the sloughing off of the hooves of dent glutathione peroxidases (GSHPx-l, GSHPx-2, GSHPx- horses to their consumption of certain plants in the regions. 3 and GSHPx-4), three selenium-dependent iodothyronine The soils of those areas are now known to contain the deiodinases, three thioredoxin reductases, selenoprotein P, highest concentrations of selenium in the world. Soils rich in selenoprotein Wand selenophosphate synthetase. The gluta- selenium are referred to as being seleniferous, and the thione peroxidases, and possibly selenoprotein P and seleno- condition of chronic selenium toxicity is known as selenosis. protein W, are antioxidant proteins. The selenium-dependent In the 1970s, a human cardiomyopathy endemic to certain iodothyronine deiodinases convert thyroxine to triiodothyro- areas of China was shown to be linked to dietary selenium nine, thus regulating thyroid hormone metabolism. The deficiency. This disorder, known as Keshan disease, is thioredoxin reductases reduce intramolecular disulfide bonds endemic to those areas of China with some of the most and regenerate vitamin C from its oxidized state, among selenium-poor soils in the world. Keshan disease is now other things. treated and prevented by selenium supplementation. ACTIONS AND PHARMACOLOGY Kashin-Beck disease, also known as "big joint disease," is an ACTIONS osteoarthropathy that is found in areas in China where the Selenium has antioxidant activity. Selenium may also have soil is selenium-poor. It is also linked to dietary selenium immunomodulatory, anticarcinogenic and anti-atherogenic deficiency. Kashin-Beck disease is found in Tibet, Siberia activities. It may have activity in detoxification of some and North Korea, also in areas where the soil is selenium- metals and other xenobiotics and activity in fertility enhance- poor and in which dietary selenium-deficiency is endemic. ment in males. 564/ SELENIUM PDR FOR NUTRITIONAL SUPPLEMENTS MECHANISM OF ACTION possible stimulatory effects of selenium on antibody The antioxidant activity of selenium is mainly accounted for production. by virtue of its role in the formation and function of the selenium-dependent glutathione peroxidases (GSHPx). Glu- The possible anticarcinogenic activity of selenium may be tathione peroxidases use reducing equivalents from gluta- accounted, for, in part, by its antioxidant activity as well as thione to detoxify hydroperoxides. There are four different its possible immune-enhancing activity. Selenium has been glutathione peroxidases. GSHPx-1 is present in most cells of shown to upregulate apoptosis in tumor cells in vitro and the body. GSHPx-2 (originally known as GSHPx-GI) is increase macrophage killing and protect against oxidative mainly found in the cells of the gastrointestinal tract. DNA damage, again, in vitro. Animal studies suggest that GSHPx-3 is an extracellular glutathione peroxidase. GSHPx- selenium may have anti-angiogenic activity. A possible 4 is a membrane-bound hydroperoxide glutathione peroxi- mechanism for selenium's possible anti-angiogenic activity dase. GSHPx-4 is also known as phospholipid hydroperoxide is its inhibitory effect on the expression of vascular or PHGPx. GSHPx-4 can detoxify phospholipid hydroperox- endothelial growth factors (VEGFs). This has been observed ides and, along with d-alpha-tocopherol, 'helps prevent in some animal studies. Selenium, in cell culture, has also oxidative damage to membranes. GSHPx-3, the extracellular been found to inhibit the gelatinolytic activity of matrix glutathione peroxidase, eliminates peroxides in the extracel- metalloproteinase-2 (MMP-2). lular fluid. Some epidemiological studies have shown an inverse rela- Glutathione peroxidases detoxify hydrogen peroxide and tionship between coronary heart disease and selenium intake. fatty acid-derived hydroperoxides. This is the antioxidant The possible anti-atherogenic activity of selenium may be role of these enzymes. However, recent research indicates accounted for, in part, by its antioxidant activity. Glutathione that reactive oxygen species play important roles in signal peroxidase may protect low density lipoprotein (LDL) from transduction processes. Therefore, by affecting the concen- oxidation. Oxidized-LDL is thought to be a crucial etiologi- trations of reactive oxygen species in cells, the glutathione cal factor in atherogenesis. Selenium may decrease platelet peroxidases may also be considered to play regulatory roles aggregation. Selenium deficiency results in lipoperoxide in signal transduction. accumulation. Lipoperoxides impair prostacyclin synthesis and promote thromboxane synthesis, which can increase Antioxidant activity of selenium can also be accounted for platelet aggregation. by its role in the selenium-dependent thioredoxin reductases. Selenium has been demonstrated to antagonize the effects of Tht::se enzymes reduce intramolecular disulfide bonds and regenerate ascorbic acid from dehydroascorbic acid. Thiore- a number of toxic metals, including cadmium and arsenic. doxin reductases can also affect the redox regulation of a Selenium inhibits the growth stimulatory effect of cadmium variety of factors, including ribonucleotide reductase (the on human prostatic epithelium in vitro. The mechanism of enzyme that converts ribonucleoside diphosphates to deoxy- the possible antagonistic action of selenium against various ribonucleoside diphosphates), the glucocorticoid receptor toxic metals and other xenobiotics is unclear. One possibility and the transcription factors AP-I and NF-KappaB. is that it forms inactive complexes with these substances. Selenium may have fertility enhancing effects for males. SeleI}ium deficiency appears to depress the effectiveness of Phospholipid hydroperoxide glutathione peroxidase various components of the immune system. In humans, (GSHPx-4), in addition to its antioxidant role in sperm,also selenium deficiency has been associated with depressed IgG appears to be responsible for maintaining the structure of and IgM antibody titers. In animal models, selenium sperm, at least in mouse sperm. ~eficiency has resulted in depressed neutrophil activity, decreased Candidacidal activity by neutrophils
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