Selenium essential elements

Selenium is an essential element and is beneficial at low concentrations, serving as an antioxidant. Lack of selenium affects thyroid function, and selenium deficiencies have been linked to Keshan Disease (34). Selenium at high levels, however, is toxic. Hydrogen selenide (which is used in semiconductor manufacturing) is extremely toxic, affecting the mucous membranes and respiratory system. However, the toxicity of most organ oselenium compounds used as donor compounds for organic semiconductors is not weU studied. 

Tellurium is not an essential element, and teUurium compounds are in general more toxic than their selenium counterparts. MetaUic teUurium is known to have a teratogenic effect in rats, though no studies have been done on the toxicity of teUurium donor compounds (35). 

Under unusual circumstances, toxicity may arise from ingestion of excess amounts of minerals. This is uncommon except in the cases of fluorine, molybdenum, selenium, copper, iron, vanadium, and arsenic. Toxicosis may also result from exposure to industrial compounds containing various chemical forms of some of the minerals. Aspects of toxicity of essential elements have been pubhshed (161). 

Water soluble Se in the North West region varied from 0.0002-0.0429 mg/kg. Water soluble Se accounted for 2.13-6.34% of the total Se in the soils of North China. Selenium is an essential element to animals and humans. When water soluble Se in soils is less than 0.003 mg/kg, Se deficiency in animal and human beings may occur. EDTA-extractable Se in the alkali desert soils of North China was in the range of 0.011-0.090 mg/kg this was about 5-11% of the total Se in the soils. Selenium deficiency was mostly found in the Loess Plateau and Tibet region. NH4OAc-cxtractable Ni in soils from Beijing was 0.29 mg/kg. 

Although known for its toxicity, but unlike antimony and arsenic, selenium is an essential element which has been identified as part of several prokaryotic and eukaryotic proteins in the form of the amino acid, selenocysteine. Selenocysteine has been referred to as the 21st amino acid since gene products required for its incorporation into protein were discovered in bacteria (Stadtman, 1996). Aspects of the mechanism of selenocysteine insertion during protein synthesis in eukaryotes are currently being investigated (Low and Berry, 1996). The two strands of current selenium research are... 

Some elements are essential to the composition or function of the body. Since the body is mostly water, hydrogen and oxygen are obviously essential elements. Carbon (C) is a component of all life molecules, including proteins, lipids, and carbohydrates. Nitrogen (N) is in all proteins. The other essential nonmetals are phosphorus (P), sulfur (S), chlorine (Cl), selenium (Se), fluorine (F), and iodine (I). The latter two are among the essential trace elements that are required in only small quantities, particularly as constituents of enzymes or as cofactors (nonprotein species essential for enzyme function). The metals present in macro amounts in the body are sodium (Na), potassium (K), and calcium (Ca). Essential trace elements are chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn), magnesium (Mg), molybdenum (Mo), nickel (Ni), and perhaps more elements that have not yet been established as essential. 

Se 0.02 Toxic to plants at concentrations as low as 0.025 mg L-1 and toxic to livestock if forage is grown in soils with relatively high levels of added selenium. As essential element to animals but in very low concentrations... 

Selenium is an essential element for humans and is a constituent of selenoamino acids contained in selenoenzymes, such as glutathione peroxidases. This group of enzymes catalyzes the reduction of peroxides and thereby takes part in the anti-oxidative defense of the body. A severely low Se intake may cause heart disease (Keshan s disease). Therefore, possible adverse health effects are associated with a low intake of this element and justify the focus on the low percentile Se intake in the population. The food groups that contribute the most to the Se intake are meat (including offal) followed by bread and cereals, and fish, as shown in Figure 9.8. 

Selenium is one of the few substances that have been shown to cause adverse human health effects as a consequence of exposure through drinking-water, although it is an essential element and in many parts of the world there is a deficiency. It is, therefore, important to consider selenium in developing new sources in areas where selenium is suspected. Where selenium is present, monitoring at the treatment works would be appropriate. 

The signs of selenium deficiency include skeletal myopathy and cardiomyopathy, and selenium deficiency continues to be reported in cases in which this essential element has not been added to parenteral nutrition solutions during long-term administration (69). 

Similar demands for speciation of trace elements exist for food analysis. Substantial differences in the biological availability are known for several essential elements and depend on the form in which they are present in the diet. The chemical bases for these differences are known for cobalt, iron, and chromium but not for zinc, copper, and selenium. The importance of speciation in food analysis is best demonstrated by the example of iron. That element, when part of heme compounds, is well absorbed, and there is little influence on the absorption by other factors in the diet. Nonheme iron, on the other hand, is not readily absorbed and, in addition, is subject to many influences from dietary ingredients those influences are poorly understood and probably not completely known (14). 

Selenium is an essential element but is toxic when excessive quantities are ingested. Exposure of horses is usually through consumption of seleniferous (accumulator or indicator) plants (e.g.. Astragalus spp.). Exposure to high quantities of selenium over a short time causes diarrhea (which is often foul smelling and contains air bubbles), neurological and cardiovascular effects, and respiratory difficulty. Death in these horses is due to respiratory failure. Chronic exposure to low levels of excessive selenium is characterized by hoof abnormalities at the coronary bands and by discoloration and loss of hair. The hoof deformities are painful and cause lameness. 

Selenium (Se) is an essential element for humans, being a constituent of the en2yme glutathione peroxidase and believed to be closely associated with vitamin E in its functions. 

The remaining candidates for the title of essential elements, in order of their position in the periodic table (Fig. 1.3), are boron, fluorine, silicon, arsenic, selenium, bromine, tin, and iodine. 

Selenium is both a toxic and an essential element. The toxicity depends greatly on the species. Selenium is a cumulative toxic substance and can be a serious health hazard when present in high concentrations in food and water. However, at very low levels, pg/kg it is recognized as an essential trace element in animal nutrition. 

---