Selenium essentiality

Soluble selenium, normally the selenate (Carlsen et al., 1991), is readily taken up by plants and its assimilation is analogous to that of sulphate (Dawson and Anderson, 1989). The question of selenium essentiality in plants has been discussed by Anderson and Scarf (1983). A number or organoselenium compounds have been identified in plants (Adriano, 1986 Peterson et al., 1981 Anderson and Scarf, 1983 Harborne, 1988). 

Figure V-23 Logarithm of the total pressure of selenium (essentially Se2(g)) as a function of temperature. The diagram is based on experimental series 8 to 13 reported in Table 10 in [87SVE/GRO],...

Elemental selenium has been said to be practically nontoxic and is considered to be an essential trace element however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. 

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). 

Selenium recovery from electronic, xerographic, or other scrap for sale on the open market remains small. This was less than 100 t/yr in 1995. The xerographic industry recovers essentially all of the selenium it uses in photoreceptor dmms by extremely efficient recycling programs (see... 

In 1956 selenium was identified (123) as an essential micronutrient iu nutrition. In conjunction with vitamin E, selenium is effective iu the prevention of muscular dystrophy iu animals. Sodium selenite is adrninistered to prevent exudative diathesis iu chicks, a condition iu which fluid leaks out of the tissues white muscle disease iu sheep and infertility iu ewes (see Eeed ADDITIVES). Selenium lessens the iacidence of pneumonia iu lambs and of premature, weak, and stillborn calves controls hepatosis dietetica iu pigs and decreases muscular inflammation iu horses. White muscle disease, widespread iu sheep and cattle of the selenium-deficient areas of New Zealand and the United States, is insignificant iu high selenium soil areas. The supplementation of animal feeds with selenium was approved by the U.S. EDA iu 1974 (see Eeed additives). Much of selenium s metaboHc activity results from its involvement iu the selenoproteia enzyme, glutathione peroxidase. 

The unusual physical complaints and findings in workers overexposed to teUurium include somnolence, anorexia, nausea, perspiration, a metallic taste in the mouth and garlic-like odor on the breath (48). The unpleasant odor, attributed to the formation of dimethyl teUuride, has not been associated with any adverse health symptoms. Tellurium compounds and metaboUc products have been identified in exhaled breath, sweat, urine, and feces. Elimination is relatively slow and continuous exposure may result in some accumulation. No definite pathological effects have been observed beyond the physical complaints outlined. Unlike selenium, teUurium has not been proved to be an essential biological trace element. 

Selenium is a vital microelement for people. It has dual properties. Selenium is an essential nutrient at low concentration levels and it becomes toxic at higher concentration levels. Deficiency of selenium results in weakness and hard diseases. Selenium is a building material of many hormones and ferments it neutralizes free radicals, radioactive radicals in organism. The range of selenium safety concentration in food and water is very narrow. The daily normal amount of human consumption of selenium is 10-20 p.g, maximum safe concentration of selenium in water is 5-10 p.g/1. It becomes toxic at 20-30 p.g and bigger content in different objects. 

The bad guys, shown in pink in Figure 2.8, are toxic, often lethal, even in relatively small quantities. Several of the essential trace elements become toxic if their concentrations in the body increase. Selenium is a case in point. You need about 0.00005 g/day to maintain good health, but 0.001 g/day can be deadly. That s a good thing to keep in mind if you re taking selenium supplements. 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

The polarographic behavior of selenium in aqueous solutions has been studied in great detail, being in fact far simpler than that of sulfur because the selenium compounds exhibit electrochemical activity essentially in only two different... 

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

Essential hypertension, whose prevalence is increased nearly two-fold in the diabetic population, may be another source of free-radical activity. The vascular lesions of hypertension can be produced by free-radical reactions (Selwign, 1983). In the recent Kuopio Ischaemic Heart Risk Factor Study in Finnish men, a marked elevation of blood pressure was associated with low levels of both plasma ascorbate and serum selenium (Salonen etal., 1988). A few studies report a hypotensive effect of supplementary ascorbate in patients with hypertension, but the actual changes in both systolic and diastolic pressure after ascorbate were not statistically significant in comparison with placebo (Trout, 1991). 

Selenium dioxide is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

Other agents, including selenium, vitamin E, lycopene, green tea, nonsteriodal anti-inflammatory agents, isoflavones, and statins, are under investigsation for prostate cancer and show promise. Selenium is a naturally occurring trace element that is an essential nutrient in the human diet.8 However, none of these agents is currently recommended for routine use outside a clinical trial. 

Trace elements are essential cofactors for numerous biochemical processes. Trace elements that are added routinely to PN include zinc, selenium, copper, manganese, and chromium. There are various commercial parenteral trace element formulations that can be added to PN admixtures (e.g., MTE-5 ). Zinc is important for wound healing, and patients with high-output fistulas, diarrhea, burns, and large open wounds may require additional zinc supplementation. Patients may lose as much as 12 to 17 mg zinc per liter of gastrointestinal (GI) output (e.g., from diarrhea or enterocutaneous fistula losses) however, others have demonstrated that 12 mg/day may be adequate to maintain these patients in positive zinc balance.18 Patients with chronic diarrhea, malabsorption, and short-gut syndrome may have increased selenium losses and may require additional selenium supplementation. Patients with severe cholestasis should have copper and manganese... 

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