Selenium biological importance

The organoselenium compounds, their chemistry, and methods of preparation are discussed in the literature (3,4,7,11,12,14,59). Selenium-containing polymers are of interest because of their semiconducting and plastic properties. The biologically important compounds include selenoaminocarboxylic acids and derivatives, selenium-containing peptides, and selenium derivatives of pyrimidines, purines, cholines, steroids, coenzyme A, and other compounds (4,14). The biochemical and medical aspects and uses of oiganoselenium compounds have also been discussed (13,14,60—62). 

Accurate determination of the biological important element, selenium, in blood serum by isotope dilution analysis using ICP-QMS with octopole collision cell (Agilent 7500ce, Tokio, Japan) is described by Schaumloffel and coworkers.55 A recovery of selenium from human serum reference material was only 78 % when hydrogen was applied as collision gas but 96.7 % using xenon as a non-reactive collision gas to eliminate isobaric interferences. A detection limit of 3.3p,gl-1 was achieved.55... 

Fig. 2. Periodic table indicating relative abundance and biological importance of elements. Highlighted elements are the 30 most abundant crustal elements with concentrations ranging from 46 % for oxygen to 0.1 yg/g for selenium. Bold faced elements have known biological functions with indicating limited data and controversy. Data taken from Wood and Wang, 1983.

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). 

Very recently, a comprehensive review on the chemistry of biologically important synthetic organoselenium compounds has been published. " However, biochemically oriented Se NMR investigations are still rare those which appeared prior to 1995 have been reviewed before. The following section contributes to this aspect and quotes later work on selenium-containing natural products or selena analogues thereof. 

Selenium substitutes for sulfur in amino acids to form seleno-cysteine, cystine, and methionine. The selenium-containing antioxidant glutathione peroxidase is biologically important, and selenium is a necessary trace nutrient in warm-blooded animals. Grazing animals develop a form of muscular dystrophy and other disorders when grazing in areas in which the selenium has been depleted with selenium-depleted diets, people develop... 

Selenium is biologically important to all classes of organisms and is essential to humans a deficiency causes serious diseases. However, an excess of selenium also causes serious diseases, including loss of hair... 

The major biologically important molecules are formed primarily from 0 gen, carbon, hydrogen, and nitrogen. Some elements found in the body are crucial for life, although they are present in very tiny amounts (trace elements). Some of these trace elements include chromium, copper, fluorine, iodine, and selenium. 

The importance and understanding of selenium s role in nutrition, and the development of industrial uses have made selenium an important metalloid and it is now a significant element industrially, biologically and also environmentally. 

Obviously the redox poise in biological systems is very important and the movement of selenium through this process has been investigated for denitrifiers such as Paracoccus denitrificans,159 a specialized selenate-respiring bacterium Thauera selenatis which used selenate as the sole electron acceptor,160,161 and phototrophic bacteria which produced different reduced forms of selenium when amended with either selenite or selenate and even added insoluble elemental Se.162 As noted above, Andreesen has commented on the importance of redox active selenocysteines135 and Jacob et al.136 note the importance of the thioredoxin system to redox poise. 

Like selenium, the process of reduction/oxidation cycling in biological systems is important and changes in the oxidation state are often an easy means of determining bioreduction for added tellurium oxyanions. The general order of... 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

Selenium is readily available in a variety of foods including shrimp, meat, dairy products, and grains, with a recommended daily intake of 55 to 70 jug. It occurs in several forms with Se+6 being biologically most important. Selenium is readily absorbed by the intestine and is widely distributed throughout the tissues of the body, with the highest levels in the liver and kidney. It is active in a variety of cellular functions and interacts with vitamin E. Selenium appears to reduce the toxic effects of metals such as cadmium and mercury and to have anticarcinogenic activity. Selenium produces notable adverse effects both in deficiency and excess thus recommended daily intake for adults is approximately 70 Jg/day but should not exceed 200 pg/day. 

---