Selenate/selenite toxicity

Maier, K.J. and Knight, A.W. (1993) Comparative acute toxicity and bioconcentration of selenium by the midge Chironomus decorus exposed to selenate, selenite, and seleno-DL-methionine, Archives of Environmental Contamination and Toxicology 25 (3), 365-370. 

Boisson, F., M. Gnassia-Barelli, and M. Romeo. 1995. Toxicity and accumulation of selenite and selenate in the unicellular marine algae Cricosphaera elongata. Arch. Environ. Contamin. Toxicol. 28 497-493. Bovee, E.C. and T.L. O Brien. 1982. Some effects of selenium, vanadium and zirconium on the swimming rate of Tetrahymena pyriformis a bioassay study. Univ. Kans. Sci. Bull. 52 (4) 39 44. 

Selenium, in the form of selenate or selenite, is toxic to D. desulfuricans (Tomei et al. 1995) and Wolinella succinogenes (Tomei et al. 1992) at elevated levels. At sublethal levels of 0.1-1. OmM selenite or 10 mM selenate, minimal levels of growth is observed with both D. desulfuricans and Wolinella succinogenes. With both selenate and selenite, colloidal elemental selenium (Se°) is produced inside the cell and released into the culture fluid after cell death. This reduction of Se(VI) and Se(IV) by these anaerobes is not coupled to growth and proceeds by mechanisms that have not yet been identified. Selenite and selenate reduction with formation of elemental selenium by these nonrespiratory processes serve to detoxify the environment for future bacteria and may be important for the geochemical cycle of selenium. 

In large areas of the world, the soils contain very little selenium in forms that can be taken up by plants. Crops produced in these areas are, therefore, very low in selenium. A selenium (deficiency in livestock is a serious problem. A deficiency causes a form of muscular dystrophy in younger animals and poor reproductive qualities in the adult animals. For prevention, sodium selenate or sodium selenite, sometimes augmented with vitamin E. is added in proper proportions to feedstuffs Some areas, including the Plains and Rocky Mountain states in the United States have soils that are rich in available selenium. Tn regions like these, selenium toxicity is a problem. The situation is particularly senous in Arizona, California, Montana, Nevada, New Mexico, and South Dakota. 

Interest in selenium concentrations in the environment and in foodstuffs has been due to the dual role of selenium as an essential nutrient at low concentration levels and as a toxic substance at high levels of concentration. The narrow range between the two levels requires accurate methods for measurements of selenium concentrations. Selenium can exist in a number of different chemical forms and oxidation states. Inorganic selenium can be present in the environment as selenate (Se ), selenite (Se ), elemental selenium (Se°) and selenide (Se2-). 

Selenium occurs in the earth s crust as metal selenides [5], and the element and its compounds are used in electronic applications and in the production of glass, ceramics, and stainless steel [11]. In trace amounts, selenium is an essential nutrient, but in higher quantities or when certain species are present, the element becomes toxic. The most important forms of selenium are selenate (Se042-), and selenite (Se032""), and selenide (Se2 ). Selanates are relatively soluble, whereas selenites and elemental selenium are virtually insoluble [12]. 

McAdam, P.A. and Levander, O.A. 1987. Chronic toxicity of dietary selenium fed to rats as D- or L-selenomethionine, selenite or selenate. Nutr. Res. 7, 601-610. 

As Se is one of the few elements absorbed by plants in sufficient amounts that can be toxic to livestock, soils containing more than 0.5 mg Se kg are considered as seleniferous because the forages produced on such soils absorb Se more than the maximum permissible level suitable for animal consumption. Se binding onto soils and sediments depends upon the pH, Eh, Se species, competing anions, hydrous oxides of iron, and the type of clay minerals. Se in contaminated soils and water exists mainly as water-soluble selenate (SeO ) and selenite (SeOf ). 

How have selenium-accumulators been able to absorb so much Se without any damage to themselves These plants are able to separate inorganic S (as sulphate) from inorganic Se (as selenate or selenite), when they enter in the plants, and to channel the Se into the synthesis of nonprotein amino acid analogues, which are not therefore incorporate into protein synthesis. The adapted plants then sequester them in the vacuoles of the leaves, where they are perfectly harmless to the plants but intensely harmful to any unsuspecting grazing animals. In the non-adapted plants selenium toxicity may be attributed to the replacement of cysteine by selenocysteine and the production of disfunctional proteins in which S-S bond between polypeptide chains are replaced by the more labile Se-Se bonds. Fig (14). 

Selected Compounds Hydrogen selenide (H2Se) Sodium selenate (Na2Se04) Sodium selenite (Na2Se03) Selenium chloride (Se2Cl2). The toxicity of compounds varies substantially Chemical Abstracts Service Registry Number CAS 7782-49-2 Synonyms Vandex C.I. 77805 CHEMiCAL/PHARMACEUTiCAL/Other Class Metals Chemical Formulas Se" Se " Se ... 

---