Total selenium content

It is well known that the toxicity of many elements depends on the physicochemical forms they assume. So, for instance, determining the total content of a certain element in a sample is definitely not sufficient to measure its toxicity. Selenium is a case in point in small amounts this element is essential to human health. But the transition from the necessary amount (about 70pg of selenium per day for an adult) to a toxic dose (about 800 pg of selenium per day) is relatively easy. In rats, moreover, the fatal dose of Se(IV) compounds is 3.2 mg kg 1 of body mass, whereas for dimethyl selenide it is 1600 mg kg 1 of body mass. Nonorganic selenium compounds [Se(IV) and Se(VI)] are believed to be the most toxic ones, whereas in the environment selenium occurs most commonly bound to amino acids (selenomethionine and selenocysteine). The least toxic forms seem to be the volatile methyl compounds of selenium, which are metabolites of a detoxication process. 

The average selenium content of the earth s crust is about 0.8 mg/kg (Aubert and Pinta, 1977). Carbonated rocks contain higher Se (0.8 mg/kg) than metamorphic and sedimentary rocks (0.6 mg/kg) and igneous rocks (0.5 mg/kg) (Aubert and Pinta, 1977). Most of the world s soils contain 0.03-2 mg/kg Se (Ure and Berrow, 1982) with a mean of 0.4 mg/kg. In the U.S., the total Se concentration ranges from trace amounts to 82 mg/kg (Adriano, 2001). 

Freeman, H.C., G. Shum, and J.F. Lithe. 1978. The selenium content in swordfish (Xiphias gladius) in relation to total mercury content. Jour. Environ. Sci. Health A13(3) 235-240. 

Camara, C., Cobo, M.G., Palacios, M.A., Munoz, R. and Quevauviller, Ph. (1998) Feasibility study for the certification of the contents of inorganic selenium (IV) and (VI) (selenite and selenate) species and total inorganic selenium in artificial freshwater reference materials. Analyst, 123, 947. 

Hydride generation AAS is a well-established technique for the determination of selenium because of its selectivity and sensitivity. The detection limits are improved by concentrating the hydride prior to the transfer to the spectrometer. Cryogenic condensation in a U-tube at liquid nitrogen temperature is a preferred method of pre-concentration. However, selenium hydride is formed essentially only from Se. The selectivity for Se has been combined with various chemical preparation steps to determine the sum of SeIV and Se and the total selenium content of samples. Se concentrations are measured by the difference between the (Se + Se ) and Se contents. The difference between total selenium and (Se + Se ) contents represents the concentrations of Se° and Se2- species. These methods have been used for determinations of selenium species in natural waters. Soluble organic matter in some groundwater samples interferes with the hydride generation determination of selenium. Isolation of humic substances by their adsorption on resins has overcome this problem. 

Table 8.3 Effects of selenium treatment on ischemia/reperfused (I/R) cardiac tissue contents of malondialdehyde (MDA), reduced glutathione (GSH), oxidized glutathione (GSSG), and total and phospho nuclear factor kappa B (NF-kB)...

Since oxidative stress has been reported to produce subcellular redistribution and activation of NF-kB, we measured total NF-kB content in cytosolic and particulate fractions of the hearts subjected to ischemia/reperfusion. In another set of experiments, the activation of NF-kB protein was studied by measuring the total NF-kB and phospho-NF-KB in the heart homogenate. Elevated ratios of NF-kB in particulate versus cytosolic fraction and of phospho-NF-KB and total NF-kB in the hearts subjected to ischemia/reperfusion were reduced by selenium. As oxidative stress has been reported to produce subcellular redistribution and activation of NF-kB and antioxidants have been shown to prevent this alteration (Cargnoni et al. 2002), it is possible that our observation on selenium-induced NF-kB translocation might be due to its antioxidant action. It should be noted that since selenium treatment could reduce the tumor necrosis factor-cc (TNF-ct)-mediated activation of NF-kB (Kim and Stadtman 1997), it suggests a possible involvement of a reduction in the formation of TNF-tx in the hearts subjected to ischemia/reperfusion by selenium. 

In past years, on line chromatographic coupling techniques such as HPLC and CE coupled to ICP-MS with the isotope dilution technique have been used for element quantification in speciation analysis. An interesting application of the isotope dilution technique in medical research was proposed recently by Prange and co-workers, who added highly enriched " S, Cu, Zn and Cd spikes to the interface of the CE-ICP-MS system. The authors separated isoforms of metallothionein (e.g., of rabbit liver) by capillary electrophoresis and quantified S, Cd, Cu and Zn concentrations in isoforms by ICP-SFMS using the isotope dilution technique. A new selenized yeast reference material (SELM-1) for methionine, selenomethionine (SeMet) and total selenium content has also certified by an intercomparison exercise. ... 

Plasma from mammalia contains a selenoprotein that carries more than 50% of the selenium present. In rat, about 8% of the total selenium content is associated with this protein, selenoprotein P. A change in the selenium status of the rat leads to a rapid change of the selenium level attached to selenoprotein P, preferential to changes of other selenoproteins such as plasma glutathione peroxidase. ... 

Total Selenium As mentioned earlier, stable isotope dilution is a powerful tool in trace element analysis. Let us first look at how it can be used to determine the total selenium content of a sample. In the following section we will develop the method further for stable isotopes in metabolic tracer studies. 

From 2 ratio measurements and the known amount of Se spike added, one can quantitatively identify a) the tracer present in the sample, and b) the amount of natural selenium present with it. As a bonus, one also gets the total selenium content of the sample, which is simply the sum of these. 

Several techniques are available to assess selenium bioavailability in soils. The most widely used is the water-soluble concentration (Fordyce et al., 2000b Jacobs, 1989 Tan, 1989). In most soils, only a small proportion of the total selenium is in dissolved form (0.3-7%), and water-soluble selenium contents are generally <0.1mgkg (Table 8). 

A batch of Se-labeled and enriched yeast was characterized with regard to isotopic composition and content of selenium species for later use in a human absorption study based on the method of enriched stable isotopes. SeMet constituted 53% of the total Se content in the yeast. Oxidation of SeMet to selenomethionine-Se-oxide (SeOMet) occurred during sample preparation... 

Torres MA, Verdoy J, Alegrf A, et al. 1999. Selenium contents of human milk and infant formulas in Spain. Sci Total Environ 228 185-192. 

---