Selenium tracer studies

Tracer studies showed that selenium is recovered quantitatively in... 

Numerous trace elements are known to be nutritionally essential in man In order to assess the essentiality, dietary availability, and metabolic fate of these, means of labeling for subsequent identification are needed In animal studies, radioisotopes are often used for this purpose, but their use in human studies is generally contraindicated due to the radiation hazards An alternate method is to use stable isotopes of the elements, which overcomes this limitation A method will be described for conveniently measuring the stable isotopes of selenium, permitting their use as metabolic tags in tracer studies Using one stable isotope as the tracer and another as internal standard, one can quantitatively identify in a sample the tracer, natural (unenriched) selenium present with it, and total selenium Some of the kinds of information obtainable from metabolic tracer studies will be discussed ... 

Stable Isotopes of selenium (as well as those of other elements) can provide a means of addressing these questions by their employment In metabolic tracer studies The same Information can be obtained as when employing radiotracers In animal studies, but without the associated radiation hazards In human studies For example, stable Isotopes of selenium can be biologically Incorporated Into test foods and these used to monitor selenium bloavallablllty (6,7) ... 

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. 

Tracer Studies. In addition to using as an Internal standard as described above, a second enriched Isotope of selenium can be used as a metabolic tag. Let us take as an example the use of enriched as a tracer. We have used a batch of 76se for this purpose, with the relative abundances shown in Table III. 

We can extend this concept to tracer studies in humans as well Let us take as an example the metabolic study of Swanson et al ( ) In this study, 3 groups of women (non-pregnant, early-pregnant and late-pregnant) were placed on a controlled diet containing 150 ug Se/day On day 8, they also ingested 150 ug Se, but 110 ug of it was natural (unenrlched) selenium and 40 ug of it was enriched 630 incorporated into egg products (7). 

Modern thermal ionization mass spectrometry (TIMS) is now sufficiently sensitive and precise to measure individual selenium-isotope abundances (e.g., Se/ Se) in solid samples or residues so that it can be used to study environmental cycling/distri-butions (Johnson era/., 1999). Microbial reduction of selenate leads to isotopically lighter selenite, i.e., the reduction has a Se/ Se fractionation factor, , of about — 5.5%o (Johnson et al., 1999). INAA has been used to determine different selenium isotopes, especially Se in plant tracer studies. 

Selenium.— 0 tracer studies of oxygen exchange between selenate and water indicate a rate law... 

The uptake of trace metals from the soil by plants and animals can be studied with high sensitivity by radiotracer techniques. In these applications, it is important that the chemical form of the radiotracer is identical with that of the trace element to be studied. For example, in agriculture, the uptake of trace elements necessary for plant growth can be investigated. Essential trace elements, such as Se, are of special interest. By use of radioactively labelled selenium compounds the transfer of this element from soil to plants and animals can be measured. For the investigation of the transfer of radionuclides (radioecology), addition of tracers is, in general, not needed. 

Dietary levels of selenium and the individual s selenium nutritional status are the most important factors that influence the route and rate of selenium excretion. Selenium excretion in expired air is only significant when exposures to selenium are high. Rats injected subcutaneously with sodium selenite at doses of 2.2-5.4 mg selenium/kg excreted 41-62% of the administered selenium in exhaled air, whereas rats injected with sodium selenite at doses of 0.005-0.9 mg selenium/kg excreted only 0.2-11% of the administered selenium in expired air (McConnell and Roth 1966 Olson et al. 1963). As the amount of administered sodium selenite increased, the percent of the administered selenium excreted in the urine decreased (from approximately 22-33% of the administered selenium at doses of 0.005-0.9 mg selenium/kg to 3-14% of the administered selenium at doses of 3.1-5.4 mg selenium/kg) (McConnel and Roth 1966). Selenium in the feces was not measured in this study. Burk et al. (1972) found that as the dietary level of sodium selenite was increased, a larger proportion of an injected tracer dose of selenium (as sodium selenite) was excreted. At a dietary level of 0.005 mg selenium/kg, approximately 60% of the injected selenium had been excreted in the first 35 days following administration. At a dietary level of 0.05 mg selenium/kg, over 94% of the injected selenium had been excreted over the same period of time. 

To study how living species interact with the environment, ecology, one can use radioactive tracers to follow the uptake of a trace metal (e.g. cobalt) from the soil by plants, and by animals after having eaten the plant. In agriculture, this is useful in studying the uptake of trace elements necessary for plant growth. For exan >le, it has been found that sheep need plants containing selenium in order to combat white muscle disease. The... 

The lack of influence of cadmium on GSH-Px activity in our studies is consistent with the results of Prohaska et al. (1977) but is at variance with those of Omaye and Tappel (1975). The reasons for this disagreement are not readily apparent. In work with rat testis cytosol, two different GSH-Px activities were found when assayed with cumene hydroperoxide (Prohaska et al, 1977). The larger MW species eluted with the void volume on Sephadex G-150 columns and incorporated Se from selenite, but the smaller one (42,000 MW) did not incorporate Se from radioactive selenite. When CdCl2 was given to rats injected 4 wk previously with a tracer dose of Se-selenite, Sephadex G-150 chromatography of testis cytosol showed that most of the ° Cd was eluted in a major peak of 34,000 MW, but very little Cd was found in association with a major Se peak (140,000 MW) or with GSH-Px activity. Thus, under homeostatic conditions, cadmium does not appear to bind to the major selenium-containing protein or to GSH-Px in the testes. 

A mechanism has been proposed for the reaction of cyanide ion with thio-cyanogen, selenocyanogen, sulphur dicyanide, and selenium dicyanide in acetonitrile on the basis of studies using C as a tracer ... 

Vandael, P. (2004) Stable isotope-enriched selenite and selenate tracers for human metabolic studies a fast and accurate method for their preparation from elemental selenium and their identification and quantification using hydride generation atomic absorption spectrometry. 

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