Chemical Speciation of Selenium


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Chemical Speciation of Selenium

Chemical speciation is "the process of identification and quantification of the different forms or phases in which one element is present in a material" or as "the description of the amounts, types of species, forms or phases present in a material " (Lima et al., 2001). However, recent studies suggest chemical speciation as the individual chemical forms of an element, which together, constitute the total concentration of the element in the sample.

A study was carried out to correlate the difference in the biological activities based on the source of the selenium (i.e. selenium-enriched garlic and selenium-enriched yeast), and speciation differences in the chemical forms of selenium present in the two natural products.

Analytical speciation studies were carried using selenium-enriched garlic sample that yielded 296 µg/g and selenium yeast sample that yielded 1922 µg/g selenium. Data suggested that bulk of the selenium was found to be in the form of γ-glutamyl-Se-methyl-L selenocysteine (73% of the total selenium content) in selenium garlic, and in the form of selenomethionine (85% of the total selenium content) in selenium yeast (Table 1).

  296 µg/g Se Garlic (%) 1922 µg/g Se Yeast (%)
Selenate 2.0 0.0
Selinite 0.0 1.0
Selenoanthionine 0.0 1.5
Selenocystine 0.5 0.5
Selenocystanthionine 0.5 1.0
Se-methylselenocysteine 3.0 0.5
Selenomethionine 13.0 85.0
γ-glutamyl-Se-methyl-L-selenocysteine 73.0 0.5
Se-adenosyl selenohomocysteine 0.0 3.0
γ-glutamyl selenomethionine 4.0 0.0
% sum of eluted Se 96.0 93.0
Table 1: Percentage composition of selenium species in selenium-enriched garlic and yeast samples


In addition, in vivo experiments were carried as a part of biological activities to determine tissue selenium accumulation, reduction of premalignant lesions in the mammary gland, and mammary cancer prevention bioassay.

Results from tissue selenium accumulation experiment showed a consistently lower total tissue selenium accumulation in rats fed with diet containing supplemental Se-garlic at different levels as compared to rats receiving Se-yeast. However, despite this difference, mammary cancer chemoprevetion studies showed that Se-yeast was only half as active as Se-garlic (Fig. 1).

Fig. 1: Effect of supplementation with Se-Yeast and Se-Garlic on mammary cancer in an animal model.

Researchers also postulated gamma-glutamyl-Se-methylselenocysteine serves mainly as a carrier of Se-methylselenocysteine. They also assumed that the superior activity of Se-garlic when compared to Se-yeast could be due to fact that Se-garlic delivers predominantly selenomethionine. Hence, it was believed that both these selenium compounds may have different efficacies owing to differences in the metabolism and their subsequent disposition in tissues (Ip et al., 2000).

These findings further corroborated chemopreventive activity of Se-enriched garlic as observed by the same research group earlier (Ip et al., 1992). In this study, treatment showed a significantly reduced tumor incidence and tumor development in a rat mammary tumor model (Fig. 2). However, the tissue selenium levels were found to be lower in animals fed Se-enriched garlic than in those fed with the same amount of selenium from inorganic selenite.

Fig. 2: Effect of selenium-enriched garlic on tumor incidence and tumor development in an animal model.