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Researchers have worked out on the bioavailability of selenium (Se) and its implication for animal agriculture. Se is a trace element that is essential for the growth and development of the immune system and metabolism. However, it does not work alone but it gets incorporated into the protein.
Selenium-containing proteins or enzymes are called selenoproteins or selenoenzymes respectively. Some of the most important selenoenzymes are glutathione peroxidase, iodothyronine deiodinases, thioredoxin reductase, glutathione-S-transferase, etc.
These selenoenzymes play important roles in the regulation of redox reactions, detoxification, and immune system as well as suppression of viral infections. Therefore, deficiency or low Se status can lead to the varieties of pathological changes and disorders associated with the function of one of the particular selenoproteins/selenoenzymes.
Bioavailability and different forms of selenium
Bioavailability of Se can vary widely depending upon the Se content of the soil. Humans and animals can be deficient of Se if they feed plants grown in the selenium-deficient soil. In the case of selenium deficiency, dietary selenium supplement is recommended that was first permitted for use about 40 years ago. During that period, selenium supplements became one of the major dietary supplements of animals to enhance their health and productivity.
Se can exist in four different oxidation states such as elemental (Se0), senelide (Se-2), selenite (Se+4) and selenate (Se+6), however, the naturally inorganic soluble form is selenite and selenate that account for the majority of the Se.
Selenium-containing amino acids
Se is a member of the oxygen family and it is much more similar to the sulfur, however, they are not completely interchangeable in the biological system. Both sulfur and Se are found in proteins in the form of amino acids. Sulfur-containing amino acid includes cysteine, homocysteine, methionine, serine, taurine while selenium-containing amino acid includes selenocysteine and selenomethionine.
Selenocysteine and selenomethionine both are the source of Se for the synthesis of selenoproteins/selenoenzymes. Incorporation of selenocysteine and selenomethionine into the selenoproteins is important for the optimal activity that may be lost if seleno-amino acid is replaced by their corresponding sulfur-containing amino acids. Therefore, understanding the synthesis of selenoproteins is necessary to understand the Se homeostasis and disease involving the defective selenoproteins.
Incorporation of selenium into selenoproteins
Se can be incorporated into the selenoproteins during translation in the ribosome where transfer RNA specific for Se-containing amino acid recognizes the specific codon and incorporates them into the growing polypeptide. Selenocysteine is incorporated in the selenoproteins in an animal system while there is no evidence of incorporation of selenomethionine in animals.
Se either in an organic form or in an inorganic form gets absorbed in the gut wall where selenite is passively absorbed while organic Se is actively absorbed. Intravenous transport of Se is accomplished by two types of selenoproteins such as selenoproteins-P and extracellular glutathione peroxidase. However, the excess of selenium is excreted through the feces and urine.
It acts as an antioxidant in the form of selenoproteins and because of that, it is considered as a complement of the vitamin E in mammals and birds. Both selenium and vitamin E prevents peroxidation of unsaturated fatty acids in the cell membrane and thus are most important. Se and vitamin E are similar in the function and therefore if there is a low level of vitamin E in the body Se is supplemented to prevent the lipid peroxidation.
The other role of Se are; 1) production of glutathione peroxidase enzyme that is required for the regeneration of reduced glutathione from the glutathione disulfide and 2) the production of thyroid hormones in which selenoprotein “iodothyronine deiodinase” enzyme that catalyzes the conversion of triiodothyronine to thyroxine.
There are many ways to measure the selenium status such as measurement of plasma selenium concentration, measurement of glutathione peroxidase enzyme activity, use of stable isotopes of selenium, etc.
Dietary selenium supplement
The soil is the main source of Se for the plants while plants are the ultimate source of Se for the animals. To improve the selenium status of plants on selenium-deficient soils, Se-enriched fertilizers are recommended to use. In addition to these, dietary Se is also supplemented as animal feeds to ensure that requirements are met. Se-enriched meat, milk, and egg are the best and same sources of dietary Se source for humans. However, some other selenium supplements such as sodium selenite, selenomethionine, selenohomolanthionine are also used as selenium supplements.
Reference: Agriculture (Selenium Biochemistry and Bioavailability: Implications for Animal Agriculture)
Article DOI: 10.3390/agriculture5041277