[When comparing different yeast selenium products, it is necessary to consider the deposition efficiency of selenium in different yeast components containing proteins and peptides due to strain specificity]
By Richard Murphy*
*Richard Murphy works at Alltech Ireland
© 2013 Feedstuffs. Vol. 85, No. 52, December 23, 2013.
The absorption and utilization of selenium depends mainly on the existence of selenium, which is an important reason for its bioavailability and effectiveness. There are two main ways to add selenium source to feed: one is inorganic salts such as sodium selenite and selenate; the other is organic, such as selenium-enriched yeast.
Yeast selenium is a natural selenium source with the highest bioavailability by controlling the fermentation process to bind selenium to yeast. Although many products on the market are advertised as organic products, the facts revealed by the production methods and the chemical principles behind them are not the case.
Selenium is absorbed in the small intestine, and the absorption of selenium-containing amino acids is through the transport mechanism of amino acids and small peptides, while inorganic selenium such as sodium selenite is mainly absorbed by passive diffusion, so its absorption efficiency is low. After the absorption of organic selenium, selenomethionine and other selenium-containing amino acids and selenium-containing peptides can participate in the synthesis of body proteins through non-specific substitution of methionine, and some organic selenium can be quickly stored in the body. When needed, the body can be quickly Absorption and utilization.
Although selenomethionine is widely believed to be the major mode of selenium in selenium yeast, recent studies have shown that selenium yeast contains more than 60 selenium-containing substances, including more than 20 metabolites that have never been reported before. (Arnaudguilhem et al., 2012). Studies have shown that these different selenium-containing amino acids are important sources of intracellular synthetic selenoproteins. In contrast, after inorganic selenium is absorbed in the small intestine, only a small portion is directly utilized or methylated, and then most of it is excreted.
Selenium amino acid content varies from tissue to tissue, suggesting that selenium plays different roles in different tissues and participates in different metabolic pathways (Table 1). The content of selenocysteine ​​in the body affects the activity of selenase, because selenocysteine ​​is an important component of the active center of selenoproteinase.
The increase of selenomethionine content in the body not only enhances the absorption and deposition of selenium, but also constitutes an endogenous selenium pool, so that the body can be quickly utilized in the absence of selenium absorption or under oxidative stress. On the other hand, livestock and poultry can store organic selenium (such as selenomethionine) in tissues (meat, eggs, milk, etc.), which not only can improve the selenium level of the body, but also provide consumers with more nutritious and more absorbable. Functional foods utilized.
Selenium yeast utilization efficiency
It is well known that even with the same selenium-tolerant yeast, different yeast strains have their own unique biochemical and genetic characteristics, and research reports in this area are numerous. One of the trials studied three selenium-enriched yeast products on the market. By measuring the yeast composition of each product, the specific conditions of selenium deposition in different yeasts were compared.
Selenium-containing components were separated from polysaccharides and proteins by various hydrolysis and enzymatic methods, and then selenium-separating components of different molecular sizes were separated by chromatography-inductively coupled plasma mass spectrometry. Product differences (Figure 1). The test results show that the composition of different yeast selenium products is very different.
Although it is widely believed that all selenium yeast products are not much different, in fact, the amount of selenium deposited between different yeast products is completely different. Because of the genetic level, different yeast strains vary greatly, which is why the deposition and distribution of selenium in different yeast cells are different. Therefore, we have reason to believe that different yeast strains perform differently at different levels (such as shelf life, bioavailability, toxicity, etc.). So we should think of them as different selenium products, rather than confusing all the selenium yeast products.
Selenium metabolism
Whether it is organic selenium or inorganic selenium will be converted into selenide by metabolism, and then converted into selenocysteine ​​to participate in the synthesis of selenoprotein. However, selenomethionine is involved in the synthesis of body proteins but does not require this intermediate step. Therefore, the biological role of selenium depends not only on the content of selenium but also on the chemical structure of the selenium-containing compound.
For organic selenium products such as yeast selenium, the bioavailability depends mainly on the absorption and utilization of selenoproteins or peptides in the product. Figure 2 lists the main metabolic pathways for selenium.
It is important to distinguish between total selenomethionine and free selenomethionine, since only free selenomethionine can be used for non-specific synthesis of proteins. Regardless of the total selenium content or the total selenomethionine content, in addition to the free selenomethionine form, other forms of selenium must first be converted to selenide in order to participate in the whole body's selenium metabolism.
There has always been a misconception in the feed industry that the higher the total selenomethionine content in selenium-enriched yeast, the better. There is currently no scientific basis for this view, and no data on selenoprotein utilization and digestibility in commercial products has been published. This misconception is based on the perception that selenomethionine is an active component of selenium yeast. The content of selenomethionine in different products is not the same, and the utilization rate and digestibility are also different due to the difference of yeast strains, and the free selenomethionine released in different products is also different (Fig. 3).
Industry studies have also addressed this question by measuring the digestibility of selenoproteins and peptides in selenium-enriched yeast, which were determined by two-dimensional chromatography and mass spectrometry in in vitro gastrointestinal digestion experiments (Reyes et al., 2006). The results showed that approximately 90% of total selenium was absorbed after digestion in the gastrointestinal tract, but only 34% was free selenomethionine. The remaining selenium is present in small, medium and large molecular selenium-containing peptides. These proteins can be further detected and identified by two-dimensional chromatography-mass spectrometry. Interestingly, most selenium-containing compounds exist as selenium-containing small peptides after simulated digestion.
In fact, although the gastrointestinal tract is highly efficient in digesting selenoproteins and peptides in selenium yeast, the efficiency of converting these digested products into freely available free selenomethionine is very low. Moreover, it is worth noting that the effectiveness of the other 60 selenium-containing compounds in selenium yeast has undergone the same digestion process and is also a major factor affecting the efficiency of selenium bioavailability.
Although it is not realistic to directly compare different selenium yeast products based on the bioavailability of selenium sources, we can compare the published tissue deposition data and use it as a basis to assess the digestibility and bioavailability of individual products. .
As part of the EU product registration process, each selenium yeast production company is required to submit detailed documentation, including the efficacy, safety and toxicity of its products. The European Food Safety Authority then evaluates the effectiveness and safety of the product in all aspects, not only for the safety of the user, but also for the safety of consumers and the environment. The European Food Safety Authority has published a set of officially recognized tissue deposition data, which is included in every EU-approved selenium yeast product. Although this set of data does not directly compare the test results of different products, we can see that the degree of selenium deposition by yeast selenium has obvious tissue specificity and strain specificity.
In fact, this set of data also reveals that selenium yeast products are completely different and cannot be confused. Different yeast strains have different locations and efficiencies in the deposition of selenoproteins and peptides, so their digestibility and bioavailability are also very different, which is the main reason for the difference in selenium deposition between different products.
Conclusion
When comparing different selenium yeast products, we should consider the difference in yeast strains in different products, as this is the main reason for the different deposition and deposition sites of selenium in individual proteins and peptides. These differences ultimately affect the digestion and release of seleno-containing amino acids and the differences in bioavailability of different products. The content of selenomethionine is not a necessary factor to increase the relative bioavailability of the selenium source.
Finally, due to the difference in utilization efficiency of selenoproteins and peptides, different yeast selenium products cannot be confused.
References
Arnaudguilhem, C., K. Bierla, L. Ouerdane, H. Preud'homme, A. Yiannikouris and R. Lobinski. 2012. Selenium metabolomics in yeast using complementary reversed-phase/hydrophilic ion interaction (HILIC) liquid chromatography electrospray hybrid quadrupole Trap/Orbitrap mass spectrometry. Analytica Chimica Acta, 757:26-38.
Encinar, JR, M. Sliwka-Kaszynska, A. Polatajko, V. Vacchina and J. Szpunar. 2003. Methodological advances for selenium speciation analysis in yeast. Analytica Chimica Acta, 500 171-183.
Reyes, LH, JR Encinar, JM Marchante Gayon, JI Garcia Alonso, A. SanzMedel. 2006. Selenium bioaccessibility assessment in selenized yeast after "in vitro" gastrointestinal digestion using two dimensional chromatography and mass spectrometry. Journal of Chromatography A, 1110:108-116
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