An HPLC method has been validated for the analysis of L-(+)-Selenomethionine on a Thermoquest PDA System using a Phenomenex Aqua Column and standardized chromatographic conditions. The method provides accurate values of L-(+)- Selenomethionine content and measures the levels of free methionine in commercial samples of L-(+)Selenomethionine.
Selenium Supplements: Relative bioavailability and health benefits
Selenium is a vital trace element nutrient with multiple roles in the growth and functioning of living cells in higher animals and humans. This element is unevenly distributed in the earth’s crust.
The primary nutritional source is the soil from which it is absorbed by plants and enters the food chain. Geographical variations in the selenium status of populations therefore exist, necessitating selenium supplementation. The recommended levels for selenium supplementation in humans are 50-200 mcg/day.
Selenium supplements include:
- Inorganic salts such as sodium selenite and sodium selenate
- Selenium/amino acid chelates/aspartate
- L(+)-Selenomethionine
- Selenium yeast (where selenium is present as L-(+)Selenomethionine).
Inorganic selenium salts and chelated forms of selenium (chelates, aspartates) are broken down into elemental selenium. It is important to note that elemental selenium is not bioavailable and may have toxic effects at levels only four to five times the amount normally ingested in the human diet.
L(+)-Selenomethionine contains selenium in a molecularly integrated form and is therefore directly incorporated into the proteins in the body in place of the amino acid, methionine. Amino acid chelates and inorganic selenite / selenate are degraded by compounds such as ascorbic acid to elemental selenium which is not bioavailable. Metabolism of L(+) Selenomethionine. L(+)-Selenomethionine has the sulfur atom in L(+)-Methionine replaced by selenium. It is converted to selenocysteine in the body. Selenocysteine is then incorporated into selenoproteins. The sequence is schematically represented in the figure below:
It is reported that the replacement of methionine by selenomethionine in the protein structure does not induce any functional changes in the protein molecule. In fact, selenium in the protein structure protects DNA from oxidation more efficiently than the original sulfur in methionine. The carbon-selenium bond is more easily broken during photochemical reactions as compared to the carbon-sulfur bond. Thus L-(+)Selenomethionine preferentially "accepts" the energy from light. Therefore topical or orally administered L-(+)Selenomethionine offers greater protection to the skin against damage by ultraviolet light1.
One study reports that selenium levels in the red blood cells of subjects treated with selenomethionine (in the form of selenium yeast) increased by 100% after 16 weeks supplementation2. Neither selenite nor selenate supplementation produced significant increases under the same conditions. Thus selenomethionine and yeast containing selenomethionine are the appropriate forms of selenium for use in nutritional supplements and foods including infant formulae3.
However, if selenomethionine is supplied in the form of selenium yeast, it is important to ascertain that the selenium in the yeast is present in the form of L-(+)-Selenomethionine. Good commercial samples of selenium yeast typically contain 1000-2000 ppm of selenium, most of which is in the form of L-(+)-Selenomethionine. However, some samples may contain substantial amounts of inorganic selenium compounds instead of L-(+)-Selenomethionine
References:
- Burke, K.E. et al.(1992) Nutr.Cancer.17: 123-127
- Alfthan, G. et al. (1991) Am. J. Clin. Nutr. 55:120-125
- Schrauzer, G.N. (1998) Journal of Medicinal Food, 1(3):201-205
