Selenium can be divided into 2 categories: organic and inorganic. The organic forms contain carbon, while the inorganic forms do not. The primary inorganic forms of selenium are selenite (SeO3) and selenate (SeO4). Selenite and selenate are not commonly found alone in nature; they are usually complexed with sodium to form sodium selenite (Na2SeO3) and sodium selenate (Na2SeO4)1.
Selenomethionine is the most common organic form of selenium. The structure of selenomethionine is shown above the structure of the amino acid methionine in the figure below.
In comparing the structures of selenomethionine or methionine, you can see that the only difference is that selenium has been substituted for the sulfur (S) atom in methionine. Selenocysteine is considered the 21st amino acid by some, because there is a codon that directs its insertion into selenoproteins. Like selenomethionine versus methionine, the only difference between selenocysteine and cysteine is the substitution of selenium for sulfur. The last organic form is methylselenocysteine (aka Se-methylselenocysteine). Notice that its structure is like selenocysteine, but with a methyl group added (like the name suggests).
The selenium content of plants is dependent on the soil where they are grown. As shown below, soil selenium levels vary greatly throughout the United States, meaning that the selenium content of plant foods also greatly vary.
The above map is interactive, so to see the soil selenium levels in a certain county or state, click on it in the link below.
–USGS Soil Selenium Levels
Inorganic forms of selenium are commonly used in supplements. Selenomethionine is the most common organic form of selenium in supplements and food. It is found in cereal grains such as wheat, corn, and rice as well as soy. Yeast are typically used to produce selenomethionine for supplements.
It should be noted that selenomethionine accumulates at much higher levels in the body than other forms of selenium. This is because it can be nonspecifically incorporated into body proteins in place of methionine. However, despite accumulating at higher levels, selenomethionine is less effective than the methylselenocysteine in decreasing cancer incidence or growth in animal models3. However, methylselenocysteine is not commonly consumed, because it is a form that plants accumulate to prevent selenium from becoming toxic to themselves. Thus, plants need to be grown in the presence of high selenium levels to accumulate meaningful amounts of methylselenocysteine.
References & Links
1. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
3. Lindshield B, Ford N, Canene-Adams K, Diamond A, Wallig M, et al. (2010) Selenium, but not lycopene or vitamin E, decreases growth of transplantable dunning R3327-H rat prostate tumors. PloS One 5(4): e10423-e10423.
USGS Soil Selenium Levels – http://tin.er.usgs.gov/geochem/doc/averages/se/usa.html