The L-selenomethionine enantiomer is the main form of selenium found in Brazil nuts, cereal grains, soybeans, and grassland legumes, while Se-methylselenocysteine, or its γ-glutamyl derivative, is the major form of selenium found in Astragalus, Allium, and Brassica species.

Selenium and methionine also play separate roles in the formation and recycling of glutathione, a key endogenous antioxidant in many organisms, including humans.

Selenium and sulfur are chalcogens that share many chemical properties so the substitution of methionine with selenomethionine may have only a limited effect on protein structure and function.

However, the incorporation of selenomethionine into tissue proteins and keratin in cattle, birds, and fish causes alkali disease.

[6] The incorporation of heavy atoms such as selenium helps solve the phase problem in X-ray crystallography.