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Se-Methylselenocysteine (SeMSC) is a naturally occurring seleno-amino acid that is synthesized by plants such as garlic, astragalus, onions, and broccoli. It cannot be synthesized by higher animals. Unlike selenomethionine, which is incorporated into proteins in place of methionine, SeMSC is not incorporated into any proteins, thereby being fully available for the synthesis of selenium-containing enzymes such as glutathione peroxidase. Selenomethionine is the major seleno-compound in cereal grains (wheat grain, maize, and rice), soybeans, and enriched yeast. Seleno-compounds present in plants may have a profound effect upon the health of animals and human subjects. It is now known that the total Se content cannot be used as an indication of its efficacy, but knowledge of individual selenocompounds is necessary to fully assess the significance. Thus, speciation of the seleno-compounds has moved to the forefront. Since animals and man are dependent upon plants for their nutritional requirements, this makes the types of seleno-compounds in plants even more critical. Se enters the food chain through incorporation into plant proteins, mostly as selenocysteine and selenomethionine at normal Se levels. There are two possible pathways for the catabolism of selenomethionine: (1) a transsulfuration pathway via selenocystathionine to produce selenocysteine, which in turn is degraded to H2Se by the enzyme beta-lyase and (2) a transamination-decarboxylation pathway. It was estimated that 90% of methionine is metabolized through this pathway and thus could be also the major route for selenomethionine catabolism (PMID: 14748935 , Br J Nutr. 2004 Jan;91(1):11-28.). Selenomethionine is an amino acid containing selenium. The L-isomer of selenomethionine, known as Se-met and Sem, is a common natural food source of selenium. In vivo, selenomethionine is randomly incorporated instead of methionine and is readily oxidized. Its antioxidant activity arises from its ability to deplete reactive species. Selenium and sulfur are chalcogen elements that share many chemical properties and the substitution of methionine to selenomethionine may have no effect on protein structure and function. However, the incorporation of selenomethionine into tissue proteins and keratin in horses causes alkali disease. Alkali disease is characterized by emaciation, loss of hair, deformation and shedding of hooves, loss of vitality, and erosion of the joints of long bones.

Selenohomocysteine (CAS: 29412-93-9) is the precursor of selenocysteine, which is synthesized by the catalysis of cystathionine beta-synthase (EC 4.2.1.22) and cystathionine gamma-lyase (EC 4.4.1.1) in mammalian systems (both enzymes require pyridoxal phosphate). Selenohomocysteine (lactone) has been found to be a competitive and irreversible inhibitor of lysyl oxidase; this may relate to the development of connective tissue defects seen in homocystinuria. L-Selenohomocysteine also can serve as a substituent donor in the beta-replacement reaction to yield selenocystathionine (PMID: 10609891, 9405445, 6456763, 3338973).

A D-alpha-amino acid compound having methylselanylmethyl as the side-chain." []

An alpha-amino acid compound having methylselanylmethyl as the side-chain." []

Zwitterionic form of Se-methyl-L-selenocysteine." []

An L-alpha-amino acid zwitterion obtained from L-selenohomocysteine by transfer of a proton from the carboxy group to the amino group. It is the major species at pH 7.3." []