Methionine is an essential amino acid and as a result humans are unable to synthesise selenomethionine. However, Selenomethionine is synthesised in plants via bonding of inorganic forms of selenium to the essential amino acid methionine (particularly in brassica and allium vegetables). Selenomethionine is absorbed from the small intestine via the sodium dependent neutral amino acid transport system, the same pathway followed by methionine. Selenite and senelate are inorganic forms of selenium that are readily incorporated into selenoproteins, but only selenomethionine is incorporated into body proteins, particularly in tissues with high rates of protein synthesis such as skeletal muscle, erythrocytes (mainly in haemoglobin) and the gastrointestinal mucosa. This tissue selenomethionine acts as a store of selenium and is able to be released into circulation. Selenomethionine is able to maintain the activity of selenium dependent enzymes for longer periods when compared to inorganic forms, because it is stored in this way.
Selenomethionine is metabolised to selenohomocysteine and selenocystathionine by the same enzymes involved in methionine metabolism to homocysteine and cystathionine. Selenomethionine therefore does not have its own metabolising enzymes, and is dependent on the vitamins B6, B12 and folate which are necessary enzymes co-factors. Selenocystathionine is metabolised further to selenocysteine which can be degraded in the liver to serine and selenide. Selenide can then be utilised for selenoprotein synthesis or methylated to dimethyl selenide and the trimethylselenium ion, and then exhaled or excreted. Use of selenomethionine for glutathione peroxidise synthesis is dependent of methionine status, with greater incorporation of selenium with increasing intakes of methionine, until a steady state is reached. This is presumably because selenomethionine substitutes for methionine in proteins when methionine is deficient and is therefore not present to synthesise glutathione peroxidase. Supplementation of methionine to individuals with low methionine status increases red blood cell glutathione peroxidise activity.
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