Chemical Properties
d,l-Methionine has a characteristic odor. It is an essential amino acid and is also used as a nutrient and dietary
supplement.
Chemical Properties
White crystalline powder
Chemical Properties
White, crystalline platelets or powder having a characteristic odor.
One g dissolves in about 30 mL of water. It is soluble in dilute
acids and in solutions of alkali hydroxides. It is very slightly
soluble in alcohol, and practically insoluble in ether. It is optically
inactive. The pH of a 1 in 100 solution is between 5.6 and 6.1.
This substance may be prepared by addition of methanethiol to
acrolein; by chemical conversion of methylthiopropionic aldehyde.
Occurrence
High levels of methionine can be found in eggs, sesame seeds, Brazil nuts, fish, meats and some other plant seeds; methionine is also found in cereal grains. Most fruits and vegetables contain very little of it. Most legumes are also low in methionine. Racemic methionine is sometimes added as an ingredient to pet foods.
Uses
An essential nonpolar amino acid with oxidative stress defense properties
Uses
DL-Methionine is sometimes given as a supplement to dogs; it helps keep dogs from damaging grass by reducing the pH of the urine.
Methionine is allowed as a supplement to organic poultry feed under the US certified organic program.
Definition
ChEBI: A sulfur-containing amino acid that is butyric acid bearing an amino substituent at position 2 and a methylthio substituent at position 4.
Preparation
By addition of methanethiol to acrolein; by chemical conversion of methylthiopropionic aldehyde.
Brand name
Pedameth (Forest).
Biosynthesis
As an essential amino acid, methionine is not synthesized de novo in humans, who must ingest methionine or methioninecontaining proteins. In plants and microorganisms, methionine is synthesized via a pathway that uses both aspartic acid and cysteine. First, aspartic acid is converted via β-aspartyl-semialdehyde into homoserine, introducing the pair of contiguous methylene groups. Homoserine converts to O-succinyl homoserine, which then reacts with cysteine to produce cystathionine, which is cleaved to yield homocysteine. Subsequent methylation of the thiol group by folates affords methionine. Both cystathionine-γ-synthase and cystathionine- β-lyase require pyridoxyl-5′-phosphate as a cofactor, whereas homocysteine methyltransferase requires vitamin B12 as a cofactor.
Biotechnological Production
DL-Methionine is the second amino acid that is almost exclusively manufactured
by chemical synthesis. The process used today was originally developed by
Werner Schwarze at Degussa in the 1940s, and has been continually improved and
refined since. Today DL-methionine is manufactured in several plants, each with a
capacity of more than 100,000 tonnes. To operate the process on an industrial scale
also requires back-integration into the key hazardous raw materials acrolein,
methyl mercaptan, and hydrogen cyanide. After the formation of the
hydantoin, the key step is alkaline hydrolysis of the hydantoin, to produce
methionine directly in up to 95 % yield based on acrolein.
Biological Functions
Together with cysteine, methionine is one of two sulfurcontaining proteinogenic amino acids. Its derivative S-adenosyl methionine (SAM) serves as a methyl donor. Methionine is an intermediate in the biosynthesis of cysteine, carnitine, taurine, lecithin, phosphatidylcholine, and other phospholipids. Improper conversion of methionine can lead to atherosclerosis.
This amino acid is also used by plants for synthesis of ethylene. The process is known as the Yang Cycle or the methionine cycle.
Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code (tryptophan, encoded by UGG, is the other). The codon AUG is also the most common eukaryote "Start" message for a ribosome that signals the initiation of protein translation from mRNA when the AUG codon is in a Kozak consensus sequence. As a consequence, methionine is often incorporated into the N-terminal position of proteins in eukaryotes and archaea during translation, although it can be removed by post-translational modification. In bacteria, the derivative Nformylmethionine is used as the initial amino acid.
General Description
DL-Methionine is an essential amino acid containing sulphur. Methionine consists of an asymmetric carbon and exists as D (dextrogyre) and L (levogyre) optical isomers. The L-methionine is considered as biologically active. The racemic mixture of D and L-isomers forms DL-methionine, which is the commercially available methionine.
Biochem/physiol Actions
Methionine offers protection against disorders related to hair, skin and nails. It elevates lecithin production in the liver and thereby reduces cholesterol level. It naturally serves as a heavy metal chelating agent, regulating ammonia concentration in the urine. This ameliorates bladder irritation. Methionine is also known to induce hair growth.
Purification Methods
Crystallise it from hot water or EtOH. Also purify it by dissolving it in H2O and passing through an Amberlite IR-120 resin (NH4+ form). The eluate is concentrated and then passed through Amberlite IR-4B resin, and this eluate is evaporated to dryness. The residue is washed with EtOH, then Me2CO, dried and recrystallised from aqueous EtOH (colourless plates) [Baddiley & Jamieson J Chem Soc 4283 1954]. [Greenstein & Winitz The Chemistry of the Amino Acids J. Wiley, Vol 3 p 2125 1961, Beilstein 4 IV 3190.]
Other biochemical pathways
Although mammals cannot synthesize methionine, they can still use it in a variety of biochemical pathways:
Generation of homocysteine Methionine is converted to S-adenosyl methionine (SAM) by (1) methionine adenosyltransferase.
SAM serves as a methyl-donor in many (2) methyl transferase reactions, and is converted to S-adenosylhomocysteine (SAH).
(3) Adenosyl homocysteinase converts SAH to homocysteine.
There are two fates of homocysteine: it can be used to regenerate methionine, or to form cysteine.
Regeneration of methionine
Methionine can be regenerated from homocysteine via methionine synthase in a reaction that requires Vitamin B12 as a cofactor.
Homocysteine can also be remethylated using glycine betaine (NNN-trimethyl glycine, TMG) to methionine via the enzyme betainehomocysteine methyltransferase (E.C.2.1.1.5, BHMT). BHMT makes up to 1.5% of all the soluble protein of the liver, and recent evidence suggests that it may have a greater influence on methionine and homocysteine homeostasis than methionine synthase.
