L-methionine - Sulfur-containing essential amino acid.
L-methionine is an α-amino acid that is used in the biosynthesis of proteins. It is a sulfur-containing essential L-amino acid that is important in many body functions.
Methionine is required for normal growth and development of humans.
Methionine is one of two sulfur-containing proteinogenic amino acids, the other being cysteine. Excluding the few exceptions where methionine may act as a redox sensor, methionine residues do not have a catalytic role. This is in contrast to cysteine residues, where the thiol group has a catalytic role in many proteins. The thioether does however have a minor structural role due to the stability effect of S/π interactions between the side chain sulfur atom and aromatic amino acids in one-third of all known protein structures.
This lack of a strong role is reflected in experiments where little effect is seen in proteins where methionine is replaced by norleucine, a straight hydrocarbon sidechain amino acid which lacks the thioether.
Methionine is required for normal growth and development of humans, other mammals, and avian species. In addition to being a substrate for protein synthesis, it is an intermediate in transmethylation reactions, serving as the major methyl group donor in vivo, including the methyl groups for DNA and RNA intermediates.
Methionine is also required for growth and tissue repair. As a sulphur-containing amino acid, methionine improves the tone and pliability of skin, hair, and strengthens nails. It’s also involved in many detoxifying processes in the body. The sulphur provided by methionine protects cells from pollutants, slows cell aging, and is essential for absorption and bio-availability of selenium and zinc. Methionine chelates heavy metals, such as lead and mercury, aiding their excretion. It also acts as a lipotropic agent and prevents excess fat buildup in the liver.
Methionine is also required for synthesis of cysteine. Methionine is accepted as the metabolic precursor for cysteine. Only the sulfur atom from methionine is transferred to cysteine; the carbon skeleton of cysteine is donated by serine.
Eggs are a good source of methionine.
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. However, it is the combination of methionine and lysine which is considered for completeness of a protein. Racemic methionine is sometimes added as an ingredient to pet foods.
Loss of methionine has been linked to senile greying of hair.
Loss of methionine has been linked to senile greying of hair. Its lack leads to a buildup of hydrogen peroxide in hair follicles, a reduction in tyrosinase effectiveness, and a gradual loss of hair color.
Methionine is an intermediate in the biosynthesis of cysteine, carnitine, taurine, lecithin, phosphatidylcholine, and other phospholipids. Improper conversion of methionine can lead to atherosclerosis.
Atherosclerosis is a disease in which the inside of an artery narrows due to the build of plaque. Initially there are generally no symptoms. When severe it can result in coronary artery disease, stroke, peripheral artery disease, or kidney problems depending on the arteries which are affected. Symptoms, if they occur, generally do not begin until middle age.
Overconsumption of methionine is related to cancer growth.
Overconsumption of methionine, the methyl group donor in DNA methylation, is related to cancer growth in a number of studies.
Methionine is known to exacerbate psychopathological symptoms in schizophrenic patients, there is no evidence of similar effects in healthy subjects.
The role of methionine as a precursor of homocysteine is the most notable cause for concern. A "loading dose" of methionine (0. 1 g/kg) has been given, and the resultant acute increase in plasma homocysteine has been used as an index of the susceptibility to cardiovascular disease.
Although this procedure results in vascular dysfunction, this is acute and unlikely to result in permanent damage. However, a 10-fold larger dose, given mistakenly, resulted in death.
Longer-term studies in adults have indicated no adverse consequences of moderate fluctuations in dietary methionine intake, but intakes higher than 5 times normal resulted in elevated homocysteine levels.
These effects of methionine on homocysteine and vascular function are moderated by supplements of vitamins B-6, B-12, C, and folic acid.
In infants, methionine intakes of 2 to 5 times normal resulted in impaired growth and extremely high plasma methionine levels, but no adverse long-term consequences were observed.
Furthermore, Methionine is found to be associated with glycine N-methyltransferase deficiency, homocystinuria, and methylenetetrahydrofolate reductase deficiency, which are inborn errors of metabolism.
There is apparent consensus concerning normal sulfur amino acid (SAA) requirements. WHO recommendations amount to 13 mg/kg per 24hrs in healthy adults. This amount is roughly doubled in artificial nutrition regimens. In disease or after trauma, requirements may be altered for methionine, cysteine, and taurine. Although in specific cases of congenital enzyme deficiency, prematurity, or diminished liver function, hypermethionemia or hyperhomocysteinemia may occur, SAA supplementation can be considered safe in amounts exceeding 2-3 times the minimal recommended daily intake.