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Methionine is an important amino acid that helps to initiate the translation of messenger RNA by being the first amino acid incorporated into the N-terminal position of all proteins. Methionine is a source of sulfur, required by the body for normal metabolism and growth. Methionine assists in the breakdown of fats, helps to detoxify lead and other heavy metals, helps diminish muscle weakness, and prevents brittle hair. Methionine reacts with adenosine triphosphate (ATP) to contribute to the synthesis of many important substances, including epinephrine and choline. Measurement of methionine is included in the Amino Acids, Plasma and Amino Acids, Urine tests along with 23 other amino acids.

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Methionine (Met/M) is an aliphatic, sulfur-containing, essential amino acid, and a precursor of succinyl-CoA, homocysteine, cysteine, creatine, and carnitine. Recent research has demonstrated that methionine can regulate metabolic processes, the innate immune system, and digestive functioning in mammals. It also intervenes in lipid metabolism, activation of endogenous antioxidant enzymes, and the biosynthesis of glutathione to counteract oxidative stress. In addition, methionine restriction prevents altered methionine/ transmethylation metabolism, thereby decreasing DNA damage and carcinogenic processes, and possibly preventing arterial, neuropsychiatric, and neurodegenerative diseases.

S-adenosyl-l-methionine (SAM or SAMe) is a compound that is formed from a reaction of methionine and adenosine triphosphate (ATP). SAM is the main methyl donor in the human body, involved in a variety of biochemical pathways.

Methionine is converted to SAM by methionine adenosyltransferase. SAM then serves as a methyl donor in many methyltransferase reactions and is converted to S-adenosylhomocysteine (SAH). Adenosylhomocysteinase converts SAH to homocysteine.

There are two fates of homocysteine.

  1. Methionine can be regenerated from homocysteine via methionine synthase. It can also be remethylated using glycine betaine to methionine via the enzyme Betaine-homocysteine methyltransferase (BHMT).
  2. Homocysteine can be converted to cysteine. Cystathionine-β-synthase combines homocysteine and serine to produce cystathionine.

Methionine and its derivatives fulfill many important functions in the body.

Source of sulfur: Methionine is an important source of sulfur for numerous compounds, such as the amino acids cysteine and taurine. Sulfur is used by the body for healthy hair, skin, and nail growth, to increase the liver's production of lecithin, reduction of liver fat, protection of the kidneys, the excretion of heavy metals, and regulate the formation of ammonia in the urine.

S-adenosyl-l-methionine (SAM) reactions: SAM, a derivative of methionine, is involved in the synthesis of epinephrine, choline, and other substances, and is an important methyl donor.

Lipotropic: Methionine is lipotropic, helping to prevent fat accumulation in the liver and typically aiding the detoxification of metabolic wastes and toxins.

Paracetamol overdose treatment: Methionine is used in the treatment of paracetamol poisoning to prevent liver damage.

Arthritis treatment: SAM is used to provide relief from arthritis and has fewer side effects than common anti-inflammatory drugs like ibuprofen and aspirin.

Depression: SAM is prescribed more than any antidepressant and is considered effective, quick acting, and with fewer side effects, and may boost the activity of key brain chemicals involved in mood, such as norepinephrine, dopamine, and serotonin.

Liver function: Methionine levels are used in determining the liver’s concentration of sulfur-containing compounds and SAM is used in improving and normalizing liver function. It is used in the treatment of cirrhosis and alcohol damage and is key to the production of glutathione. It also inactivates estrogens to prevent suppressed bile flow in pregnant women or those on oral contraceptives.

Neurological disorders: SAM is considered to improve the binding of neurotransmitters to receptor sites in the brain and the regeneration of neuron axons following injury.

Methionine also plays a role in the synthesis of phosphatidylcholine and other phospholipids. Improper conversion of methionine can lead to atherosclerosis.

While virtually all protein-containing foods have some methionine, the amount varies widely. Eggs, fish, and some meats contain high amounts of this amino acid. It is estimated that around 8% of the amino acids in egg whites are sulfur-containing amino acids (methionine and cysteine). Plant proteins usually have even lower quantities of these amino acids.

Despite the low intake among vegetarians, research has shown that they have higher blood concentrations of methionine than those who eat meat and fish. This finding led the researchers to conclude that dietary content and blood concentrations of methionine are not always directly related.

Although methionine has important roles in the body, some research shows the benefits of diets that are low in this amino acid. Some cancer cells are dependent on dietary methionine to grow. In these cases, limiting your dietary intake could be beneficial to help starve cancer cells. Since proteins from plants are often lower in methionine than animal proteins, some researchers believe that plant-based diets could be a tool to fight some cancers. Additionally, several studies in animals show that reducing methionine can increase lifespan and improve health.

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