Generalities and Functions
There methionine (abbreviated Met) is an alpha-amino acid with the chemical formula HO2CCH (NH2) CH2CH2SCH3 and the structural formula shown in the "image below. It is classified as an amino acid apolar and falls within the group of so-called essential amino acids.
Together with cysteine, methionine is one of the proteinogenic sulfur amino acids.
Excluding the few exceptions in which methionine acts as a redox sensor, it is possible to define that its residues do not play any catalytic role; on the contrary, in the cysteine residues, the group thiol it can take on the role of a catalyst.
There are also metabolically active derivatives of methionine. The most important is undoubtedly the S-adenosyl Methionine (SAM), a cofactor that plays the role of a methyl donor. Some enzymes use SAM to initiate a radical reaction (radical SAM enzymes), which is essential in chemotaxis bacterial.
Methionine (together with lysine) participates in the synthesis of carnitine, is involved in the production of melatonin, has a chelating function on certain heavy metals and, if present in sufficient quantities, acidifies the pH of the urine, helping to prevent some types of kidney lithiasis.
It is necessary to bear in mind that the element sulfur is indispensable for the production of articular cartilage, of which methionine and cysteine are rich. If this is insufficient, negative effects can occur in the long term. Furthermore, people who suffer from rheumatoid arthritis, who are deficient in sulfur, risk complaining of a worsening of their pathological condition.
Methionine and Foods
Methionine is present in good quantities: in eggs, sesame seeds, Brazil nuts, fish, meats and other seeds (including cereals).
Most fruits and vegetables, as well as legumes, contain very little.
It is also good to remember that the biological function of methionine subordinates to cystine, therefore, from the plastic point of view, a protein can be completed only when both are present. Sometimes, the racemic methionine it is added as an ingredient in puppy food for pets.
Restrictions of Methionine
It is scientifically proven that reducing methionine consumption can extend the life of some organisms.
A 2005 study on the restriction of methionine consumption, unrelated to the decrease in total caloric intake, resulted in an increase in lifespan in mice.
Another research published in "Nature" showed that by adding only methionine to the diet of fruit flies, already subjected to dietary restriction (including other essential amino acids), they restored fertility without reducing the "increase in average life (typical of calorie restriction). This has led researchers to speculate that methionine adversely affects lifespan only in combination with another or more essential amino acids.
Several works have shown that dietary restriction of methionine inhibits the pathological processes of aging in mice and colon carcinogenesis in rats.
In humans, dietary restriction of methionine could be achieved by following a vegan-type diet. Being completely based on plant foods, this diet is generally low in methionine; however, some types of nuts and legumes contain a fair amount of them.
A 2009 study in rats showed that "in rat liver cells, the dietary supplement of methionine increases the mitochondrial production of ROS, promoting oxidative DNA damage within the mitochondria; it is a mechanism that would justify its "hepatotoxicity".
However, since methionine is an essential amino acid, it must never be removed from the diet of healthy individuals; this would however increase the risk of premature death. For example, rats fed a methionine-free diet develop steatohepatitis (fatty liver) e anemia, moreover, they show a weight loss equal to 2/3 of their weight in just 5 weeks. Administration of methionine is able to rapidly improve the clinical picture of the deficiency.
Methionine could also be essential in reversing methylation damage (possibly caused by repeated stress exposure) on glucocorticoid receptors in the central nervous system, with non-negligible implications on depressive symptoms.
Other Health Implications
Loss of methionine has been linked to age-related graying of the hair. Its deficiency would induce a buildup of hydrogen peroxide in the hair follicles, a reduction in the effectiveness of tyrosinase and a gradual loss of hair color.
Methionine is an intermediate in the biosynthesis of cysteine, carnitine, taurine, lecithin, phosphatidylcholine and other phospholipids.
Methionine can be obtained from homocysteine, an intermediate product of cysteine synthesis, whose accumulation in the blood is considered a possible risk factor for atherosclerosis. Improper conversion of methionine can therefore lead to an increase in the possibility of cardiovascular compromise.
Veterinary Uses
DL-methionine is sometimes used as a food supplement for dogs. This amino acid, which significantly lowers the pH of the urine, would help them maintain their health even by eating potentially contaminated grass. In other words, in dogs racemic methionine reduces the possibility of kidney lithiasis and exerts a chelating action on heavy metals such as: mercury, lead and cadmium, removing them from the body.
Methionine is also known for its ability to increase urinary excretion of quinidine. However, antibiotics aminoglycosides used to treat urinary tract infections work best in alkaline conditions, so urinary acidification with methionine may reduce its effectiveness.
If the dog follows a diet that in itself acidifies the urine, supplementation with methionine is contraindicated.
Methionine supplements
Despite controversy over the alleged health effects of methionine, the amino acid is commonly sold as a dietary supplement.
Methionine is proposed to users for its function as a precursor of SAM (S-Adenosyl Methionine) and for its catalyzing effect in methylation reactions. In practice, methionine supplements should favor the elimination of metabolic waste products; a sort of purifier that affects drugs, hormones, etc.
Being a product that acidifies the urine, methionine supplements are often combined with blueberry to prevent or attempt to treat urinary infections (such as cystitis); moreover, it seems to prevent the formation of kidney stones that develop in the presence of basic urine (concretions of calcium phosphate, calcium carbonate, magnesium phosphate and struvite).
Methionine supplements are also sold as hair and nail fortifying agents; the structural effect would be attributable to the ability of methionine to form sulphurous chains linked together and, from some studies, it seems that its intake can accelerate its growth.
Many other functions are also attributed to the integration of this amino acid; however, many of these have not been demonstrated. In fact, if the metabolic implications of this amino acid and its derivatives are clear, it is not as clear whether increasing its circulating levels can affect positively on the related physiological processes.
Obviously, since it is an amino acid particularly present in foods of animal origin but lacking in those of vegetable type, methionine is recommended above all in the case of a vegan diet, in order to guarantee the minimum recommended intake.
Methionine is commonly available in 500mg capsules, to be taken once a day and away from meals; commercial bodies suggest its association with some vitamins of the B complex (cobalamin, pyridoxine and folic acid) to favor the physiological synthesis of articular cartilage.
Complex with selenium, methionine is used as an organic form of this mineral with potential antioxidant activity.
Side effects
The pharmacological excess of methionine can induce hyperhomocysteinemia, neoplastic changes and brain damage.
Also, for people with hepatic impairment, excessively increasing methionine intake could worsen the pre-existing condition.
Likewise, those who tend to metabolic acidosis should avoid taking this amino acid which tends to acidify.
