METHIONINE

CAS Number: 59-51-8 
63-68-3 (L-isomer) 
348-67-4 (D-isomer)
EC Number:  200-432-1
IUPAC name: Methionine

PROPERTIES

Chemical formula: C5H11NO2S
Molar mass: 149.21 g·mol−1
Appearance: White crystalline powder
Density: 1.340 g/cm3
Melting point: 281 °C (538 °F; 554 K) decomposes
Solubility in water: Soluble
Acidity (pKa): 2.28 (carboxyl), 9.21 (amino)

APPLICATIONS

Methionine is a unique amino acid. 
Methionine contains sulfur and can produce other sulfur-containing molecules in the body. 
Also, Methionine is involved in starting protein production in your cells.

Methionine is an amino acid. 
Amino acids are the building blocks that our bodies use to make proteins. 
Methionine is found in meat, fish, and dairy products, and it plays an important role in many cell functions.

Methionine is used to prevent liver damage in acetaminophen (Tylenol) poisoning. It is also used for increasing the acidity of urine, treating liver disorders, and improving wound healing. Other uses include treating depression, alcoholism, allergies, asthma, copper poisoning, radiation side effects, schizophrenia, drug withdrawal, and Parkinson's disease.

In acetaminophen poisoning, methionine prevents the breakdown products of acetaminophen from damaging the liver. 
Methionine may also act as an antioxidant and help to protect damaged tissues.

DL-Methionine is sometimes given as a supplement to dogs; DL-Methionine helps reduce the chances of kidney stones in dogs. 
Methionine is also known to increase the urinary excretion of quinidine by acidifying the urine. 
Aminoglycoside antibiotics used to treat urinary tract infections work best in alkaline conditions, and urinary acidification from using methionine can reduce its effectiveness. 

If a dog is on a diet that acidifies the urine, methionine should not be used.
Methionine is allowed as a supplement to organic poultry feed under the US certified organic program.
Methionine can be used as a nontoxic pesticide option against giant swallowtail caterpillars, which are a serious pest to orange crops.
Methionine is commonly taken by mouth to treat liver disorders and viral infections along with many other uses. 
But there is limited scientific research that supports these uses.

Methionine chelates heavy metals, such as lead and mercury, aiding their excretion. 
Methionine also acts as a lipotropic agent and prevents excess fat buildup in the liver. 

L-methionine is the L-enantiomer of methionine. 
L-methionine has a role as a nutraceutical, a micronutrient, an antidote to paracetamol poisoning, a human metabolite and a mouse metabolite. 
Also, L-methionine is an aspartate family amino acid, a proteinogenic amino acid, a methionine and a L-alpha-amino acid.

Methionine is a sulfur-containing amino acid that improves the tone and elasticity of the skin, promotes healthy hair and strengthens the nails.
Methionine supplements are commonly taken to treat various infections and disorders, but there is limited scientific research to support the efficacy of the supplements for the treatment of diseases. 
However, methionine is thought to be effective in the treatment of Tylenol (acetaminophen) poisoning.

Methionine (Met), an essential amino acid in the human body, possesses versatile features based on its chemical modification, cell metabolism and metabolic derivatives. 
Benefitting from its multifunctional properties, Methionine holds immense potential for biomedical applications.

First, given the unique structural characteristics of Methionine, two chemical modification methods are briefly introduced. 
Subsequently, due to the disordered metabolic state of tumor cells, Methionine is used in cancer treatment and diagnosis. 
Furthermore, the efficacy of S-adenosylmethionine (SAM), as the most important metabolic derivative of Methionine, for treating liver diseases. 

The sulfur in methionine provides the body with many potential health benefits.These may include:

-Nourishing the hair, skin, and nails
-Protecting the cells from pollutants
-Facilitating the detoxifying process
-Slowing down the aging process
-Helping with the absorption of other nutrients (such as selenium and zinc)
-Aiding in the excretion of heavy metals (such as lead and mercury) helping the body’s excretion process
-Preventing excess fat buildup in the liver (by acting as a lipotropic agent—one that facilitates the breakdown of fats)
-Lowering cholesterol levels by increasing lecithin production in the liver.

Methionine is commonly taken for other disorders, but there is a lack of clinical research study results to back up the safety and efficacy of its use in these conditions:

-Herpes simplex and herpes zoster (shingles)
-Symptoms of menopause
-Inflammation of the pancreas
-Liver problems
-Depression
-Alcoholism
-Urinary tract infections (UTI’s)
-Asthma and allergies
-Schizophrenia

USE IN ANIMAL FEED

Human medical uses for synthetic methionine were eclipsed in the 1950s by realisation of its value in animal feeding. 
A rise in the use of soy and maize as the base fodder for livestock in the 1940s introduced dietary shortages in animals in essential amino acids such as methionine and lysine, difficult to remedy with increasingly scarce and expensive supplies of whole protein feed components, such as fishmeal. 

Methionine can limit protein production in diverse livestock species raised on feed, particularly in poultry diets. 
Feathers have high cysteine content and dietary deficiency in methionine and cysteine causes birds to peck other birds’ feathers and restricts the uptake of other amino acids in the diet.

DESCRIPTION

Methionine (symbol Met or M) is an essential amino acid in humans. 
As the substrate for other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical role in the metabolism and health of many species, including humans. 
Methionine is encoded by the codon AUG.

Methionine is also an important part of angiogenesis, the growth of new blood vessels. 
Supplementation of Methionine may benefit those suffering from copper poisoning.
Overconsumption of methionine, the methyl group donor in DNA methylation, is related to cancer growth in a number of studies.
Methionine was first isolated in 1921 by John Howard Mueller.

Methionine is an amino acid. 
Amino acids are the building blocks that our bodies use to make proteins. 
Methionine is found in meat, fish, and dairy products. 
Methionine plays an important role in the many functions within the body.

Methionine is an amino acid found in many proteins, including the proteins in foods and those found in the tissues and organs of your body.
In addition to being a building block for proteins, Methionine has several other unique features.
One of these is its ability to be converted into important sulfur-containing molecules.

Sulfur-containing molecules have a variety of functions, including the protection of your tissues, modifying your DNA and maintaining proper functioning of your cells.
These important molecules must be made from amino acids that contain sulfur. 
Of the amino acids used to make proteins in the body, only methionine and cysteine contain sulfur.

Although your body can produce the amino acid cysteine on its own, methionine must come from your diet.
Additionally, methionine plays a critical role in starting the process of making new proteins inside your cells, something that is continuously occurring as older proteins break down.
For example, Methionine starts the process of producing new proteins in your muscles after an exercise session that damages them.

Methionine is one of nine essential amino acids in humans (provided by food), Methionine is required for growth and tissue repair. 
A sulphur-containing amino acid, methionine improves the tone and pliability of skin, hair, and strengthens nails. 
Involved in many detoxifying processes, sulphur provided by methionine protects cells from pollutants, slows cell aging, and is essential for absorption and bio-availability of selenium and zinc.

L-methionine is a conjugate base of a L-methioninium. 
L-methionine is a conjugate acid of a L-methioninate. 
Moreover, L-methionine is an enantiomer of a D-methionine. 
L-methionine is a tautomer of a L-methionine zwitterion. 

Methionine (L-methionine) is a nutritional supplement as well as an essential amino acid found in food. 
Methionine is required for normal growth and repair of body tissues; it cannot be made by the body, but must be obtained from the diet; thus, it is considered an “essential” amino acid. 

There are two types of methionine—L-methionine (which is naturally-occurring) and D-methionine. 
Each contains the same chemical make-up, but the molecules are mirror images. 
A mixture of the two is called DL-methionine.

Methionine, sulfur-containing amino acid obtained by the hydrolysis of most common proteins. 
First isolated from casein (1922), methionine accounts for about 5 percent of the weight of egg albumin; other proteins contain much smaller amounts of methionine. 
Methionine is one of several so-called essential amino acids for mammals and fowl; i.e., they cannot synthesize it. 

In microorganisms it is synthesized from the amino acids cysteine and aspartic acid.
Methionine is important in methylation (the process by which methyl, or -CH3, groups are added to compounds) and is also a precursor of two other amino acids, cystine and cysteine.

Methionine is one of nine essential amino acids that humans must derive from food and is the initiating amino acid for protein synthesis. 
As the principal sulphur-containing amino acid, it is a precursor for all other sulphur-containing amino acids and their derivatives, such as homocysteine and glutathione.
Methionine is a key component of one-carbon metabolism, donating methyl groups to epigenetic and detoxification pathways in the form of S-adenosylmethionine. 
Shortly after its identification as an essential amino acid in the 1930s, methionine was recognised as a limiting amino acid (ie, it limits the rate of protein synthesis) involved in human malnutrition, making it a key input to the diet of humans and of the animals that are reared for food.

Since the 1950s, methionine has increasingly become an anthropogenic factor embedded within our food systems because of its mass manufacturing from fossil fuels. Synthetic methionine is mainly produced by petrochemical synthesis in the bioavailable chemical forms of DL-methionine and α-hydroxy analogues.
Synthetic methionine products are added as a standard component to composed feeds used in industrial animal husbandry (mixtures of grains, minerals, amino acids, and other additives designed to match livestock species and developmental stage). 
Methionine is the leading limiting amino acid in poultry diets, and a stimulant of protein production in dairy cows, fish, and pigs. 

A necessary complement to grains naturally low in this limiting amino acid, synthetic methionine has been a key enabling technology in converting soy and maize into biomass of domesticated animals.
This conversion is particularly evident in the prodigious expansion in poultry biomass that has seen the modern broiler chicken become a “signal of a human reconfigured biosphere”.

Synthetic methionine is used for economic and environmental reasons, mainly because the optimised conversion of feed to protein reduces the overall amount of crude protein uptake, therefore lessening food production cost while also minimising nitrogen excretion. 
Because of its relatively low cost and the efficiency of using calibrated mixtures of amino acids instead of whole protein, there is a rapidly expanding worldwide market for synthetic methionine.8 More than 1 million tons a year of synthetic methionine now feeds into the world food supply, fundamentally changing the availability of this historically limiting amino acid.

Previously limited to biosynthesis in plants, fungi, and prokaryotes, more methionine is now entering the human food supply than at any time in history (in the form of inexpensive animal protein from meat, fish, milk, and eggs). 
Unintended effects might lie not in the synthetic origin of methionine itself, but in its use on an industrial scale. 
Biological sulphur assimilation from sulphate to methionine is energetically costly and a function that is done by a subset of species in an ecosystem. 
Industrial-scale methionine synthesis has opened a way to bypass long-evolved ecological interdependencies by shifting the way that sulphur is assimilated and circulates through food webs. 

This shift is reminiscent of how the Haber–Bosch process has transformed the nitrogen economy of the world by fixing dinitrogen and redistributing nitrogen to the ground, water, and people.
In this Personal View we argue for a planetary health perspective on the entwined human health and environmental implications of mass producing this pivotal and historically limiting amino acid. 
Although synthetic methionine is regarded as a routine feature of animal husbandry to scientists working in applied agricultural research, knowledge about it has remained siloed. 

Renewed scrutiny of methionine's role in cellular metabolism and human physiology brings urgency to querying the health effects of these industrial practices. 
Sulphur metabolites are emerging as important factors in diet and longevity studies, and methionine restriction could largely account for the health and lifespan effects previously attributed to calorie restriction. 
Restriction of other amino acids does not yield comparable results. 
Renewed interest in cancer metabolism has intensified investigation of cancer cell types that are methionine-dependent.

A 2020 epidemiological analysis in the USA suggests that sulphur amino acids (ie, methionine and cysteine) are currently being consumed, on average, at amounts associated with increased cardiometabolic risk, and that the sulphur amino acids intake optimal for long-term health might be substantially lower than consumed in the average US diet.
Despite its importance in regulating metabolism, the historical shift in methionine availability and prevalence in the food supply has not been systematically assessed.

In this Personal View, we trace synthetic methionine from the global scale of its manufacturing to that of cellular metabolism, beginning with a history of production trends. 
We then estimate the contribution of industrially sourced methionine to human physiology. 
Using the USA as an example, we model the proportion of synthetic methionine in the human body, the first such estimate of its kind. 
We discuss why this proportion might be relevant to human health, placing our modelling results in the context of biomedical studies in epigenetics, cancer biology, and longevity.

BIOCHEMICAL DETAILS

Methionine (abbreviated as Met or M; encoded by the codon AUG) is an α-amino acid that is used in the biosynthesis of proteins. 
Methionine contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), a carboxyl group (which is in the deprotonated −COO− form under biological conditions), and an S-methyl thioether side chain, classifying it as a nonpolar, aliphatic amino acid.

In nuclear genes of eukaryotes and in Archaea, methionine is coded for by the start codon, meaning it indicates the start of the coding region and is the first amino acid produced in a nascent polypeptide during mRNA translation.

A PROTEINOGENIC AMINO ACID 

Together with cysteine, methionine is one of two sulfur-containing proteinogenic amino acids. 
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.
It has been conjectured that norleucine was present in early versions of the genetic code, but methionine intruded into the final version of the genetic code due to the fact it is used in the cofactor S-adenosyl methionine (SAM-e).
This situation is not unique and may have occurred with ornithine and arginine.

ENCODING

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). 
In reflection to the evolutionary origin of its codon, the other AUN codons encode isoleucine, which is also a hydrophobic amino acid. In the mitochondrial genome of several organisms, including metazoa and yeast, the codon AUA also encodes for methionine. 
In the standard genetic code AUA codes for isoleucine and the respective tRNA (ileX in Escherichia coli) uses the unusual base lysidine (bacteria) or agmatidine (archaea) to discriminate against AUG.

The methionine codon AUG is also the most common start codon. 
A "Start" codon is 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 N-formylmethionine is used as the initial amino acid.

DERIVATIVES: S-adenosyl-methionine

The methionine-derivative S-adenosyl methionine (SAM-e) is a cofactor that serves mainly as a methyl donor. 
SAM-e is composed of an adenosyl molecule (via 5' carbon) attached to the sulfur of methionine, therefore making it a sulfonium cation (i.e., three substituents and positive charge). 
The sulfur acts as a soft Lewis acid (i.e., donor/electrophile) which allows the S-methyl group to be transferred to an oxygen, nitrogen, or aromatic system, often with the aid of other cofactors such as cobalamin (vitamin B12 in humans). 

Some enzymes use SAM-e to initiate a radical reaction; these are called radical SAM-e enzymes. 
As a result of the transfer of the methyl group, S-adenosyl-homocysteine is obtained. 
In bacteria, this is either regenerated by methylation or is salvaged by removing the adenine and the homocysteine, leaving the compound dihydroxypentandione to spontaneously convert into autoinducer-2, which is excreted as a waste product / quorum signal.

BIOSYTHESIS

As an essential amino acid, methionine is not synthesized de novo in humans and other animals, which must ingest methionine or methionine-containing proteins. 
In plants and microorganisms, methionine biosynthesis belongs to the aspartate family, along with threonine and lysine (via diaminopimelate, but not via α-aminoadipate). 
The main backbone is derived from aspartic acid, while the sulfur may come from cysteine, methanethiol, or hydrogen sulfide.

First, aspartic acid is converted via β-aspartyl-semialdehyde into homoserine by two reduction steps of the terminal carboxyl group (homoserine has therefore a γ-hydroxyl, hence the homo- series). 
The intermediate aspartate-semialdehyde is the branching point with the lysine biosynthetic pathway, where it is instead condensed with pyruvate. 
Homoserine is the branching point with the threonine pathway, where instead it is isomerised after activating the terminal hydroxyl with phosphate (also used for methionine biosynthesis in plants).

Homoserine is then activated with a phosphate, succinyl or an acetyl group on the hydroxyl.
In plants and possibly in some bacteria, phosphate is used. 
This step is shared with threonine biosynthesis.
In most organisms, an acetyl group is used to activate the homoserine. 
This can be catalysed in bacteria by an enzyme encoded by metX or metA (not homologues).

In enterobacteria and a limited number of other organisms, succinate is used. 
The enzyme that catalyses the reaction is MetA and the specificity for acetyl-CoA and succinyl-CoA is dictated by a single residue. 
The physiological basis for the preference of acetyl-CoA or succinyl-CoA is unknown, but such alternative routes are present in some other pathways (e.g. lysine biosynthesis and arginine biosynthesis).
The hydroxyl activating group is then replaced with cysteine, methanethiol, or hydrogen sulfide. 

A replacement reaction is technically a γ-elimination followed by a variant of a Michael addition. 
All the enzymes involved are homologues and members of the Cys/Met metabolism PLP-dependent enzyme family, which is a subset of the PLP-dependent fold type I clade. They utilise the cofactor PLP (pyridoxal phosphate), which functions by stabilising carbanion intermediates.
If it reacts with cysteine, it produces cystathionine, which is cleaved to yield homocysteine. 

The enzymes involved are cystathionine-γ-synthase (encoded by metB in bacteria) and cystathionine-β-lyase (metC). 
Cystathionine is bound differently in the two enzymes allowing β or γ reactions to occur.
If it reacts with free hydrogen sulfide, it produces homocysteine. 

This is catalysed by O-acetylhomoserine aminocarboxypropyltransferase (formerly known as O-acetylhomoserine (thiol)-lyase. 
It is encoded by either metY or metZ in bacteria.
If it reacts with methanethiol, it produces methionine directly. Methanethiol is a byproduct of catabolic pathway of certain compounds, therefore this route is more uncommon.

If homocysteine is produced, the thiol group is methylated, yielding methionine. 
Two methionine synthases are known; one is cobalamin (vitamin B12) dependent and one is independent.
The pathway using cysteine is called the "transsulfuration pathway", while the pathway using hydrogen sulfide (or methanethiol) is called "direct-sulfurylation pathway".

Cysteine is similarly produced, namely it can be made from an activated serine and either from homocysteine ("reverse trans-sulfurylation route") or from hydrogen sulfide ("direct sulfurylation route"); the activated serine is generally O-acetyl-serine (via CysK or CysM in E. coli), but in Aeropyrum pernix and some other archaea O-phosphoserine is used.
CysK and CysM are homologues, but belong to the PLP fold type III clade.

CHEMICAL SYTHESIS

The industrial synthesis combines acrolein, methanethiol, and cyanide, which affords the hydantoin. 
Racemic methionine can also be synthesized from diethyl sodium phthalimidomalonate by alkylation with chloroethylmethylsulfide (ClCH2CH2SCH3) followed by hydrolysis and decarboxylation.

ORIGINS of SYNTHETIC METHIONINE

The search for a means to produce large quantities of purified methionine for supplementing diets originated in the realisation of its status as an essential and limiting amino acid. 
The distinction between essential and non-essential amino acids was established in the 1930s through dietary experiments using hydrolysed amino acid compositions in place of whole protein. 

Restricting single amino acids such as methionine led to an appreciation of their importance to the diets of humans and animals. 
William Rose in his seminal 1949 paper on the amino acid requirements of humans, noted that methionine is more likely than any amino acid to be limiting in diets around the world, citing data from nutritional anthropology studies of the global incidence of kwashiorkor. 
Shortage of protein in the diet had been isolated as a primary cause of the so-called war dropsy or nutritional oedema, which was widespread in Europe following World War 1. 

In 1945 it once again became an immediately pressing post-war issue, particularly in Europe.
Researchers had long noted the paradox of being unable to treat people with severe malnutrition with refeeding in late stages when digestion was deeply compromised. Practitioners of intravenous and tube-feeding, now with the knowledge of amino acids, saw better results in patients presenting with burns or malnutrition with amino acids solutions than with whole foods or whole proteins. 
However, amino acids such as methionine had to be hydrolysed from whole protein, which was difficult and relatively expensive. 
Therefore, the initial drive for a synthetic process was to provide an plentiful limiting amino acid for the medical treatment of human starvation.

Petrochemical synthesis of methionine was enabled by the commercial manufacture of acrolein through the oxidation of propylene, which began in 1938. 
In 1947 acrolein was combined with methyl mercaptan to yield methionine, and a number of early publications and patents show that this method was an intense research focus for laboratories at fine chemical companies in the years following World War 2. 

The main process still in use today requires hydrogen sulphide and hydrogen cyanide. 
It generates a number of hazardous waste products, including pyridine, acetonitrile, cyanide, phenols, and benzene. 
Alternative biocatalytic and fermentation-based processes have been actively explored, albeit yields are often limited by the complex feedback regulation of de novo methionine biosynthesis.

HUMAN NUTRITION

Requirements
The Food and Nutrition Board of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. 
For methionine combined with cysteine, for adults 19 years and older, 19 mg/kg body weight/day.

This translates to about 1.33 grams per day for a 70 kilogram individual.

Dietary sources
High levels of methionine can be found in eggs, meat, and fish; sesame seeds, Brazil nuts, and some other plant seeds; and cereal grains. 
Most fruits and vegetables contain very little. Most legumes, though protein dense, are low in methionine. 
Proteins without adequate methionine are not considered to be complete proteins. 
For that reason, racemic methionine is sometimes added as an ingredient to pet foods.

Restriction
Some scientific evidence indicates restricting methionine consumption can increase lifespans in fruit flies.

A 2005 study showed methionine restriction without energy restriction extends mouse lifespans.
This extension requires intact growth hormone signaling, as animals without intact growth-hormone signaling do not have a further increase in lifespan when methionine restricted. 
The metabolic response to methionine restriction is also altered in mouse growth hormone signaling mutants.

A study published in Nature showed adding just the essential amino acid methionine to the diet of fruit flies under dietary restriction, including restriction of essential amino acids (EAAs), restored fertility without reducing the longer lifespans that are typical of dietary restriction, leading the researchers to determine that methionine “acts in combination with one or more other EAAs to shorten lifespan.” 
Restoring methionine to the diet of mice on a dietary restriction regimen blocks many acute benefits of dietary restriction, a process that may be mediated by increased production of hydrogen sulfide.

Several studies showed that methionine restriction also inhibits aging-related disease processes in mice and inhibits colon carcinogenesis in rats. 
In humans, methionine restriction through dietary modification could be achieved through a plant-based diet.

Restriction of dietary methionine reduces levels of its catabolite S-adenosylmethionine (SAM-e), resulting is a subsequent loss of histone methylation.
An active process mediated by a specific, preserved methylation of H3K9 preserves the memory of the original methylation profile, allowing the epigenome to be restored when dietary methionine levels return.

A 2009 study on rats showed "methionine supplementation in the diet specifically increases mitochondrial ROS production and mitochondrial DNA oxidative damage in rat liver mitochondria offering a plausible mechanism for its hepatotoxicity".

However, since methionine is an essential amino acid, it cannot be entirely removed from animals' diets without disease or death occurring over time. 
For example, rats fed a diet without methionine and choline developed steatohepatitis (fatty liver) and anemia, and lost two-thirds of their body weight over 5 weeks. Administration of methionine ameliorated the pathological consequences of methionine deprivation.
Short-term removal of only methionine from the diet can reverse diet-induced obesity and promotes insulin sensitivity in mice, and methionine restriction also protects a mouse model of spontaneous, polygenic obesity and diabetes.

HEALTH 

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 raises the intracellular concentration of GSH, thereby promoting antioxidant mediated cell defense and redox regulation. 
It also protects cells against dopamine induced nigral cell loss by binding oxidative metabolites.

Methionine is an intermediate in the biosynthesis of cysteine, carnitine, taurine, lecithin, phosphatidylcholine, and other phospholipids. 
Improper conversion of methionine can lead to atherosclerosis due to accumulation of homocysteine.

FOOD SOURCES of METHIONINE 

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

This value is about 5% in chicken and beef and 4% in dairy products. 
Plant proteins usually have even lower quantities of methionine and cysteine.
Some research has also examined the overall amount of the sulfur-containing amino acids (methionine and cysteine) in different types of diets.

The highest content (6.8 grams per day) was reported in high-protein diets, while lower intakes were present for vegetarians (3.0 grams per day) and vegans (2.3 grams per day).
Despite the low intake among vegetarians, other research has shown that they actually 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.
However, these studies did find that vegans have both low dietary intake and low blood concentrations of methionine.

SIDE EFFECTS and TOXICITY

Using a single amino acid supplement may lead to negative nitrogen balance. 
This can lessen how well your metabolism works. 
It can also make your kidneys work harder. 
In children, taking single amino acid supplements may also cause growth problems.

You should not take high doses of single amino acids for long periods of time.

Methionine can cause:

-Nausea
-Vomiting
-Dizziness
-Drowsiness
-Low blood pressure
-Irritability

Methionine may also make liver problems worse. 
Talk to your healthcare provider before using Methionine if you have severe liver disease.  
People with bipolar disorder should not take methionine supplements.
Women who are pregnant or breastfeeding shouldn’t use methionine supplements.
Toxicity of methionine is rare.
People with homocystinuria type I, an inherited disease, shouldn’t use methionine supplements.
If you take methionine supplements without enough folic acid, vitamin B-6, and vitamin B-12, it can increase the conversion of methionine to homocysteine. 
This may increase your risk for heart disease.

SYNONYMS

2-amino-4-(methylthio)butanoic acid
(S)-2-Amino-4-(methylmercapto)butyric acid 
L-2-Amino-4-(methylthio)butanoic acid
L-methionine
63-68-3
methionine
h-Met-oh
D-methionine
DL methionine
DL-methionine
L-2-amino-4-(methylthio) butyric acid
a-amino-g-methylthiol-n-butyric acid