Methionine is an essential sulfur-containing amino acid, and its main function is to act as a building block for protein synthesis while also serving as a methyl donor and as a precursor for cysteine, glutathione and taurine. The human body cannot make methionine in adequate amounts, so it must be supplied through dietary protein or through fortified formulations. The grade traded in the largest volumes is DL-methionine, CAS 59-51-8, a racemic mixture of the D- and L-forms supplied as a white crystalline powder, while the optically active L-methionine, CAS 63-68-3, is chosen where a defined stereochemistry is required.
Identity and Typical Specifications
| Property | DL-Methionine |
|---|---|
| Chemical name | 2-amino-4-(methylthio)butanoic acid |
| Molecular formula | C5H11NO2S |
| Molecular weight | 149.21 g/mol |
| CAS number, DL-form | 59-51-8 |
| CAS number, L-form | 63-68-3 |
| Appearance | White crystalline powder |
| Sulfur content | About 21% by weight |
| Solubility | Soluble in water and in dilute acid or alkali; solubility rises with temperature |
| Assay | Agreed contractually and generally set at 98% or higher on a dried basis for feed and food grades |
Sulfur accounts for roughly one fifth of the molecule by mass, which is why methionine is the first-choice source of organic sulfur in many formulated diets. Impurity limits, heavy-metal limits and microbiological criteria belong in the purchase specification, and each batch is released against a certificate of analysis.
Core Functions in Human and Animal Nutrition
Inside the cell, methionine is incorporated into newly synthesized proteins in muscle, skin, hair and other tissues. Its activated form, S-adenosylmethionine, is the principal methyl donor for reactions that regulate gene expression and that produce creatine, phosphatidylcholine and other methylated compounds. Methionine also opens the transsulfuration pathway, which yields cysteine and supports the synthesis of glutathione, a central intracellular antioxidant, as well as taurine. Through polyamine synthesis, its decarboxylated derivative contributes to cell division and tissue repair.
Protein synthesis: supplies the sulfur amino acid backbone for tissue protein, hair and wool keratin.
Methyl donation: S-adenosylmethionine transfers methyl groups in dozens of enzymatic reactions.
Antioxidant defence: cysteine derived from methionine is a building block of glutathione.
Growth and immunity: supports feathering, muscle growth and the immune response in production animals.
Feed Applications and Inclusion Practice
In compound feed, DL-methionine is a standard nutritional additive used to balance the amino acid profile of poultry, swine and aquaculture diets. Methionine is frequently the first limiting amino acid in cereal- and soybean-meal-based rations, so supplementing it raises feed efficiency, supports weight gain and promotes the growth of feathers and other keratin-rich tissue. In the feed industry the product is a nutritional supplement rather than a veterinary drug, and inclusion levels are set from the nutritional requirement of the target species, the digestible amino acid profile of the other ingredients and the performance target of the farm.
Food, Cosmetic and Pharmaceutical Uses
In food processing, methionine is valued for its antioxidant behaviour, which slows the oxidation of lipids in selected baked goods and meat products and thereby extends shelf life. It can also be used to fortify foods and clinical nutrition products where the sulfur amino acid supply is low. In cosmetics and personal care, methionine powder appears in anti-aging serums, creams and colour cosmetics, where it acts as an antioxidant and as a stabiliser that protects oxidation-sensitive ingredients in the formula.
In pharmaceutical production the compound is used as an active ingredient and as a component of cell culture media, where the L-form is generally preferred because it matches the configuration used by mammalian cells. Methionine is also an important chiral building block for fine chemicals, because its defined three-dimensional structure gives chemists a reliable starting point for asymmetric synthesis and its reactive sulfur atom can be oxidised, alkylated or used to introduce a methylthio group. Derivatives of methionine additionally serve agricultural chemistry as intermediates for crop protection products and as biostimulant ingredients.
Quality Control, Packaging and Storage
Consistency between batches is the property most valued by industrial buyers. A typical release programme covers identity by spectroscopy, assay by titration or liquid chromatography, water content by Karl Fischer titration, residue on ignition, heavy metals such as lead and arsenic, and microbiological counts for food and pharmaceutical grades.
The powder is packed in multi-wall paper bags with an inner liner, or in fibre drums, and should be stored in a dry, cool, well-ventilated area away from strong oxidising agents and direct sunlight. Containers should stay closed between withdrawals because the material absorbs moisture and can cake when humidity is high.
Frequently Asked Questions
Q: What is the main function of methionine?
Its main function is to serve as a building block for protein synthesis, while it also acts as a methyl donor through S-adenosylmethionine and as a precursor for cysteine and glutathione.
Q: What is the CAS number of DL-methionine and L-methionine?
DL-methionine carries CAS 59-51-8 and the pure L-isomer carries CAS 63-68-3.
Q: Is methionine an essential amino acid?
Yes. Humans and animals cannot synthesise it in sufficient quantities, so it must be supplied by the diet or by supplementation.
Q: Is methionine in feed a veterinary drug?
No. In animal nutrition it is a nutritional additive used to balance the amino acid profile of the ration, not a therapeutic drug.
Q: Why is methionine added to food products?
It is added mainly for its antioxidant effect, which slows lipid oxidation and extends the shelf life of certain products, and for nutrient fortification.
Q: Which natural sources contain methionine?
Meat, dairy products, fish, eggs, beans and nuts are the usual dietary sources, with animal proteins generally providing the highest concentrations.