Methionine

α-amino-γ-methylmercaptobutyric acid, CH₃SCH₂CH₂CH(NH₂)COOH; a sulfur-bearing monocarboxylic amino acid. Methionine exists in D- and L- forms and in a racemic DL- form. L-methionine is a component of most vegetable and animal proteins. It was isolated in 1922 from the products of the acid hydrolysis of casein.

In mammals and man, methionine is a donor of methylene groups in the body. In its S-adenosyl form (active methionine, a product of the reaction of methionine with ATP in the presence of Mg²+ ions), methionine participates in enzymic transmethylation processes, which lead to the formation of choline, adrenalin, and other biologically important substances. It also serves as a source of sulfur in the biosynthesis of cysteine.

The initial substance in the biosynthesis of methionine is aspartic acid, which undergoes a series of conversions to methionine’s immediate precursor, homocysteine. This series of conversions can occur only in certain microorganisms and plants. Homocysteine can also undergo methylation in mammals, enzymically or by direct transfer of a methyl group from donor molecules.

Methionine is an essential amino acid, the daily adult human requirement for which is 2.5–3 g. Methionine deficiency in the diet of animals and man leads to impairment of protein biosyn-thesis, retardation of growth and development, and severe functional disorders. Synthetic methionine, produced industrially from propylene, is used medicinally and to enrich fodders and food. The D- and L- forms of methionine are of equal value, since they are capable of interconversion in the body.