methionine (mĕthĪˈənēn) [key], organic compound, one of the 20 amino acids commonly found in animal proteins. Only the L-stereoisomer appears in mammalian protein. It is one of the several essential amino acids needed in the diet; the human body cannot synthesize it from simpler metabolites. It is an important source of dietary sulfur. Methionine reacts with adenosine triphosphate to form S-adenosyl methionine, a potent donor of methyl groups (composed of one carbon and three hydrogen atoms); S-adenosyl methionine is the principal methyl donor in the body and contributes to the synthesis of many important substances, including epinephrine and choline (see acetylcholine; vitamin). Since methionine is the only essential amino acid not present in significant amounts of soybeans, it is produced commercially as an additive for soybean meal. Methionine was isolated from casein (milk protein) in 1922, and its structure was proved by laboratory synthesis in 1928.