L-methionine tablets. Bottle and tablets of the essential amino acid L-methionine. Methionine is a naturally occurring sulphur-containing amino acid, chemical formula C5H11NO2S. It is an essential constituent of the diet of mammals. There are 20 amino acids which make up all the proteins in humans. Of these, 12 can be synthesised in the body. The other 8 are essential amino acids and must be obtained from the diet; of these L-methionine is necessary for methylating reactions in the body.