Amino Acid Supplement Free-Form Powder L-Methionine is an essential sulfur-containing, free-form amino acid. It contributes to the synthesis of S-adenosyl-L-methionine (SAMe), which is necessary for normal brain function, and L-cysteine, which is a component of glutathione, an important antioxidant molecule in the body. L-Methionine is also a transporter of the antioxidant mineral, selenium.