L-Methionine is a sulfur-containing, essential amino acid that is converted in the body to S-adenosylmethionine, a precursor to phosphatidyl choline and creatine. Phosphatidyl choline, an important component of cell membranes, supports healthy brain function. Creatine forms creatine phosphate, a high-energy compound found in high concentration in the muscles. Creatine phosphate helps regenerate ATP during exercise, providing energy for short bursts of maximal muscle power.