Amino Acid Supplement Free-Form Powder L-Cysteine is a crystalline free-form amino acid that is active in many body processes. L-Cysteine works in the liver to protect the body from harmful substances and may help to prevent oxidation of tissues. L-Cysteine, along with L-glutamic acid and glycine, is the rate-limiting precursor in the synthesis of glutathione peroxidase (one of the body's main antioxidant enzymes), which minimizes lipid peroxidation (fats turning rancid).