L-Cysteine is a crystalline free-form amino acid that is active in many body processes. L-Cystine works in the liver to protect the body from harmful substances and may Enhance to Beneficial Effects oxidation of tissues. L-Cystine, 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).