L-Cysteine CasNo.: 52-90-4 is a semi-essential amino acid found naturally in the human body. Abundant in protein-rich foods, L-cysteine is also sold as a dietary supplement (sometimes just called cysteine). Along with the amino acids glutamine and glycine, cysteine is a building block of the powerful antioxidant glutathione. The body can make cysteine from the amino acids methionine and serine, but if these are in short supply, supplementing with L-cysteine can fill the gaps.