– L-Cysteine-d3 is a deuterated compound of L-Cysteine. L-Cysteine has a CAS number of 52-90-4. L-Cysteine is a non-essential sulfur-containing amino acid in humans. L-Cysteine is important for protein synthesis, detoxification, and diverse metabolic functions. Found in beta-keratin, the main protein in nails, skin, and hair, L-Cysteine is important in collagen production, as well as skin elasticity and texture. Also required in the manufacture of amino acid taurine, L-Cysteine is a component of the antioxidant glutathione, and plays a role in the metabolism of essential biochemicals such as coenzyme A, heparin, and biotin.