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L-cystine

L-cystine is convertible to and from cysteine. Cystine is a nonessential amino acid commonly derived from the hydrolysis of protein, whereas cysteine is a nonessential amino acid derived from cystine itself. Both of these materials are commonly used in food and other nutritional products as nutrient and dietary supplements. Cysteine is unstable in solution, so that it is difficult to add cysteine directly to a liquid nutrient composition such as an infusion. Use of N-acetyl-L-cysteine in place of cysteine has been put into practice. On the other hand, L-cystine is considered to be equivalent to cysteine from a nutriological standpoint, and it is extremely stable as the oxidized form of cysteine. The sulfur containing amino acid cysteine and its stable dimeric disulfide product cystine, which is formed therefrom readily upon oxidation, do not belong to the classic essential amino acids, but nevertheless cystine is considered an indispensible component in amino acid preparations provided for the nutrition of babies, for chronical uremia, hepatic cirrhosis and of the injured. L-cystine is believed to minimize the cross-linking of free radicals that age the skin, harden the arteries, deposit aging pigments, and cause arthritis and mutagenic disorders such as cancer. Additionally, L-cystine is the precursor of glutathione, the body's primary antioxidizer and detoxifier that protects against such substances as lead, mercury, radiation, pesticides, and tobacco smoke. L-Cystine is also important in red and white cell formation and facilitating oxygen transport. The amino acid cystine is usually obtained by acid hydrolysis of keratinous waste products such as, for example, human hair, and is purified by reprecipitation.

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