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Cysteine is classified as a nonessential amino acid, but cysteine may be essential for infants, the elderly, and individuals with certain metabolic diseases or malabsorption syndromes. Cysteine is an important structural and functional component of many proteins and enzymes. Cysteine is named after cystine, its oxidized dimer. Cysteine is potentially toxic and is catabolized in the gastrointestinal tract and blood. In opposition, cysteine is absorbed during digestion as cystine, which is more stable in the gastrointestinal tract. It is cystine that travels to cells, where it is reduced to two cysteine molecules upon cell entry. Measurement of cysteine is included in the Amino Acids, Plasma and Amino Acids, Urine tests along with 23 other amino acids.

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Cysteine (Cys/C) is a sulfur-containing amino acid. It is unstable in the air. In proteins, it usually exists as a cystine by forming a disulfide bond between two cysteine residues, carefully protected inside of the protein to function as a stabilizer for the high-order structure of the protein, or an active center for its bioactivity.

Cysteine itself is a powerful antioxidant and has the potential to trap reactive oxygen species (ROS) and is promising as an antiaging active. Because cysteine tends to be absorbed into cells where it cannot exhibit its antioxidant property, N-acetyl cysteine (NAC) is often used instead for this purpose. In addition, cysteine has a skin-whitening property as well. During the biosynthesis process of melanin, cysteine accelerates the pathway directed to the formation of pheomelanin, which produces yellowish or reddish colors and blocks the formation of eumelanin which produces dark colors.

Cysteine is an amino acid important for making protein, and other metabolic functions. It is found in beta-keratin, the main protein in nails, skin, and hair. Cysteine is important in the creation of collagen which affects skin elasticity and texture.

L-cysteine is a non-essential amino acid that can be synthesized in the body from L-methionine and L-serine. It is conditionally essential for preterm infants. It is an important precursor for the synthesis of proteins such as glutathione, taurine, coenzyme A, and inorganic sulfate. Some anti-inflammatory properties have been shown by L-cysteine and are important for the protection against various toxins.

Cysteine can be endogenously synthesized mainly in the liver, from homocysteine issued from the transmethylation of methionine. Our body's cysteine is created from methionine, an essential amino acid. Some of the foods that have cysteine are pork, duck, turkey, chicken, yogurt, cheese, oat flakes, wheat germ, lunch meat, beef liver, etc.

Cysteine is used as a constituent in the food, pharmaceutical, and personal care industries.

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