Cystine

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Cystine is an oxidized dimeric form of cysteine. It is formed by linking two cysteine residues via a disulfide bond (cys-S-S-cys) between the -SH groups.

Cystine was discovered in 1810 but was not recognized as a component of proteins until it was isolated from the horn of a cow in 1899.1 Cystine is found in high concentrations in the cells of the immune system, skeletal and connective tissues, skin, digestive enzymes, and in hair. Hair and skin are 10-14% cystine.

Contents 1 Stability 2 Dietary importance 3 Nutritional sources 4 Side effects 5 Physical characteristics 6 See also 7 References 8 External links

[edit] Stability

The disulfide link is readily reduced to the corresponding thiol (-SH), for example by β-mercaptoethanol. It can also be broken by high temperatures (above about 150 °C, especially at low moisture levels (below about 20%)^[1].

[edit] Dietary importance

Glutamate and glycine are readily available in most North American diets, but the availability of cysteine makes it be the rate-limiting substrate for the synthesis of the critically important antioxidant glutathione within the cells of our body. It is the sulfhydryl (thiol) group (SH) of cysteine that serves as proton-donor and is responsible for the biological activity of glutathione.

The free amino acid cysteine does not represent an ideal delivery system to the cell. It is potentially toxic and is easily catabolized spontaneously in the gastrointestinal tract and in the blood plasma. However, cysteine which is absorbed during digestion as cystine (two cysteine molecules linked by a disulfide bond) is more stable than the free amino acid cysteine. The disulfide bond is pepsin and trypsin-resistant (two important digestive enzymes in the stomach), but may be split by heat, low pH, and mechanical stress. Cystine travels safely through the GI tract and blood plasma and is promptly reduced to the two cysteine molecules upon cell entry.

Cystine is the preferred form of cysteine for the synthesis of glutathione in cells involved in the immune function including macrophages and astrocytes. Lymphocytes and neurons prefer cysteine for glutathione production. Optimizing glutathione levels in macrophages and astrocytes with cystine allows these cells to provide cysteine to lymphocytes and neurons directly upon demand.

[edit] Nutritional sources

Supplemental N-acetyl cysteine is a good source of cystine, but the dose of this supplement is limited by side effects. One of the richest nutritional sources of cystine in the diet is undenatured whey proteins from milk. The disulfide-bonded cysteine is not digested or significantly hydrolized by the stomach, but is transported by the blood stream to the tissues of the body. Here, within the cells of the body, the weak disulfide bond between the two cysteine molecules is cleaved, and the cell has 2 molecules of cysteine from which glutathione can be manufactured.^[2]

[edit] Side effects

Nutritional sources of cystine are virtually free of the toxic side effects associated with the single molecule of cysteine, N-acetyl cysteine. The greatest dietary source of cystine is bio-active, unpasteurized or low-heat pasteurized undenatured whey proteins

[edit] Physical characteristics

Cystine has a melting point of 247 °C to 249 °C

[edit] See also

• Cystinuria

[edit] References

1. ^ M.A. Aslaksena, O.H. Romarheima, T. Storebakkena and A. Skrede (28 June 2006). "Evaluation of content and digestibility of disulfidenext term bonds and free thiols in unextruded and extruded diets containing fish meal and soybean protein sources". Animal Feed Science and Technology 128 (3-4): 320-330.
2. ^ Glutathione Information for Physicians

[edit] External links

• Links to external chemical sources.
• Cystine precipitation in the urinary tract (Cystinuria)

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