CYSTINE

'Cystine' is the amino acid described by the formula (SCH₂CH(NH₂)CO₂H)₂. This colorless solid melts at 247 -249 °C. It forms upon oxidation of a pair of cysteine molecules. It was discovered in 1810 by William Hyde Wollaston but was not recognized as a component of proteins until it was isolated from the horn of a cow in 1899.^[1] As a component in most proteins, cystine significant determines the tertiary structure of most proteins. Cystine is partially responsible for the formation of a gluten matrix in bread, along with hydrogen bonding and hydrophobic interactions. The acid hydrolysis of two kilograms of human hair affords about 100 grams of cystine.^[2]

Contents

Properties

Dietary importance

Nutritional sources

In animal feed

Side effects

See also

References

External links

Properties

The disulfide link is readily reduced to give the corresponding thiol, cysteine. This reaction is typically effected with thiols such as mercaptoethanol or dithiothreitol.
:(SCH₂CH(NH₂)CO₂H)₂ + 2 RSH → 2 HSCH₂CH(NH₂)CO₂H + RSSR

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.
Cystine has been claimed to represent an ideal precursor for delivery of cysteine to the cell. 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.

Nutritional sources

Supplemental N-acetyl cysteine is claimed to be a 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 is cleaved to give cysteine, from which glutathione can be synthesized.

In animal feed

Cystine can also be broken by high temperatures (above about 150 °C, especially at low moisture levels (below about 20%)^[3].

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.

See also

★ Cystinuria

★ Cysteine

References

1. "cystine." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 27 July 2007 www.britannica.com/eb/article-9028437/cystine
2. Gortner, R. A.; W. F. Hoffman, W. F. “l-Cystine” Organic Syntheses, Collected Volume 1, p.194 (1941). http://www.orgsyn.org/orgsyn/pdfs/CV1P0194.pdf
3. Evaluation of content and digestibility of disulfidenext term bonds and free thiols in unextruded and extruded diets containing fish meal and soybean protein sources, M.A. Aslaksena, O.H. Romarheima, T. Storebakkena and A. Skrede, , , Animal Feed Science and Technology,

External links