Cystine is an organic compound described by the formula (SCH2CH(NH2)CO2H)2. This colourless amino acid 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.() Cystine is found in most proteins, and makes a significant contribution to 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.()
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
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.
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%).()
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.