Cystine

Chemical structure of cystine formed from L-cysteine (under biological conditions)

3D representation of cystine with the disulfide bond shown in yellow

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.

Stability:

The disulfide link is readily reduced to the corresponding thiol (-SH), and this weak bond can be broken by heat and mechanical agitation. Milk is an important source of cysteine. The milk pasteurization process can involve temperatures up to 160 degrees farenheit, and this is more than sufficient to break the bond. Additionally the the high speed mechanical agitation involved in the milk homogenation process is capable of breaking this weak bond.

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. (1)

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. (1)

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. (1)

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

Physical Characteristics:

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

References:

(1) Glutathione Information for Physicians