Cysteine and cystine are two related forms of a single amino acid. The bio-molecule called cystine is formed as a sculpture bridged dimer from the joining together of two molecules of cysteine. The singular form of the sulphur containing amino acid cysteine prefers conversion to the dimer L-cystine form if another molecule of cysteine is present in the solution - the cystine dimer tends to have greater stability and sulphur link bridges are readily formed in solution. The stable cystine dimer can also be destabilized by certain chemical reactions and separation of the stable cystine structure into two cysteine molecules results. Cysteine is the most important sulfur containing amino acid; it is a major constituent of the proteins that make up hair as well as the skin. Cysteine and the dimer cystine are also involved in major detoxification processes in the body.

The protein called alpha-keratin found in fingernails, toenails, skin, and hair contains large amounts of the amino acid cysteine and the cystine dimer. Elasticity and texture in the skin is promoted by the presence of cysteine, the amino acid is also very important in the bio-chemical production of the protein collagen in the body. The amino acid cysteine and its dimer are also observed in many other proteins performing different functions in the body; several digestive enzymes also contain cysteine as a major constituent amino acid.

One major function performed by the amino acid cysteine in the body is in the detoxification of dangerous toxins from the body, as well as protecting the body from all sorts of radiation induced cellular level damage. The amino acid cysteine is very effective in scavenging dangerous molecules called free radical as and when they are formed during cellular oxidation in the body. In the anti-oxidant and free radical destroying role, the amino acid performs best combined with the mineral selenium and the essential vitamin E. The important bio-molecule glutathione is also another compound that contains cysteine. This important chemical substance aids in the process of detoxification occurring in the liver, it carries this out by quickly binding with all the potentially toxic substances produced during metabolism in the liver. The presence of glutathione in the liver helps it recover from damage induced by the presence of alcohol, it protects the liver from the toxic by-products of many kinds of drugs, and different toxic compounds such as those from cigarette smoke and automobile exhausts.

The solubility of cysteine in water is greater than the solubility of the cystine dimer - this is due to the greater tensile strength of the bonds in the dimer. Cysteine and its dimer can be used in supplemental form during the treatment process of many disorders affecting the body. Cysteine is normally formed form L-methionine in the body itself and is classed as a non-essential amino acid - not required in diet. Cysteine synthesis in the body requires the presence of vitamin B6, if there is some form of chronic disease in the body, the synthesis may be interrupted. Higher than normal doses of the amino acid cysteine is therefore necessary for many people suffering from chronic illnesses may need higher than normal doses of cysteine. Doses of 1,000 milligrams taken thrice a day for dosage periods lasting a month is typical for a single treatment stage, this can be repeated.

Disorders such as rheumatoid arthritis, the hardening of the arteries, and all mutagenic disorders like cancer are treated using a variety of biochemical means including supplementation with cysteine. Post surgical healing is promoted by cysteine, the rate of recovery from severe burns is also boosted by supplements of cysteine and other amino acids. Cysteine helps chelates heavy metals in the body if they are present, and it can also bind with soluble iron, thus aiding in the rapid absorption of iron. Cysteine also aids in adding bulk to muscles and promotes the burning of fat in the body - it is thus a helpful aid in weight loss programs. The presence of excess mucus in the respiratory tract is also rapidly cleared by cysteine and this ability to break down mucus is of great help during the treatment of severe respiratory disorders. The use of L-cysteine as an amino acid supplement in the treatment of disorders such as bronchitis and emphysema, as well as tuberculosis may also prove to be beneficial to the patient. Cysteine in general, hastens the healing rate of the body from respiratory disorders and enables the body to play an important role increasing the activity of white blood cells - these cells are the main forefront in the body’s ability to fight off diseases and disorders of all kinds.

Supplemental doses of L-cysteine can be substituted by supplementing with the dimer cystine or even with the N-acetyl form of cysteine called N-acetylcysteine. The form of cysteine called the N-acetylcysteine form is helpful in prevention of adverse side effects accruing from chemotherapy and the side effects of long term radiation therapy in cancer patients. The presence of N-acetylcysteine in the body tends to increase the levels of the compound glutathione found in the lungs and other vital organs like the kidneys, the liver, and the tissues of the bone marrow. The molecule is also credited with a potent anti-aging effect, it is believed to aid in reducing the total accumulation of age spots in the body. The supplements of N-acetylcysteine for example, have been shown to be much more effective at increasing the levels of glutathione than other known supplements of cystine or even supplements of glutathione.

The use of supplemental cysteine by individuals suffering from diabetes is not encouraged and such patients must be cautious about using supplemental cysteine in any form, as the amino acid is capable of inactivating insulin if present in large amounts in the body of a person. Cysteine must also be avoided by all individuals suffering from what is termed cystinuria, which is a very rare genetic condition, that can induce the formation of cystine kidney stones in the kidneys.

Supplements of cysteine could potentially help bring an improvement in disorders associated with an allergy or in case of sinusitis and related conditions. The good growth and condition of fingernails, toenails and hair is due to cysteine. The supplements of cysteine can also help alleviate disorders like psoriasis.

Cysteine can easily be synthesized from the amino acid methionine and other basic chemical building blocks in the body. Most high protein food sources contain abundant cysteine; it is also the amino acid from which NAC is derived.

Studies conducted on individuals infected with the HIV suggests that blood levels of cysteine and glutathione appears to be low in such people - there is thus a possible immune system role for this bio-molecule. The proper functioning of the immune system is connected to the bio-availability of cysteine; the immune system may thus be weakened if there is a deficiency in this amino acid. The low levels of cysteine in HIV positive people may be caused either by the low levels of cysteine contributing to immune system disorder or it could be a direct result of the severe immune system suppression which is the main hallmark of the HIV induced condition.