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These two amino acids are closely related; each molecule of cystine consists of two molecules of cysteine joined together. Cysteine is very unstable and is easily converted to L-cystine; however, each form is capable of converting into the other as needed. Both are sulfur- containing amino acids that aid in the formation of skin and are important in detoxification.
L-cysteine is also known as L-2-amino-3-mercaptopropanoic acid, 2-amino-3-mercaptopropanoic acid, beta-mercaptoalanine, 2-amino-3 mercaptopropionic acid and alpha-amino-beta-thiolpropionic acid. Its IUPAC abbreviation is Cys, and its one-letter abbreviation, used when spelling out protein structures, is C.
L-cysteine is a white, solid substance that is soluble in water. It is hygroscopic and slowly decomposes and oxidizes. In solution, it undergoes oxidation to L-cystine, which is a dimer of L-cysteine. N-acetyl cysteine is a preferred delivery form of L-cysteine because of greater stability and possible higher absorbability.
Cystine is a crystalline, sulfur-containing amino acid, formed from two molecules of the amino acid cysteine. It can be converted to cysteine by reduction and was discovered in 1810 but was not recognized as a component of proteins until 1899, when it was isolated from animal horn - it is particularly abundant in skeletal and connective tissues, hair and digestive enzymes. The steps followed in the formation of cystine and cysteine, is from methionine to cystathionine and then to cysteine to cystine.
Cysteine is a sulfur containing non-essential amino acid and is closely related to cystine, as cystine consists of two cysteine molecules joined together. It is an unstable nutrient and easily converts to cystine, but this does not cause a problem, since both can convert into the other - as required by the body.
Cysteine is present in alpha-keratin, the chief protein constituent of the fingernails, toenails, skin, and hair. Cysteine aids in the production of collagen and promotes the proper elasticity and texture of the skin. It is also found in a variety of other proteins in the body, including several of the digestive system.
Cysteine helps to detoxify harmful toxins and protect the body from radiation damage. It is one of the best free radical destroyers, and works best when taken with selenium and vitamin E. Cysteine is also precursor to glutathione, a substance that detoxifies the liver by binding with potentially harmful substances there. It helps to protect the liver and brain from damage due to alcohol, drugs, and toxic compounds in cigarette smoke. Cystine may be effective in preventing hangovers.
Since cysteine is more soluble than cystine, it is used more readily in the body and is usually best for treating most illnesses. This amino acid is formed from L-methionine in the body. Vitamin B-6 is necessary for cysteine synthesis, which may not take place as it should in the presence of chronic disease. Cystine is required for proper vitamin B-6 utilization. Therefore, people with chronic illnesses may need higher than normal doses of cysteine, as much as 1,000 mg three times daily for a month at a time.
L-cysteine may have anti-inflammatory properties that can protect against various toxins, and that supplementation may be recommended for and be helpful in the treatment of osteoarthritis and rheumatoid arthritis. Supplementation with L-cysteine is recommended in the treatment of hardening of the arteries, and mutagenic disorders such as cancer. Cystine promotes healing after surgery, wounds, and severe burns, chelates heavy metals, and binds with soluble iron, aiding in iron absorption. This amino acid also promotes the burning of fat and the building of muscle.
Because of its ability to break down mucus in the respiratory tract, L-cysteine is often beneficial in the treatment of bronchitis, emphysema, and tuberculosis. It promotes healing from respiratory disorders and plays an important role in the activity of white blood cells, which fight disease. Cystine helps break down mucus deposits in cystic fibrosis.
Cystine or the N-acetyl form of cysteine (N-acetyl cysteine) may be used in place of L-cysteine. N-acetyl cysteine aids in preventing side effects from chemotherapy and radiation therapy. Because it increases glutathione levels in the lungs, kidneys, liver, and bone marrow, it has an anti-aging effect on the body - reducing the accumulation of age spots, for example. N-acetyl cysteine has been shown to be more effective at boosting glutathione levels than supplements of cystine or even of glutathione itself.
People who have diabetes should be cautious about taking supplemental cysteine because it is capable of in-activating insulin. Persons with cystinuria, a rare genetic condition that leads to the formation of cystine kidney stones, should not take cysteine. Cysteine also assists in the supply of insulin to the pancreas, which is needed for the assimilation of sugars and starches.
Cysteine has also been found that it may help in strengthening the protective lining of the stomach as well as intestines, which may help prevent damage caused by aspirin and similar drugs. L-cysteine serves as a precursor for synthesis of proteins, glutathione, taurine, coenzyme A and inorganic sulfate. Glutathionine itself has a number of biochemical functions, including maintenance of normal cellular redox state. Certain conditions, e.g. an acetaminophen overdose, can deplete hepatic glutathione, and this can be life-threatening. The antidote to an acetaminophen overdose is L-cysteine, in the delivery form of N-acetyl cysteine. The L-cysteine derived from N-acetyl cysteine helps to restore hepatic glutathione.
Cysteine is also critical to the metabolism of a number of essential biochemicals including coenzyme A, heparin, biotin, lipoic acid, and glutathione.
Sources
The body can synthesize cysteine from the amino acid methionine but is also found in high protein foods such as poultry, wheat, broccoli, eggs as well as garlic, onions and red peppers.
Molecular Description
Cysteine is only incorporated into proteins at the rate of 2.8 percent relative to the other amino acids, but the unique thiol side chain of this amino acid is often heavily involved in the three-dimensional stability of proteins and enzymes. The side chain is also often involved in the chemistry occurring at the active sites of many enzymes. Cysteine is also critical to the metabolism of a number of essential biochemicals including coenzyme A, heparin, biotin, lipoic acid, and glutathione.
Cystine is the product of an oxidation between the thiol side chains of two cysteine amino acids. As such, cystine is not considered one of the 20 amino acids. This oxidation product is found in abundance in a variety of proteins such as hair keratin, insulin, the digestive enzymes chromotrypsinogen A, papain, and trypsinogen where it is heavily involved in stabilizing the tertiary structure of these macromolecules.
Cystine Deficiency
No direct deficiencies have been reported as such.
In chronic diseases it appears that the formation of cysteine from methionine is prevented, resulting in a deficiency.
People suffering from AIDS/HIV may benefit from cysteine in proper amounts, as low levels are normally reported in people with this problem.
Toxicity & Symptoms of High Intake
People suffering from diabetes should be careful when taking supplementation, as it could inactivate insulin.
There are no reports of over dosage in those taking L-cysteine supplements. However, large doses of L-cysteine are neuroexcitotoxic in several species. Single injections of L-cysteine (0.6-1.5 g/kg) into 4-day-old pups resulted in massive damage to cortical neurons, permanent retinal dystrophy, atrophy of the brain and hyperactivity.
Cystine / Cysteine Dosage
General dosage is not known but as supplement cysteine is used at 200 mg two to three times per day. The usual supplemental dosage of L-cysteine is 500 milligrams to 1.5 grams daily. Those who supplement with L-cysteine should drink at least six to eight glasses of water daily in order to prevent cystine renal stones. Some studies indicate that an intake of 3 to 5 grams daily of vitamin C may prevent cystine stones. However, high-dose vitamin C itself may contribute to renal stones in some.
Cysteine is best taken with selenium, vitamin B-6, and vitamin E.
Zinc: L-cysteine complexes with zinc and may increase the absorption of zinc.
Vitamin C: Ascorbic acid may inhibit the oxidation of L-cysteine to L-cystine.
Supplement Forms
Capsules - 500 mg, 750 mg
Injection - 50 mg/mL
Powder
Tablets - 500 mg
Cautions
People who have diabetes should be cautious about taking supplemental cysteine because it is capable of in-activating insulin.
Persons with cystinuria, a rare genetic condition that leads to the formation of cystine kidney stones, should not take cysteine. Those who form renal stones should avoid L-cysteine supplements.
L-cysteine, like other sulfhydryl-containing substances, could produce a false-positive result in the nitroprusside test for ketone bodies used in diabetes.
L-cysteine supplementation is contraindicated in those hypersensitive to any component of the preparation.
With typical doses of 1 to 1.5 grams daily, the most commonly reported side effects have been gastrointestinal, such as nausea. There are rare reports of cystine renal stone formation.
Because of lack of long-term safety studies, L-cysteine supplementation should be avoided by children, pregnant women and nursing mothers.
MoonDragon's Health Information: Nutrition Basics - Amino Acids Index
CYSTEINE / N-ACETYL CYSTEINE SUPPLEMENT PRODUCTS
L-Cysteine Structural Support, w/ Vitamin B-6 & C, vegetarian Formula, NOW Foods, 500 mg, 100 Tabs
NOW L Cysteine functions as an antioxidant and is a powerful aid to the body in protecting against radiation and pollution. Hair and skin are made up 10-14% Cystine.
NAC (N-Acetyl Cysteine) w/ Selenium, Molybdenum, NOW Foods, 600 mg, 100 Caps
In order for us to utilize our protein intake in an efficient manner, an ample supply of the amino acid glutathione must be present in our cells - NAC (N-Acetyl Cysteine) is essential in the formation of glutathione, as it directly impacts our ability to synthesize the proteins we ingest.
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