L Cysteine
Cysteine, a nonessential amino acid, is one of the 20 building blocks of protein. Cysteine is a part of organic molecules containing an amino group, which can combine in linear arrays to form proteins in living organisms. Cysteine can be synthesized by the body and is not essential to the diet. Its key chemical feature is a thiol group that contains sulfur. Cysteine is a naturally occurring hydrophobic amino acid which has a sulfhydryl group and is found in most proteins, however only in small quantities. When it is exposed to air it oxidizes to form cystine, which is two cysteine molecules joined by a disulfide bond. One molecule of water (H2O) is the byproduct from the creation of each molecule of cystine. It can be taken as a supplement in the form of N-acetylcysteine (NAC). N-acetyl-L-cysteine (NAC) helps break down mucus and detoxify harmful substances in the body. Both cysteine and NAC have been shown to increase levels of the antioxidant glutathione. This thiol group can combine with the thiol group of another cysteine to form a disulfide bridge, which helps structural proteins and enzymes maintain their configuration. Two cysteine molecules linked by a disulfide bridge make up the amino acid cystine.

What is L- Cysteine ?
L-Cysteine is a naturally occurring amino acid that is classified as a protein amino acid. One of the main functions of l-cysteine is the promotion of stomach lining health and also the correction of situations where the absorption of essential nutrients from food sources takes place. Many people are able to obtain as much of this protein source as they need without taking any type of supplement. L-cysteine can be found in a number of foods ranging from meats to dairy and vegetable sources.

While most people do not require any additional l-cysteine in the body, there are exceptions. Infants may sometimes need l-cysteine in order to absorb nutrients from food properly. Elderly persons may need the amino acid for the same reason. When any type of ongoing disease that impacts the ability to metabolize nutrients properly, there may be a need to use a supplement in order to provide a higher quality of life.

Where it is found
Cysteine is found in most high-protein foods, including:
arw Animal sources: pork, sausage meat, chicken, turkey, duck, luncheon meat, eggs, milk, whey protein, ricotta, cottage cheese, yogurt

arw Plant sources: red peppers, garlic, onions, broccoli, brussels sprouts, oats, granola, wheat germ, lentil.

Best Form for Human Consumption
With the exception of taurine, GABA and glycine, most amino acids exist in either the D or L form. These forms are the mirror reverse images of each other. The L form represents the natural type found in living plants and animal tissues. The L form is used in human protein structures and is more compatible to human biochemistry than the D form. Only phenylalanine can be present in human protein structures in both the D and L forms.
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Benefits / Uses
Cysteine is one of the key components in all living things. Proteins are synthesized by formation of peptide bonds during ribosomal translation of messenger RNA. Cysteine plays a key role in stabilizing extracellular proteins. Cysteine strengthens the protective lining of the stomach and intestines, which may help prevent damage caused by aspirin and similar drugs. In addition, cysteine may play an important role in the communication between immune system cells. Cysteine can react with itself to form an oxidized dimer by formation of a disulfide bond. The environment within a cell is too strongly reducing for disulfides to form, but in the extracellular environment, disulfides can form and play a key role in stabilizing many such proteins, such as the digestive enzymes of the small intestine. Cysteine is occasionally converted into glucose and used as a source of energy. Cysteine is one of the few amino acids that contains sulfur. This allows cysteine to bond in a special way and maintain the structure of proteins in the body. Cysteine is a component of the antioxidant glutathione. The body also uses cysteine to produce taurine, another amino acid. Cysteine is believed to play a role in the normal growth rate of hair. Cysteine may possibly help reduce the effects of aging on the skin, assist in healing after surgery or burns, and help protect the skin from radiation injury.

Most people do not need to supplement cysteine. There is no specific pediatric recommendation for cysteine. Recommended adult doses of NAC vary depending on the health condition being treated. NAC is administered either intravenously or orally in the hospital to treat acetaminophen (paracetamol) poisoning in both children and adults. Typical dosage recommendations are in the range of 250-1500mg of NAC daily for the majority of therapeutic benefits. Those who supplement with l-cysteine should drink at least six to eight glasses of water daily in order to prevent cystine renal stones.

Possible Side effects / Precautions / Possible Interactions:
Cysteine is considered to be of low toxicity, even in high doses. High oral doses of N-Acetyl cysteine have been associated with nausea, vomiting and other gastrointestinal disturbances. NAC may enhance the blood pressure-lowering effects of ACE inhibitors, medications commonly used to treat high blood pressure. Treatment with NAC may enhance the effectiveness of immunosuppressive medications such as azathioprine, cyclophosphamide, prednisolone, or prednisone. N-acetyl cysteine may increase the effectiveness of corticosteroids, a class of drugs with anti-inflammatory activity. Topical applications of NAC may increase the effectiveness of oxiconazole, an antifungal medication used for athlete's foot. Some forms of cysteine are toxic and should be avoided. These include D-cysteine, D-cystine, and 5-methyl cysteine.

Research studies / References

arw Weast, Robert C., ed. (1981), CRC Handbook of Chemistry and Physics (62nd ed.), Boca Raton, FL: CRC Press, p. C-259, ISBN 0-8493-0462-8.

arw "Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB Recommendations 1983)", Pure Appl. Chem. 56 (5): 595-624, 1984, doi:10.1351/pac198456050595

arw Martens, Jürgen; Offermanns, Heribert; Scherberich, Paul (1981), "Facile Synthesis of Racemic Cysteine", Angew. Chem. Int. Ed. Engl. 20 (8): 668, doi:10.1002/anie.198106681

arw Karlheinz Drauz, Ian Grayson, Axel Kleemann, Hans-Peter Krimmer, Wolfgang Leuchtenberger, Christoph Weckbecker "Amino Acids"in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim. doi:10.1002/14356007.a02_057.pub2

arw Hell, Rüdiger (1997), "Molecular physiology of plant sulfur metabolism", Planta 202 (2): 138-48, doi:10.1007/s004250050112, PMID 9202491.
6. Bulaj, Grzegorz; Kortemme, Tanja; Goldenberg, David P. (1998), "Ionization-reactivity relationships for cysteine thiols in polypeptides.", Biochemistry 37 (25): 8965-72, doi:10.1021/bi973101r, PMID 9636038.

arw Sevier, Carolyn S.; Kaiser, Chris A. (2002), "Formation and transfer of disulphide bonds in living cells", Nature Rev. Mol. Cell. Biol. 3 (11): 836-47, doi:10.1038/nrm954, PMID 12415301.

arw Lill, Roland; Mühlenhoff, Ulrich (2006), "Iron-Sulfur Protein Biogenesis in Eukaryotes: Components and Mechanisms", Ann. Rev. Cell Dev. Biol. 22: 457-86, doi:10.1146/annurev.cellbio.22.010305.104538, PMID 16824008.

arw Lippard, Stephen J.; Berg, Jeremy M. (1994), Principles of Bioinorganic Chemistry, Mill Valley, CA: University Science Books, ISBN 0-935702-73-3.