L-Cysteine is a non-essential (or semi-essential), neutral, genetically
coded amino acid.
Isoelectric point (pH)
1.71, 8.33 (thiol), 10.78
CAS Registry Number
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Deutsche Version · Overview of amino acids · Homepage
Burkhard Kirste, 1994-08-23, 1998-01-23
What can high-cysteine foods do for you?
Help your body detoxify chemicals and heavy metals
Protect cells from free radical damage
Help breakdown extra mucous in your lungs
What events can indicate a need for more high-cysteine foods?
Food sources of cysteine include poultry, yogurt, egg yolks, red
peppers, garlic,onions, broccoli, Brussel sprouts, oats, and wheat germ.
For serving size for specific foods, see Nutrient Rating Chart below at the
bottom of this page.
Description Drug-nutrient interaction
Function Nutrient interaction
Deficiency Symptoms Health conditions
Toxicity Symptoms Supplements
Cooking, storage and processing Food Sources
Factors that affect function Public Recommendations
What is cysteine?
Cysteine is a sulfur-containing amino acid that occurs naturally in foods and
can also be manufactured by the body from the amino acid methionine. In
the production of cysteine, methionine is converted to S-adenosyl
methionine (SAM), which is then converted to homocysteine. Homocysteine
then reacts with serine to form cysteine. Cysteine is also found in the body
and in food as cystine, an amino acid that contains two cysteines joined
How it Functions
What is the function of cysteine?
Promoting Antioxidant Activity
As a key constituent of glutathione, cysteine has many important
physiological functions. Glutathione, formed from cysteine, glutamic acid,
and glycine, is found in all human tissues, with the highest concentrations
found in the liver and eyes. Glutathione is a potent antioxidant, protecting
fatty tissues from the damaging effects of free radicals. The antioxidant
activity of glutathione is attributed specifically to the presence of cysteine in
Glutathione also plays a vital role in the detoxification of harmful substances
by the liver and can chelate (attach to) heavy metals such as lead, mercury,
and cadmium. It is also believed that glutathione carries nutrients to
lymphocytes and phagocytes, important immune system cells.
Help Eliminate Mucous
Cysteine also has the ability to breakdown proteins found in mucous that
settles in the lungs. As a result, this amino acid may be useful in the
treatment of bronchitis and other respiratory problems.
What are deficiency symptoms for cysteine?
Cysteine deficiency is relatively uncommon, but may be seen in vegetarians
with low intake of the plant foods containing methionine and cysteine. There
is no known medical condition directly caused by cysteine deficiency, but low
cysteine levels may reduce one's ability to prevent free radical damage and
may result in impaired function of the immune system.
What are toxicity symptoms for cysteine?
Consumption of foods containing cysteine, or its precursor methionine, is not
likely to cause toxicity symptoms. However, cysteine is a brain excitoxin that
can cause damage to brain cells in susceptible individuals. Such individuals
do not metabolize the amino acid correctly, and, as a result, may be at risk
for certain neurodegenerative diseases, including multiple sclerosis,
amylotrophic lateral sclerosis (Lou Gehrig's disease) and Alzheimer's
High doses of oral N-acetyl-cysteine, such as those given to patients with
acetaminophen (Tylenol) toxicity, can cause nausea, vomiting, and diarrhea.
Intravenous administration of N-acetyl cysteine can cause allergic reactions
in a small percentage of people, characterized by skin flushing, a drop in
blood pressure, irregular heart beat, and respiratory distress. Accidental
overdose of intravenous N-acetyl-cysteine has proven to be fatal.
Impact of Cooking, Storage and Processing
How do cooking, storage, or processing affect cysteine?
There is no research available about the impact of cooking, storage or
processing on cysteine.
Factors that Affect Function
What factors might contribute to a deficiency of cysteine?
The production of cysteine involves several nutrients. As a result, dietary
deficiency of methionine, vitamin B6, vitamin B12, s-adenosyl methionine
(SAMe) and folic acid may decrease the production of cysteine.
What medications affect cysteine?
Supplemental cysteine, given intravenously as N-acetyl cysteine, may
prevent the development of tolerance to nitroglycerin, a drug used in the
treatment of chest pain. Unfortunately, the combination of nitroglycerin and
N-acetyl-cysteine can cause severe headaches.
Oral and intravenous N-acetyl cysteine is used in the treatment of
acetaminophen (Tylenol) poisoning. N-acetyl cysteine helps to metabolize
acetaminophen quickly, so that liver damage is prevented.
Supplemental cysteine, given as N-acetyl cysteine, may reduce the nausea
and vomiting caused by chemotherapy.
N-acetyl cysteine may increase the effectiveness of corticosteroids, a class of
drugs with anti-inflammatory activity.
Researchers are investigating the role of N-acetyl cysteine in preventing
heart damage caused by certain chemotherapy drugs and in enhancing the
effectiveness of interferon in treating hepatitis C.
How do other nutrients interact with cysteine?
There is no research available about how other nutrients interact with
What health conditions require special emphasis on cysteine?
Cysteine may play a role in the prevention and/or treatment of the
following medical conditions:
Acute respiratory distress syndrome
Heavy metal toxicity or exposure
Form in Dietary Supplements
What forms of cysteine are found in dietary supplements?
Many people take supplemental cysteine as a way to increase the level of
glutathione in the body. As a dietary supplement, cysteine is available as L-
cysteine hydrochloride, and more commonly as n-acetyl-cysteine (NAC).
NAC is a more water-soluble form of cysteine, and as such is believed to be
more bioavailable than oral L-cysteine hydrochloride.
What foods provide cysteine?
Cysteine is found in a variety of foods including poultry, yogurt, egg yolks,
red peppers, garlic, onions, broccoli, Brussel sprouts, oats, and wheat germ.
Food Source Analysis not Available for this Nutrient
Public Health Recommendations
What are current public health recommendations for cysteine?
In its most recent 2005 public health recommendations for amino acids
(published as the Dietary Reference Intakes for Energy, Carbohydrate, Fiber,
Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients),
National Academies Press, 2005), the National Academy of Sciences (NAS)
established a general principle for cysteine intake. The NAS lumped cysteine
together with another sulfur-containing amino acid, methionine, and
recommended that all individuals 1 year of age or greater consume 25
milligrams of cysteine plus methione (combined) for every 1 gram of food
protein. Here is how that recommendation would look for each age and
gender group, assuming RDA-level protein intake and 50% of sulfur-
containing amino acid needs supplied by cysteine:
Children 1-3 years: 163 mg of cysteine
Children 4-8 years: 238 mg of cysteine
Males 9-13 years: 425 mg of cysteine
Males 14-18 years: 650 mg of cysteine
Males 19 years and older: 700 mg of cysteine
Females 9-13 years: 425 mg of cysteine
Females 14 years and older: 575 mg of cysteine
Pregnant or lactating females: 888 mg of cysteine
Breuille D, Obled C. Cysteine and glutathione in catabolic states. Nestle Nutr
Workshop Ser Clin Perform Programme 2000;3:173-91; discussion 191-7 2000.
Droge W. Cysteine and glutathione in catabolic conditions and immunological
dysfunction. Curr Opin Clin Nutr Metab Care 1999 May;2(3):227-33 1999.
Droge W, Hack V, Breitkreutz R, et al. Role of cysteine and glutathione in signal
transduction, immunopathology and cachexia. Biofactors 1998;8(1-2):97-102
Groff JL, Gropper SS, Hunt SM. Advanced Nutrition and Human Metabolism.
West Publishing Company, New York, 1995 1995.
Lininger SW, et al. A-Z guide to drug-herb-vitamin interactions. Prima Health,
Rocklin, CA, 2000 2000.
Mahan K, Escott-Stump S. Krause's Food, Nutrition, and Diet Therapy. WB
Saunders Company; Philadelphia, 1996 1996.
Puerto M, Guayerbas N, Victor V. Effects of N-acetylcysteine on macrophage
and lymphocyte functions in a mouse model of premature ageing. Pharmacol
Biochem Behav 2002 Nov;73(4):797-804 2002.
Quig D. Cysteine metabolism and metal toxicity. Altern Med Rev 1998
Aug;3(4):262-70 1998. PMID:15290.
From Wikipedia, the free encyclopedia
Not to be confused with cystine, its oxidized dimer.
IUPAC name [show]
CAS number [52-90-4]
ChemSpider ID 574
Molecular formula C3H7NO2S
Molar mass 121.16 g mol−1
Supplementary data page
n, εr, etc.
Thermodynamic Phase behaviour
data Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Cysteine (abbreviated as Cys or C) is an α-amino
acid with the chemical
formula HO2CCH(NH2)CH2SH. It is a non-essential
amino acid, which means that humans can synthesize
it. Its codons are UGU and UGC. With a thiol side
chain, cysteine is classified as a hydrophobic amino
acid. Because of the high reactivity of this thiol,
cysteine is an important structural and functional
component of many proteins and enzymes. Cysteine
is named after cystine, its oxidized dimer.
o 1.1 Dietary sources
o 2.1 Industrial sources
o 2.2 Biosynthesis
3 Biological functions
o 3.1 Precursor to the antioxidant glutathione
o 3.2 Oxidation to cystine linkages
o 3.3 Precursor to iron-sulfur clusters
o 3.4 Metal ion binding
o 3.5 Post-translational modifications
6 Reducing Toxic Effects of Alcohol
o 6.1 N-acetylcysteine (NAC)
7 See also
9 External links
Although classified as a non-essential amino acid, in
rare cases, cysteine may be essential for infants, the
elderly, and individuals with certain metabolic
disease or who suffer from malabsorption syndromes.
Cysteine can usually be synthesized by the human
body under normal physiological conditions if a
sufficient quantity of methionine is available.
Cysteine is potentially toxic and
is catabolized in the gastrointestinal tract and blood
plasma. In contrast, cystine travels safely through the
GI tract and blood plasma, and is promptly reduced to
the two cysteine molecules upon cell entry.
Cysteine is found in most high-protein foods,
Animal sources: pork, sausage meat, chicken,
turkey, duck, luncheon meat, eggs, milk, whey
protein, ricotta, cottage cheese, yogurt
Vegan sources: red peppers, garlic,
onions, broccoli, brussels
sprouts, oats, granola, wheat germ
As other amino acids, cysteine has an amphoteric
(R)-Cysteine (left) and (S)-Cysteine (right) in zwitterionic form at neutral pH
See also Food safety in China#Soy sauce made from
At the present time, the cheapest source of
material from which food-grade L-cysteine may be
purified in high yield is by hydrolysis of human
hair. Other sources include feathers and pig
bristles.[citations needed]The companies producing
cysteine by hydrolysis are located mainly
in China. There is some debate as to whether or
not consuming L-cysteine derived from human
hair constitutes cannibalism. Although
many other amino acids were accessible
via fermentation for some years, L-cysteine was
unavailable until 2001
when German company Wacker
Chemie introduced a production route via
fermentation (non-human, non-animal origin). An
industrial production of DL-cysteine was realized
at Degussa AG in the 1980th.
Cysteine synthesis.Cystathionine beta synthasecatalyzes the upper reaction and cystathionine
gamma-lyase catalyzes the lower reaction.
In animals, biosynthesis begins with the amino
acid serine. The sulfur is derived from methionine,
which is converted to homocysteine through the
intermediate S-adenosylmethionine. Cystathionine
beta-synthase then combines homocysteine and
serine to form the asymmetrical
thioether cystathionine. The enzyme cystathionine
gamma-lyase converts the cystathionine into
cysteine and alpha-ketobutyrate. In bacteria,
cysteine biosynthesis again starts from serine,
which is converted to O-acetylserine by the
enzyme serine transacetylase. The enzyme O-
acetylserine (thiol)-lyase, using sulfide sources,
converts this ester into cysteine, releasing
The cysteine thiol group is nucleophilic and easily
oxidized. The reactivity is enhanced when the thiol
ionized, and cysteine residues in proteins
have pKa values close to neutrality, so are often in
their reactive thiolate form in the cell. Because
of its high reactivity, the thiol group of cysteine
has numerous biological functions.
Precursor to the antioxidant glutathione
Due to the ability of thiols to undergo redox
reactions, cysteine has antioxidant properties.
Cysteine's antioxidant properties are typically
expressed in the tripeptideglutathione, which
occurs in humans as well as other organisms. The
systemic availability of oral glutathione (GSH) is
negligible; so it must be biosynthesized from its
constituent amino acids, cysteine, glycine,
and glutamic acid. Glutamic acid and glycine are
readily available in most Western diets, but the
availability of cysteine can be the
limiting substrate.
Oxidation to cystine linkages
Oxidation of cysteine produces
the disulfide cystine. More aggressive oxidants
convert cysteine to the corresponding sulfinic
acid and sulfonic acid. Cysteine residues play a
valuable role by crosslinking proteins, which
increases the protein stability in the harsh
extracellular environment, and also functions to
confer proteolytic resistance (since protein export
is a costly process, minimizing its necessity is
advantageous). Inside the cell, disulfide bridges
between cysteine residues within a polypeptide
support the protein's secondary structure. Insulin is
an example of a protein with cystine crosslinking,
wherein two separate peptide chains are connected
by a pair of disulfide bonds.
Protein disulfide isomerases catalyze the proper
formation of disulfide bonds; the cell transfers
dehydroascorbic acid to the endoplasmic
reticulum, which oxidises the environment. In this
environment, cysteines are, in general, oxidized to
cystine and no longer functional as a nucleophiles.
Precursor to iron-sulfur clusters
Cysteine is an important source of sulfide in
human metabolism. The sulfide in iron-sulfur
clusters and in nitrogenase is extracted from
cysteine, which is converted to alanine in the
Metal ion binding
Beyond the iron-sulfur proteins, many other metal
cofactors in enzymes are bound to the thiolate
substituent of cysteinyl residues. Examples include
zinc in zinc fingers and alcohol dehydrogenase,
copper in the blue copper proteins, iron
in cytochrome P450, and nickel in the [NiFe]-
hydrogenases. The thiol group also has a
high affinity for heavy metals, so that proteins
containing cysteine willbind metals such as
mercury, lead, and cadmium tightly.
Aside from its oxidation to cystine, cysteine
participates in numerous Posttranslational
modifications. The nucleophilic thiol group allows
cysteine to conjugate to other groups, e.g.,
in prenylation.Ubiquitin ligases transfer ubiquitin
to its pendant, proteins, and caspases, which
engage in proteolysis in the apoptotic
cycle. Inteins often function with the help of a
catalytic cysteine. These roles are typically limited
to the intracellular milieu, where the environment
is reducing, and cysteine is not oxidized to cystine.
Cysteine, mainly the L-enantiomer, is a precursor
in the food, pharmaceutical, and personal care
industries. One of the largest applications is the
production of flavors. For example, the reaction of
cysteine with sugars in a Maillard reaction yields
meat flavors. L-cysteine is also used as
a processing aid for baking. Small quantities (in
the tens of ppm range) help to soften the dough
and thus reduce processing
In the field of personal care, cysteine is used
for permanent wave applications predominantly in
Asia. Again the cysteine is used for breaking up
the disulfide bonds in the hair's keratin.
Cysteine is a very popular target for site-directed
labeling experiments to investigate biomolecular
structure and dynamics. Maleimides will
selectively attach to cysteine using a
covalent Michael addition. Site-directed spin
labeling for EPR or paramagnetic relaxation
enhanced NMR also uses cysteine extensively.
In a 1994 report released by five
top cigarette companies, cysteine is one of the 599
additives to cigarettes. Like most cigarette
additives, however, its use or purpose is
unknown. Its inclusion in cigarettes could offer
two benefits: Acting as an expectorant, since
smoking increases mucus production in the lungs;
and increasing the beneficial
antioxidant glutathione (which is diminished in
Cystine, showing disulfide bond
Cysteine is required by sheep in order to produce
wool: it is an essential amino acid which must be
taken in as food from grass. As a consequence,
during drought conditions, sheep stop producing
wool; however, transgenic sheep which can make
their own cysteine have been developed.[citation
Reducing Toxic Effects of Alcohol
Cysteine has been proposed as a preventative or
antidote for some of the negative effects of
alcohol, including liver damage and hangover. It
counteracts the poisonous effects
of acetaldehyde , which is the major by-product
of alcohol metabolism and is responsible for most
of the negative aftereffects and long-term damage
associated with alcohol use (but not the immediate
effects of drunkenness). Cysteine supports the next
step in metabolism, which turns acetaldehyde into
the relatively harmlessacetic acid. In a rat study,
test animals received a LD50 dose of acetaldehyde
(the amount which normally kills half of all
animals). Those that received cysteine had an 80%
survival rate; when thiamine was added, all
animals survived. There is not yet direct
evidence for or against its effectiveness in humans
who consume alcohol at normal levels.
N-acetyl-L-cysteine (NAC) is a derivative of
cysteine wherein an acetyl group is attached to the
nitrogen atom. This compound is sometimes
considered as a dietary supplement, although it is
not an ideal source since it is catabolized in the
gut. NAC is often used as a cough
medicine because it breaks up the disulfide bonds
in the mucus and thus liquefies it, making it easier
to cough up. It is also this action of breaking
disulfide bonds that makes it useful in thinning the
abnormally thick mucus in Cystic
Fibrosis patients. NAC is also used as a
specific antidote in cases
of acetaminophen overdose.
1. ^ IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. "Nomenclature
and Symbolism for Amino Acids and Peptides". Recommendations on Organic &
Biochemical Nomenclature, Symbols & Terminology etc. Retrieved on 2007-05-17.
2. ^ J. Martens, H. Offermanns und P. Scherberich : A simple synthesis of racemic
cysteine, Angewandte Chemie International Edition in English 20 (1981) 668.
3. ^ Hell, R. 1997. "Molecular physiology of plant sulfur metabolism" Planta 202:138-
148. PMID: 9202491
4. ^ Bulaj G, Kortemme T, Goldenberg D (1998). "Ionization-reactivity relationships for
cysteine thiols in polypeptides.". Biochemistry 37 (25): 8965–
72. doi:10.1021/bi973101r. PMID 9636038.
5. ^ Roland Lill, Ulrich Mühlenhoff “Iron-Sulfur Protein Biogenesis in Eukaryotes:
Components and Mechanisms” Annual Review of Cell and Developmental Biology,
2006, Volume 22, pp. 457-486. doi:10.1146/annurev.cellbio.22.010305.104538.
6. ^ S. J. Lippard, J. M. Berg “Principles of Bioinorganic Chemistry” University Science
Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3.
7. ^ Baker D, Czarnecki-Maulden G (1987). "Pharmacologic role of cysteine in
ameliorating or exacerbating mineral toxicities.". J Nutr 117 (6): 1003–
10. doi:10.1126/science.2237411.<br> (inactive 2008-06-25). PMID 3298579.
8. ^ Hui , Nip W., Rogers, R. (2001). , p. 74.Hui et al., Y. (2001), Meat science and
applications, CRC Press, pp. 74, ISBN 0824705483
9. ^ http://quitsmoking.about.com/cs/nicotineinhaler/a/cigingredients.htm
10. ^ http://www.lef.org/protocols/prtcl-004.shtml
11. ^ Effects of cysteine on acetaldehyde
Ask The Doctor
Beauty and Skin Care
Essential Fatty Acids
Fruit and Vegetable
Joint and Bones
Vitamins & Minerals
L-Cysteine and Cysteine Information
L-cysteine is a protein amino acid that exists naturally as a protein in most living organisms. Although
most cysteine is found in proteins, small amounts of cysteine are also located in body fluids and in plants
in non-protein form. The average diet of most individuals contributes approximately 1 gram of L-cysteine
L-cysteine is considered a nonessential amino acid, meaning that sufficient amounts are produced by the
body itself. Cysteine is one of the few amino acids containing sulfur. This allows cysteine to bond in a
special way and to maintain the structure of proteins in the body.
L-cysteine aids in the synthesis of proteins, glutathione, taurine, coenzyme A, and inorganic sulfate in the
body. 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-acetylcysteine. The L-cysteine derived from N-acetylcysteine helps to restore hepatic
Cysteine can also be transformed into glucose and used by the body as a source of energy. Cysteine
strengthens the protective lining of the stomach and intestines, which may help prevent damage caused by
aspirin and similar drugs. Additionally, cysteine may play an important role in the communication
between immune system cells. Cysteine is rarely used as a dietary supplement. N-acetyl cysteine (NAC),
which contains cysteine, is more commonly used as a supplement.
There are a number of user submitted cysteine product reviews and ratings available at
Dosage and Administration
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 (see Vitamin
Healthy people do not need to supplement N-acetyl cysteine. Optimal levels of supplementation remain
unknown, though much of the research uses 250-1,500 mg per day.