Cysteine

What is it?

Cysteine is a nonessential amino acid (protein building block), meaning that cysteine can be made in the human body. Cysteine is one of the few amino acids that contains sulphur. 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 can also be converted into glucose and used 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.¹ In addition, cysteine may play an important role in the communication between immune system cells.² Cysteine is rarely used as a dietary supplement. N-acetyl cysteine (NAC), which contains cysteine, is more commonly used as a supplement.

Where is it found?

The body can synthesise cysteine from methionine and other building blocks. Cysteine, the amino acid from which NAC is derived, is found in most high-protein foods.

Who is likely to be deficient?

According to several studies, blood levels of cysteine and glutathione are low in people infected with HIV.³ ⁴ ⁵ Cysteine has a role in the proper function of the immune system, so a deficiency of this amino acid may either contribute to, or result from, immune suppression associated with HIV.

How much is usually taken?

Most people do not need to supplement with cysteine. Almost nothing is known about appropriate supplemental levels, in part because almost all clinical research has been done with N-acetyl cysteine and not cysteine itself.

Are there any side effects or interactions?

No consistent adverse effects of NAC have been reported in humans. One small study found that daily amounts of 1.2 grams or more could lead to oxidative damage.⁶ Extremely large amounts of cysteine, the amino acid NAC is derived from, may be toxic to nerve cells in rats.⁷

Adequate amounts of methionine are needed in the diet, as the precursor to cysteine, to prevent cysteine deficiency.

At the time of writing, there were no well-known drug interactions with cysteine.

References
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1. Salim AS. Sulfhydryl-containing agents in the treatment of gastric bleeding induced by nonsteroidal anti-inflammatory drugs. Can J Surg 1993;36:53-8.

2. Droge W, Eck HP, Gander H, Mihm S. Modulation of lymphocyte functions and immune responses by cysteine and cysteine derivatives. Am J Med 1991;91(suppl 3C):140S-4S.

3. Eck HP, Gander H, Hartmann M, et al. Low concentrations of acid-soluble thiol (cysteine) in the blood plasma of HIV-1 infected patients. Biol Chem Hoppe Seyler 1989;370:101–8.

4. Droge W, Eck HP, Mihm S. HIV-induced cysteine deficiency and T-cell dysfunction—a rationale for treatment with N-acetylcysteine. Immunol Today 1992;13:211–4.

5. Droge W. Cysteine and glutathione deficiency in AIDS patients: a rationale for the treatment with N-acetyl-cysteine. Pharmacology 1993;46:61–5 [review].

6. Kleinveld HA, Demacker PNM, Stalenhoef AFH. Failure of N-acetylcysteine to reduce low-density lipoprotein oxidizability in healthy subjects. Eur J Clin Pharmacol 1992;639–42.

7. Olney JW, Ho OL. Brain damage in infant mice following oral intake of glutamate, aspartate or cysteine. Nature 1970;227:609–10 [letter].

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The information presented in Healthnotes is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or chemist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires March 2007.

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