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 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 antioxidantglutathione. The body also uses cysteine to produce taurine, another amino acid.
The body can synthesize cysteine from methionine and other building blocks. Cysteine, the amino acid from which NAC is derived, is found in most high-protein foods.
According to several studies, blood levels of cysteine and glutathione are low in people infected with HIV.1 2 3 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.
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.
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.4 Extremely large amounts of cysteine, the amino acid NAC is derived from, may be toxic to nerve cells in rats.5
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.
1. 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.
2. 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.
3. Droge W. Cysteine and glutathione deficiency in AIDS patients: a rationale for the treatment with N-acetyl-cysteine. Pharmacology 1993;46:61–5 [review].
4. 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.
5. 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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