Cysteine is one of the three amino acids, along with taurine and methionine, that has the element sulfur. Cysteine can provide a portion of the sulfur amino acid requirement in adults. Cysteine can be synthesized from methionine and serine and is, therefore, a nonessential amino acid in human adults.

Cysteine metabolism
The carbons and nitrogen for the synthesis of cysteine are provided by serine and the sulfur is provided by methionine. Serine reacts with homocysteine to form cystathionine. This reaction is catalyzed by cystathionine synthase. Cleavage of cystathionine by cystathionase produces cysteine and alpha ketoglutarate, which forms succinyl CoA via propionyl CoA. Since cysteine dreives its sulfur from teh essential amino acid methionine, cysteine becomes essential if it is low in the diet and the supply of mehtioninen is inadequate for cysteine synthesis. An adequate supply of of cysteine spares methionine, that is, it decreases the amount of methionine that must be degrades to produce cysteine. When cysteine is degraded, the nitrogen is converted to urea, the carbons to pyruvate, and the sulfur to sulfate, which is excreted in the urine.

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L Cysteine medical uses
I am not certain at this time for which medical conditions cysteine supplements would be appropriate or helpful.

Cysteine Availability

Cysteine is sold as a supplement either in capsules or as anhydrous powder. You can also find cysteine sold as acetylcysteine, which has been shown to have powerful antioxidant properties. Another product contains glutathione along with cysteine in a combination capsule, although the benefit of oral glutathione is in question. See Amino Acids for information on several amino acid supplements.

Cystinuria info
Cystinuria is a common inherited amino-aciduria resulting in abnormal urinary excretion of cystine and the dibasic aminoacids, lysine, arginine and ornithine. Formation of cystine kidney stones, recurrent infections and subsequent renal failure are the main complications of the disease.

Cysteine questions
Q. Hi, what exactly is the difference between cysteine and N-acetylcysteine? Which one is better to take?
   A. Most often aceytylcysteine is the one taken since it has stronger antioxidant potential, however it depends on the condition being treated. Acetylcysteine has an additional acetyl group which makes it more potent as an antioxidant. Cysteine is an amino acid but there are hardly any studies to inform us of its clinical uses..

Q. Can you tell me if L-cysteine promotes detoxing the body of drugs taken prescription as well as narcotics?
   A. Acetylcysteine has been studied much more thoroughly in this regard than cysteine.

I have seen cysteine spelled as cystine.

This cysteine page was last updated in February 2006.