Cysteine is one of the three amino acids, along with taurine and methionine, that has the element sulfur. It 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.

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.

L Cysteine medical uses
I am not certain at this time for which medical conditions cysteine supplements would be appropriate or helpful. However, it does appear that supplementation increases levels of glutathione which has anti-aging potential.

Deficient synthesis of glutathione underlies oxidative stress in aging and can be corrected by dietary cysteine and glycine supplementation. The American Journal of Clinical Nutrition, 2011.
We tested whether glutathione deficiency occurs because of diminished synthesis and contributes to oxidative stress in aging and whether stimulating glutathione synthesis with its precursors cysteine and glycine could alleviate oxidative stress. Eight elderly and 8 younger subjects received stable-isotope infusions of [2H2]glycine, after which red blood cell (RBC) glutathione synthesis and concentrations, plasma oxidative stress, and markers of oxidant damage (eg, F2-isoprostanes) were measured. Glutathione deficiency in elderly humans occurs because of a marked reduction in synthesis. Dietary supplementation with the glutathione precursors cysteine and glycine fully restores glutathione synthesis and concentrations and lowers levels of oxidative stress and oxidant damages.
 

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.

What is the difference between cysteine and cystine?
   They are both sulfur-containing amino acids. Cysteine is the monomer and cystine is the dimer formed by two cysteine molecules. Generally, cystine is more stable than cysteine; however, both can be converted into the other form as required by the body. More supplements contain cysteine than cystine because the absorption for cysteine may be better. Cysteine is required by the skin and for detoxification of the body, while cystine is required for vitamin B6 utilization.

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.

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? Also, what is methylselenocysteine?
   A. Acetylcysteine has been studied much more thoroughly in this regard than cysteine.

I have seen cysteine spelled as cystine.