The Difference Between Cysteine & Cystine

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Cysteine and cystine are so closely related that the two amino acids can be confusing. For example, cysteine is used to make cystine, then cystine can be converted back to cysteine. Due to this relationship, they can fill the same roles. But that’s not the whole story. In spite of their similarities, they’re each responsible for different jobs that are vital for your health.

Related Amino Acids

The production of cysteine and cystine is typical of many chemical processes in your body. They must follow a specific order, and each step depends on the presence of certain substances. An amino acid called methionine is used to make cysteine, but it takes several steps to get from methionine to cysteine, and you need vitamins B-12 and B-6 to make it happen. Cysteine then proceeds through more steps to produce the antioxidant glutathione and the amino acid taurine. Cystine is made when two molecules of cysteine combine. If your body needs more cysteine, it can reverse the process and convert cystine back into cysteine.

Metabolic Differences

Cysteine and cystine both contain sulfur, which is only found in a few amino acids, but because it takes two cysteine molecules to make one cystine, they're structurally different. As a result, they have different functions. Cystine has a significant role in protein synthesis; it helps determine the protein’s final shape. Your body also needs cystine to metabolize vitamin B-6. Cysteine controls the synthesis of glutathione. If you're low on cysteine, your body can't make enough glutathione to keep you healthy.

Unique Health Benefits

Cysteine detoxifies acetaminophen, so it’s often used to prevent liver and kidney damage caused by an overdose. Glutathioine production is a vital benefit from cysteine because it's a potent antioxidant that fights inflammation, boosts your immune system, regulates genes and helps metabolize nutrients, reported a review in the Journal of Nutrition in March 2004. Cystine and cysteine both helped reduce blood levels of fats in laboratory rats with a type of cancer that affects fat metabolism. However, cystine had a different role than cysteine: It boosted levels of an enzyme that removes fats from the bloodstream, according to a report in Cytotechnology in June 2010. More studies are needed to determine if either amino acid has the same effect in people.

Recommendations and Warnings

In healthy adults, the body produces a sufficient supply of both amino acids, but infants, the elderly and adults with metabolic or malabsorption problems may need to boost their cysteine intake through high-protein foods, such as lean meat, fish, poultry and soy products. Cystine is seldom available in supplements, compared to cysteine which is commonly sold as a supplement in the form of N-acetyl-L-cysteine, or NAC. The University of Maryland Medical Center warns that other forms of cysteine may be toxic in supplemental form. Talk to your physician before taking NAC if you take prescription medications, as the two may interact. NAC may also cause nausea, vomiting and diarrhea.