What is cystine?

Cystine is a nonessential amino acid, which can be composed in your body from 2 molecules of the amino acid cysteine, so you do not need to get it from food in order to be healthy [1].

Cystine is a nonproteinogenic amino acid, which is not regularly incorporated into proteins [1], which means it is found only in some proteins as opposed to proteinogenic amino acids that are found in most proteins in the human.

Cystine Functions in the Human Body

Cystine is [1]:

  • A part of the protein alpha-keratin in the skin, hair and nails
  • Involved in the production of glutathione, which is an antioxidant
  • A source of sulfur

Cystine Supplements

Available nonprescription oral forms of cystine:

  • L-cystine
  • N-acetyl cystine

Cystine Benefits

Cystine supplements are used in treatment of certain hereditary metabolic disorders.

There is INSUFFICIENT EVIDENCE about the effectiveness of cystine supplements in the prevention or treatment of bronchitis, cystic fibrosis, chronic obstructive pulmonary disease (COPD), end-stage renal disease (ESRD), epilepsy, fibrosing alveolitis, heart related chest pain (angina) or influenza, in promoting hair growth, improving brittle nails, boosting immunity, removing toxins from the body, wound healing or relieving pain in rheumatoid arthritis [9].

Cystine Safety: Side Effects, Toxicity

The use of a single amino acid supplement may lead to negative nitrogen balance, decreasing the metabolic efficiency and increasing the workload of the kidneys. In children, taking single amino acid supplements may also harmfully affect growth.

Not enough is known about the safety of cystine supplements during pregnancy and breastfeeding, so women in these periods should avoid them.

Who else should avoid cystine supplements?

Individuals with:

  • Maple syrup urine disease (MSUD)
  • Cystinuria
  • Cystinosis

Cystinuria and Cystine Kidney Stones

Cystinuria is a hereditary metabolic disorder in which cystine is excreted in the urine, which may result in the formation of cystine kidney stones [2].

Symptoms can appear for the first time in infants, children or adolescents and may include sulfur or rotten egg-smelling urine, blood in urine, pain in the flank (usually only on one side and often radiating toward the groin) [3].

Diagnosis of cystine kidney stones is made by a CT; urine test may reveal cystine crystals [4].

Cystinuria can be decreased by a vegetarian diet, which is low in the amino acid methionine (which is metabolized to cystine in your body) [5].


Cystinosis is a hereditary metabolic disorder with accumulation of cystine within the cells, mainly in the kidneys and eyes [6]. The affected individuals usually have blond hair and blue eyes, but the disorder can be also found in some with dark hair and brown eyes [7]. Symptoms, which can appear in infants, children or adults, may include failure to thrive, vomiting, craving for salty and spicy foods and excessive urination. Main complications include kidney failure or blindness. Treatment includes a low-protein diet, cysteamine and other drugs [6]. Patients may live up to their 5th decade.

5 Responses to "Cystine"

  1. no one says:

    It is confusing when you describe that nonproteinogenic proteins are “not regularly incorporated into protein” but then state that they are parts of protein (such as this one, which sounds like it is certainly ‘regularly incorporated’). Through some further research I’ve found that this must refer to post-translational modification, correct? So that these amino acids are only added in after the protein has already pretty much been created?

    But more specificity on this would be awesome.

    Also there is a 22nd amino acid, pyrrolysine, which is in fact present in humans (and other animals), though in many places it is stated that it is not found in humans.

    I would also like to point out that one of your sources, Chempep (e.g. utilizes Wikipedia as a source. Of course, that does not automatically make it inaccurate (although it is in this case, as it says there are only 20 proteinogenic amino acids), but considering the ease with which anyone can edit wiki articles and its notorious unreliability, in addition to that this makes Chempep several steps away from the original source (making it all the more likely to be inaccurate), I would reconsider using it as a source (at least where it cites wiki).

    thank you for this great resource though

    • Jan Modric says:

      no one, currently, there is a common agreement that there are 21 known proteinogenic amino acids in humans (the 21st being selenocysteine). Pyrrolysine is synthesized by certain bacteria. If it is also found in humans, I assume, it would be possible some normal intestinal bacteria produce it and this would make it “to appear in humans.” But this does not mean it is produced by humans. So, I did not include it in the list.

      By “not regularly incorporated in the proteins” I meant that it is found only in certain proteins as opposed to proteinogenic amino acids, all of which are found practically in all proteins in the human. For example, hydroxyproline is found only in collagen and few other proteins. Yes, it is about post-translational modification, but I just wanted to emphasize this other aspect that it is found only in certain proteins. I might need to reword that to prevent confusion.

      • no one says:

        woops, sorry, did some more research, my source that gave me the idea that pyrrolysine was also included must have just stated it incorrectly (it said ‘the 21st and 22nd common to all organisms,’ but since every other thing I’m reading states otherwise, that’s clearly wrong). nevermind that one 😐 haha

        thank you for the clarification 🙂

  2. Tracey says:

    When speaking of cystinuria you write, “The affected individuals have blond hair and blue eyes.” What is the basis of this statement? Does this disorder not affect others that don’t fit this description?

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