Cystine is an amino acid which is a fundamental building block of protein in the body. Cystine and other amino acids are typically recycled within the body as proteins are broken down and rebuilt to support the biochemical processes that enable us to live and grow. Unlike other biochemicals, amino acids are usually reclaimed from the urine during processing in the kidneys, with only small amounts ending up in the final product. However, when cystine is abundant in a person’s urine, that person is said to have cystinuria (literally, “cystine in the urine”).
There are a number of reasons why a person might have cystine in their urine. For example, some diseases and conditions cause a high level of urinary excretion of amino acids in general (not specific to cystine). The medical term “cystinuria” is more specific, traditionally referring to the increased urinary presence of cystine plus three chemically similar amino acids: arginine, lysine, and ornithine. This “profile” of amino acids in cystinuric urine is the result of changes in the affected person’s genes which break the biochemical system responsible for ensuring those amino acids are reclaimed.
Most people suffering from cystinuria will recognize it as a disease of chronic cystine kidney stone formation. This is because cystine (unlike the other three amino acids lost in abundance) is poorly soluble in human urine and readily forms solid crystals which grow into cystine stones. These stones may be expelled with the urine as “sand”, “gravel”, or individual stones if sufficiently small, or they may grow in size and obstruct the inside of the kidney and urinary tract. In the latter case, surgical intervention is typically required to remove of the stone(s) and restore the kidney’s proper function.
People with cystinuria are affected with varying degrees of severity. Some may have the disease but no symptoms of stone formation while others will become frequent stone formers. Treatments aside, the mechanisms governing the severity of cystine stone formation among people affected by the disease remain largely unknown and are a topic of active research.