Cystine is a naturally occurring amino acid, a fundamental building block of proteins. It forms when two molecules of another amino acid, cysteine, link together. While typically dissolved within the body’s fluids, cystine has low solubility in water. This characteristic becomes relevant when it accumulates in excessive amounts.
Cystine’s Essential Functions
Cystine plays a significant role in maintaining the structural integrity and function of various proteins. It forms strong disulfide bonds, which are chemical links that stabilize the three-dimensional shape of proteins, including those in hair, skin, and nails. These bonds are important for proper protein folding and stability.
Beyond its structural contributions, cystine also serves as a precursor to glutathione. Glutathione is a powerful antioxidant that protects cells from damage caused by harmful molecules. This aids in the body’s natural detoxification processes.
Understanding Cystinuria
Cystinuria is a rare genetic disorder where the kidneys cannot properly reabsorb certain amino acids, including cystine, from filtered blood. Normally, kidneys reclaim nearly all cystine, but a genetic defect in individuals with cystinuria prevents this reabsorption. This leads to excessive cystine accumulation in the urine.
Since cystine is not highly soluble, its elevated concentration in urine causes it to form crystals that aggregate into hard stones. These cystine stones primarily develop in the kidneys, but can also travel to and lodge in the ureters or bladder. The condition is typically inherited in an autosomal recessive pattern, meaning an individual must inherit a mutated gene from both parents. Specific mutations in the SLC3A1 or SLC7A9 genes are responsible for this transport defect.
Symptoms of cystinuria usually arise when stones form or pass through the urinary tract. Common indicators include severe pain in the side or back. Other symptoms can involve blood in the urine, nausea, vomiting, and recurrent urinary tract infections. Symptoms often first appear in young adults.
Living with Cystinuria
Managing cystinuria primarily focuses on preventing the formation and growth of cystine stones. A cornerstone of management involves significantly increasing fluid intake to dilute cystine concentration in the urine. Adults are often advised to drink enough water to produce at least three liters of urine daily, helping to keep cystine dissolved.
Dietary adjustments also play an important part in stone prevention. Reducing dietary sodium intake can help lower cystine excretion. Limiting foods high in methionine, an amino acid the body converts to cystine, such as animal proteins, can also be beneficial.
When conservative measures are insufficient, medications are often introduced. Alkalinization agents, such as potassium citrate, are prescribed to increase the urine’s pH level, as cystine is more soluble in alkaline conditions. Additionally, thiol-binding drugs like tiopronin or D-penicillamine can be used; these medications chemically bind to cystine, forming a more soluble compound that is easier to excrete. Regular medical monitoring, including urine and blood tests, helps track the condition and adjust treatment. If stones do form and cause complications, surgical interventions may be necessary for their removal.