Cystinuria
What is cystinuria?
Cystinuria is an inherited (genetic) disorder of the transport of
an amino acid (a building block of protein) called cystine resulting
in cystinuria (an excess of cystine in the urine) and the formation
of cystine stones.
How frequent is cystinuria?
Cystinuria is one of the more common genetic disorders. Its
overall prevalence is about 1 in 7,000 in the population.
Cystinuria is the most common defect known in the transport of an
amino acid.
What is a transport defect?
Within the body, many molecules are able to pass across the
membranes that surround cells. These molecules can accomplish this
feat due to specific transport systems. These systems include special
receptors on the membrane of the cell and special carrier proteins.
The receptor recognizes the molecule and receives it on the cell
membrane. Then the molecule hitches a ride through the cell membrane
on the back of a carrier protein.
With such remarkable specificity, it is little wonder that
sometimes there are defects in transport systems. Several dozen
different diseases are now known to be due to transport defects.
What causes cystinuria?
Cystinuria is caused by the defective transport of cystine and
several other amino acids through the cells of the kidney and the
intestinal tract.
What happens with cystine in the urine?
Although cystine is not the only overly excreted amino acid in
cystinuria, it is the least soluble of all naturally occurring amino
acids. Cystine precipitates, or crystallizes out of urine and forms
stones (calculi) in the kidney, ureter, bladder, or anywhere in the
urinary tract.
The cystine stones (below) compared in size to a quarter (a U.S.
$0.25 coin) were obtained from the kidney of a young woman by
percutaneous nephrolithotripsy (PNL), a procedure for crushing and
removing the dense stubborn stones characteristic of cystinuria.
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