The concentration of free amino acids in the extracellular fluids is significantly lower than that within the cells of the body.
TRANSPORT OF AMINO ACIDS INTO CELLS
The concentration of
free amino acids in the extracellular fluids is significantly lower than that
within the cells of the body. This concentration gradient is maintained because
active transport systems, driven by the hydrolysis of ATP, are required for
movement of amino acids from the extracellular space into cells. At least seven
different transport systems are known that have overlapping specificities for
different amino acids. Because the small intestine and the proximal tubule of
the kidney have common transport systems for amino acid uptake, a defect in any
one of these systems results in an inability to absorb particular amino acids
into the gut and into the kidney tubules. For example, one system is
responsible for the uptake of cystine and the dibasic amino acids, ornithine,
arginine, and lysine (represented as “COAL”). In the inherited disorder
cystinuria, this carrier system is defective, and all four amino acids appear
in the urine (Figure 19.6). Cystinuria occurs at a frequency of 1 in 7,000
individuals, making it one of the most common inherited diseases and the most
common genetic error of amino acid transport. The disease expresses itself
clinically by the precipitation of cystine to form kidney stones (calculi),
which can block the urinary tract. Oral hydration is an important part of
treatment for this disorder. [Note: Defects in the transport of tryptophan can
result in Hartnup disorder and pellagra-like dermatologic and neurologic
symptoms.]
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