| AMINO ACIDS AS PRECURSORS OF SIGNALING MOLECULES
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Derivatives of amino acids have important roles as signaling molecules
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| A 21-year-old man came to the emergency room with severe pain in his right side and back. Subsequent investigation indicated a kidney stone, and increased concentrations of cystine, arginine, and lysine in the urine. This patient exhibited the characteristic symptoms of cystinuria. |
| Comment. Cystinuria is an autosomal recessive disorder of intestinal absorption and proximal tubular reabsorption of dibasic amino acids; it does not result from a defect in cysteine metabolism per se. Because of the transport deficiency, cysteine, which is normally reabsorbed in the proximal renal tubule, remains in the urine. The cysteine spontaneously oxidizes to its disulfide form, cystine. Because cystine has very limited solubility, it tends to precipitate in the urinary tract, forming kidney stones. The condition is generally treated by restricting the dietary intake of methionine (a biosynthetic precursor of cysteine), encouraging high fluid intake to keep the urine dilute, and, more recently, with various drugs that may convert urinary cysteine to a more soluble compound that will not precipitate. |
| In addition to their role as building blocks for peptides and proteins, several of the common amino acids serve as precursors of amino acid derivatives that function as neurotransmitters
or hormones. Some of the amino acids may be used as neurotransmitters directly, for example glycine, aspartate, and glutamate, whereas others may be converted to neurotransmitters or hormones through modification. Tyrosine is notable in that it serves as a precursor of several neurotransmitters, the catecholamines, and thyroid hormones (Chapters 37 and 40).
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