Figure 123

Over 95% of the filtered amino acid load is normally reabsorbed in the proximal tubule. The term aminoaciduria is applied when more than 5% of the filtered load is detected in the urine. Aminoaciduria can occur in the context of metabolic defects, which elevate plasma amino acid concentrations and thus increase the glomerular filtered load. Aminoaciduria can be a feature of generalized proximal tubular dysfunction caused by toxic nephropathies or Fanconi's syndrome. In addition, aminoaciduria can arise from genetic defects in one of the several amino acid transport systems in the proximal tubule. Three distinct groups of inherited aminoacidurias are distinguished based on the net charge of the target amino acids at neutral pH: acidic (negative charge), basic (positive charge), and neutral (no charge) [5].

Acidic aminoaciduria involves the transport of glutamate and aspartate and results from a defect in the high-affinity sodium-potassium-dependent glutamate transporter [6]. It is a clinically benign disorder.

Four syndromes caused by defects in the transport of basic amino acids or cystine have been described: cystinuria, lysinuric protein intolerance, isolated cystinuria, and isolated lysinuria.

Cystine actually is a neutral amino acid that shares a common carrier with the dibasic amino acids lysine, arginine, and ornithine. The transport of all four amino acids is disrupted in cystinuria. The rarer disorder, lysinuric protein intolerance, results from defects in the basolateral transport of dibasic amino acids but not cystine. Increased intracelluar concentrations of lysine, arginine, and ornithine are associated with disturbances in the urea cycle and consequent hyperammonemia [7].

Disorders involving the transport of neutral amino acids include Hartnup disease, blue diaper syndrome, methioninuria, iminogly-cinuria, and glycinuria. Several neutral amino acid transporters have been cloned and characterized. Clinical data suggest that Hartnup disease involves a neutral amino acid transport system in both the kidney and intestine, whereas blue diaper syndrome involves a kidney-specific tryptophan transporter [5]. Methioninuria appears to involve a separate methionine transport system in the proximal tubule. Case reports describe seizures, mental retardation, and episodic hyperventilation in affected patients [8]. The patho-physiologic basis for this phenotype is unclear. Iminoglycinuria and glycinuria are clinically benign disorders.

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