Overflow aminoaciduria

Much that is known about renal reabsorption mechanisms has been learned from the study of various forms of aminoaciduria. Three types of aminoaciduria have been identified (1) overflow aminoaciduria occurs when the plasma level of one or more amino acids exceeds the renal threshold (tubular capacity for reabsorption) (2) renal aminoaciduria occurs when plasma levels are normal but the renal transport system has a congenital or acquired defect and (3) no-threshold aminoaciduria occurs when excessive amounts of an amino acid, arising from an inherited metabolic block, are present in urine, but plasma levels are essentially normal because ah the amino acid is excreted. The no-threshold aminoacidurias, such as homocystinuria, are not due to congenital or acquired kidney defects but solely to saturation of the normal renal tubular reabsorption mechanisms. 

Conservation of amino acids filtered at the glomerulus is made possible by the existence of four main transport systems for specific amino acids that facilitate active reabsorption of these amino acids from the proximal tubule. A lack or deficiency of the transport system responsible for the absorption of valine, alanine, cystine, and tryptophan, and of the transport system for arginine, lysine, cystine, and ornithine, leads to excretion of these specific amino acids in urine, which is characterized as renal aminoaciduria to distinguish it from overflow aminoaciduria. In the latter situation, the production of amino acids far exceeds the proximal tubular reabsorption capacity, thus leading to overflow of amino acids into urine. This can occur due to defective metabolism of amino acids, as is the case when phenylalanine cannot be metabolized due to the deficiency of the enzyme phenylalanine hydroxylase, or to the inability to deaminate amino acids in liver disease. 

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