The renal aminoacidurias.

TIIE FINDING of increased amounts of amino acids in the urine is characteristic of several inherited as well as acquired metabolic diseases. Such aminoacidurias are usually classified as “overflow” or “renal” in origin. In the former instance, it is assumed that the concentrations of amino acids in the plasma markedly exceed the capacity of the normal renal tubule to reabsorb completely the filtered load of amino acids. In all such cases, the quantities of amino acids in the plasma will be found to be in excess of the normal. Examples of such “overflow” aminoacidurias are those found in far-advanced liver disease and also in phenylketonuria. However, the majority of aminoacidurias of clinical interest are of the renal type, in which, even with normal concentrations of amino acids in the plasma and, consequently, normal ifitered loads in the kidneys, there are increased amounts of amino acids excreted into the urine. Under these circumstances the cause of the aminoaciduria is attributed to a decreased capacity of the renal tubule to reabsorb amino acids. To appreciate fully the significance of a renal aminoaciduria, it is necessary to review the normal mechanisms whereby the renal tubule handles amino acids. Failure to do so has led to erroneous interpretation of the results of analysis for amino acids in the urine.