The class of naturally occurring amino acids comprises over 20 compounds, mostly possessing an La-amino configuration. In circulating plasma they are present in free form in small but significant amounts, and as such are readily filtered at the glomerulus. Nevertheless, the urine concentration of amino acids is normally very low, due to efficient reabsorptive mechanisms localized mostly in proximal tubules.

Under certain abnormal conditions the concentration of free amino acids in urine is greatly increased, a phenomenon appropriately described as aminoaciduria. More than 150 years ago WOLLASTON [364] had already investigated the chemical properties of a stone which had crystallized out of urine in the bladder and discovered in it a sulfur-containing substance which he named cystine, from the Greek word for bladder. As more exact methods of measurements for amino acids became available aminoacidurias were reported with increasing frequency. Today many examples of such excretory malfunction are known: Thus, generalized tubular damage is likely to lead to increased excretion of all normally reabsorbed amino acids; in other cases specific lesions affect transport of only specific compounds.