Aminoaciduria



Abnormal excretion of amino acids may occur as a single defect in proximal tubular reabsorptive function. The aminoacidurias are grouped ac­cording to the class of amino acids affected, i.e., basic, neutral, or acidic, and reflect a specific de­fect in the cellular transport system for each group. The most significant of these many dis­orders is cystinuria, which is actually a failure of reabsorption of the dibasic amino acids lysine, or­nithine, arginine, and cystine. Cystine is singled out because of its very low solubility over the uri­nary pH range of 4.5 to 7.0. Renal stone formation is the most prominent feature of the disease, al­though only half the stones are pure cystine. Renal failure due to repeated bouts of urinary tract ob­struction and/or urinary tract infection is a com­mon outcome of untreated cystinuria. No cellular accumulation of cystine occurs, a reflection of the fact that cystinuria represents a defect in cellular transport of cystine, whereas cystinosis represents a defect in cellular processing of the amino acid.
The disease can be recognized on urinalysis by the flat, hexagonal cystine crystalluria. A defini­tive diagnosis is made by the demonstration of the isolated appearance of the four dibasic amino acids in the urine. Urinary cystine excretion of more than 400 mg/gm creatinine is characteristic. The most effective treatment is forced intake of 5 L or more of water daily to keep urine cystine con­centrations below saturation levels.