Measurement of Cystine in Urine by Liquid Chromatography–Tandem Mass Spectrometry

Abstract

Cystine is a neutral sulfur-containing amino acid involved in a variety of important cellular functions, including detoxification, metabolism, and protein synthesis. Cystine is excreted through the kidneys, with the epithelial cells of the renal proximal tubules absorbing ∼99% of filtered cystine through a high-affinity luminal transport system in the proximal renal tubule, which also carries the dibasic amino acids lysine, arginine, and ornithine. Genetic mutations in the genes encoding this transport system can cause cystinuria, characterized by excessive amounts of cystine, arginine, lysine, and ornithine in the urine (1) and decreased absorption of cystine through the intestine (2). The primary clinical manifestation of cystinuria is recurrent nephrolithiasis, which can lead to urinary tract obstruction and renal insufficiency. Cystinuria accounts for 1% of renal stones, with an incidence of 1:100 000 (3)(4). The renal stones are formed through nucleation of crystals from a supersaturated solution. Treatment is therefore aimed at lowering the urinary cystine concentration to below its solubility limit of 250 mg/L. The solubility of cystine begins to decrease below pH 7; therefore, alkalinization of the urine can be used to prevent the recurrence of renal stones. Other treatments used in cystinuria include low-protein diets (<0.8 g · kg−1 · day−1) and low-sodium diets, as the presence of sodium in the urine decreases the solubility of cystine (5). Chelating agents such as captopril have also been used (6). It is important to identify patients with cystinuria to enable the correct treatment to be given to prevent recurrent renal stone formation. Different methods have been used for this, including spectrophotometric measurement of cysteine in urine with phosphotungstate used as a chromogen and by HPLC followed by ultraviolet detection. These methods are time-consuming, however: the …