Solubility of L-cystine in NaCl and artificial urine solutions

L-Cystine is the least soluble of the naturally occurring amino acids, and cystine stones, caused by a genetic disorder, account for between 1-4% of a ll urinary stones. Since the concentration of cystine in urine is the only factor related to the stone formation, a proper knowledge of cystine solubi lity in urine is necessary. Some research groups have already reported on t he solubility of L-cystine, but the results are scattering. In this work, a systematic investigation of cystine solubility under conditions most perti nent to urolithiasis was carried out. The solubilities of L-cystine were me asured at 25.0 and 37.0 degrees C, from pH 1 to 9, and in different media i ncluding (i) 0.300 mol.dm(-3) NaCl solution and (ii) artificial urine solut ions. The Joint Expert Speciation System (JESS) computer package and select ed protonation constants of cystine reported in literature were used to mod el the solubilities of cystine. Excellent agreement was obtained between th e experimentally determined solubility data and computer modelling of cysti ne solubility. The results of this work show that the presence of inorganic components has little influence on the solubility of cystine in urine.