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Cystine: a promoter of the growth and aggregation of calcium oxalate crystals in normal undiluted human urine.

Journal of Urology 2002 January
PURPOSE: Many variables are known to be associated with the formation of calcium oxalate stones. We noted that on analysis a number of patients with calcium oxalate calculi also had cystine in the stones. Some but not all of these patients showed urinary cystine excretion slightly above the normal limits, resembling heterozygous carriers. This finding raised the question of whether some recurrent stone formers may be at risk for calcium oxalate calculi when they excrete cystine in above normal concentrations.

MATERIALS AND METHODS: Pooled urine obtained from 3 pairs of age and sex matched controls was independently analyzed. Each urine sample was divided into spun and filtered, and ultrafiltered urine. A Multisizer II (Coulter Electronics Ltd, Beds, England) was used to measure particle number, diameter and volume. The metastable limit of each specimen was determined. Promotion activity was measured in spun and filtered, and ultrafiltered urine using 3 concentrations of cystine (80, 160 and 320 micromol./l.). Results were confirmed by measuring the incorporation of (14)C-oxalate into the crystals. Scanning electron microscopy was performed to study further the agglomerates as well as exclude cystine crystals. Each experiment was repeated 6 times. Crystalline material was collected for x-ray powder diffraction analysis.

RESULTS: The urine metastable limit did not change with increasing cystine concentrations. Particle diameter increased significantly from 10.6 microm in ultrafiltered urine alone to 11.6 and 13.5 microm (p < 0.05) at 160 and 320 micromol/l. cystine, respectively. In addition, particle volume also increased proportionally in a dose response manner to cystine concentration. The dose of 320 micromol/l. cystine increased the crystal growth rate 52%. 14C-oxalate experiments confirmed these results. Scanning electron microscopy at 500x magnification revealed no cystine crystals in any experiments performed. Furthermore, x-ray powder diffraction analysis of samples revealed that experimentally determined parameters matched reference values for calcium oxalate trihydrate but not for cystine, again confirming absent cystine in the samples.

CONCLUSIONS: Adding cystine to undiluted human urine resulted in the marked enhancement of calcium oxalate crystal precipitation. When considered with the finding of cystine in calcium oxalate stones in the noncystinuric population, this result implies that urinary cystine may be a risk factor for calcium oxalate calculi. Cystine was not observed in any calcium oxalate crystals, suggesting that the mechanism of crystal formation was a salting out effect.

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