Solid phase assay of urine cystine supersaturation in the presence of cystine binding drugs.

PURPOSE: We report a new kind of assay system for urine cystine supersaturation that is accurate in the presence of cystine binding thiol drugs. We measured the molar ratio of cystine dissolved per mole of drug.

MATERIALS AND METHODS: Measured amounts of cystine crystals were incubated in buffer or urine for 48 hours with stirring. The solid phase remaining was pelleted by centrifugation, extracted into a high pH buffer and measured. D-penicillamine, tiopronin and captopril were added to determine their effect on solid phase dissolution.

RESULTS: Total cystine calculated from urine and solid measurements closely matched the amounts of cystine weighed in, meaning that the assay system successfully recovered the total cystine from the 2 phases. Each drug dissolved solid cystine in a specific and fixed proportion to its molar concentration in the range of 0.2 to 0.4 mM. dissolution per mM. of drug. Solution measurements were not a reliable gauge to the actual amounts of cystine dissolved.

CONCLUSIONS: Changes in solid phase cystine accurately reflect buffer or urine supersaturation when thiol drugs are present. The solid phase assay is a technically straightforward and reliable way of assessing cystine movement into and out of urine that avoids complexity of measurement and distortions of assay systems by drugs. This assay enables one to assess the level of drug effect and the need for a change in dosing.