Urinary Excretion of Free Cystine and the Tiopronin-Cysteine-Mixed Disulfide during Long-Term Tiopronin Treatment of Cystinuria

Abstract

We report the results of a biochemical evaluation of long-term treatment of cystinuria with the SH compound tiopronin (2-mercaptopropionylglycine). The effects of tiopronin were studied by monitoring the urinary excretion of free cystine and the mixed disulfide between tiopronin and cysteine. Thirty-one patients with homozygous cystinuria were treated with tiopronin for 0.4-12 years (mean 7.8 years). The urinary concentration of free cystine was used to adjust the tiopronin dose. In 28 of the 31 patients a mean urinary cystine concentration of less than 1,200 μmol/1 (288 mg/l) was achieved with the final dose. The final daily doses of tiopronin ranged from 250 mg (1.5 mmol) to 3,000 mg (18.4 mmol; mean 1,540 mg; 9.4 mmol). In a majority of the patients the treatment reduced the 24-hour urinary free cystine excretion effectively, on average by 0.61 μmol (0.15 mg)/mg of tiopronin administered. No changes in the efficacy of tiopronin over time were observed, and the frequency of adverse effects was acceptable. To evaluate the effects of tiopronin on the metabolism of cystine we calculated the total urinary excretion of cystine as the sum of free cystine and the amount of cystine corresponding to the cysteine content of the tiopronin-cysteine disulfide. At low doses of tiopronin there was an increase in urinary excretion of the mixed disulfide as well as of total cystine, whereas higher doses were followed by an unexpected decrease in the urinary excretion of these compounds. We conclude that long-term treatment with tiopronin in cystinuria effectively reduces the urinary excretion of free cystine. Monitoring urinary cystine concentration is necessary to achieve adequate individualized doses of tiopronin. Assessment of the mixed tiopronin-cysteine disulfide and the urinary excretion of total cystine shows that tiopronin may interfere with cystine metabolism in a more complex way than through a simple disulfide exchange reaction with urinary cystine.

Therapy modifies cystine kidney stones at the macroscopic scale. Do such alterations exist at the mesoscopic and nanometre scale?

“…It is of note that the large crystals have lost their typical hexagonal shape, also suggesting the beginning of a dissolution process. Such dissolution of cystine stones has already been reported (suggested) in the literature (Lotz & Bartter, 1965;Lindell et al, 1995). Similar alterations of cystine crystals as seen with tiopronine were observed for patients (subjects 20-22, Fig.…”

Identification of 12 novel mutations in the SLC3A1 gene in Swedish cystinuria patients

“…Fifty-one of these cystinuria patients were defined as homozygous and two as heterozygous (nos. 36 and 46) by determination of urinary excretion of cystine and dibasic amino acids using ion exchange chromatography [Lindell et al, 1995] at the Department of Clinical Chemistry, Malmö University Hospital. The patients were not classified as to cystinuria subtype since urinary excretion values of obligate carriers were not available.…”

Studies on Renal Function in Patients with Cystinuria

“…15]. The presence of the mixed disulphides between cysteine and tiopronin o f £>-penicillaminc was used as a control o f compliance to medication [14], Uri nalysis with the dip-stick test and urinary cultures were made regular ly, as were blood analyses relevant to the disease and the therapy. Signs of albuminuria were followed up by a quantitative assessment.…”