Investigations of the efficacy of ascorbic acid therapy in cystinuria

We investigated ascorbic acid therapy for cystinuria in a study of seven healthy control persons and seven cystinuric patients. The study lasted 9 days. During the first period, we collected 24-h urine specimens from all subjects on 3 consecutive days. Starting on day 4, all were given 5 g ascorbic acid/day for a period of 6 days. On the last 3 days, 24-h urine specimens were again collected. Quantitative amino acid determination was performed using an HPLC method described elsewhere. During ingestion of ascorbic acid, the mean excretion of cysteine by the control group increased from 134.1 to 159 mumol/day, whereas the excretion of cystine decreased from 107.1 to 82 mumol/day. The corresponding values for the cystinuric patients increased from 352.4 to 452.1 mumol/day for cysteine and decreased from 4,131.6 to 3,663.2 mumol/day for cystine. Thus, ascorbic acid seems to have only mild reducing properties in respect to cystine.