title: A modified cyanide-nitroprusside method for quantifying urinary cystine concentration that corrects for creatinine interference.
abstract: Cystinuria, an inherited disease, is clinically diagnosed by detecting cystine in urine. A colorimetric method using sodium cyanide and sodium nitroprusside is a simple qualitative test used to detect cystinuria. Several colorimetric methods have been proposed for quantitative analysis of cystine; however, we found that none of them were satisfactory because the results were not reproducible. The causes of non-reproducible results were: (1) insufficient reduction time for conversion of cystine to cysteine, and (2) the interference of creatinine. In this report, we present a method to quantitate cystine in urine. We also found that ascorbic acid and ferric chloride, but not zinc chloride, interfered with the color reaction. Using this method, 15 normal urine samples (10 males and 5 females) and 12 cystine stone forming patients' (5 males and 7 females) urine were analyzed. The method was compared to commercially available urine controls. Only captopril showed a dose dependent response and color intensity at 521 nm. Thiola and D-penicillamine showed little effect on cystine determination.
authors.affiliation: Department of Biochemistry & Molecular Biology, and Kidney Stone Program, Department of Medicine, University of Chicago, Chicago, IL 60637, USA. ynakagaw@medicine.bsd.uchicago.edu
authors.forename: Y
authors.initials: Y
authors.lastname: Nakagawa
authors.forename: F L
authors.initials: FL
authors.lastname: Coe
mesh_terms: D000328:Adult; D016477:Artifacts; D001205:Ascorbic Acid / Q000737:chemistry; D002216:Captopril / Q000627:therapeutic use; D002712:Chlorides / Q000737:chemistry; D003124:Colorimetry / Q000379:methods*; D003404:Creatinine / Q000737:chemistry / Q000652:urine*; D003486:Cyanides / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry; D003555:Cystinuria / Q000175:diagnosis* / Q000188:drug therapy / Q000652:urine*; D004305:Dose-Response Relationship, Drug; D005260:Female; D005296:Ferrous Compounds / Q000737:chemistry; D006801:Humans; D008297:Male; D008875:Middle Aged; D009599:Nitroprusside / Q000737:chemistry; D010396:Penicillamine / Q000627:therapeutic use; D012016:Reference Values; D015203:Reproducibility of Results; D012709:Serum Albumin / Q000737:chemistry; D008625:Tiopronin / Q000627:therapeutic use; D017967:Zinc Compounds / Q000737:chemistry
chemical_list: D002712:Chlorides; D003486:Cyanides; D005296:Ferrous Compounds; D012709:Serum Albumin; D017967:Zinc Compounds; D009599:Nitroprusside; C016837:zinc chloride; D002216:Captopril; D003404:Creatinine; D008625:Tiopronin; D010396:Penicillamine; D003545:Cysteine; D001205:Ascorbic Acid; C029451:ferrous chloride