Determination of N-Acetylcysteine and Thioglycolic Acid in Human Urine

A method for the determination of total N-acetylcysteine and thioglycolic acid in human urine is described. Because these compounds are mainly excreted as disulfides, they are first reduced to the free thiols by treatment with tris(2-carboxyethyl)phosphine hydrochloride and then derivatized with 2-chloro-1-methylquinolinium tetrafluoroborate. Separation and quantitation of the 2-S-quinolinium derivatives of the thiols were achieved by reversed-phase ion-pair liquid chromatography with UV-detection at 355 nm. Because the method enables simultaneous determination of other endogenous urinary thiols, e.g. cysteine and cysteinylglycine, amounts of these compounds in urine were also studied. Detector responses were linear over the range covering most practical situations, with correlation coefficients for all four analytes better than 0.999. Recovery and imprecision (as RSD) were within 99.77–102.17 and 0.01–7.79%, respectively. The lower limit of detection was 0.25 μmol L−1 urine for thioglycolic acid and N-acetylcysteine, and 0.12 μmol L−1 urine for cysteine and cysteinylglycine. The method was used for analysis of urine samples from 29 healthy individuals to establish reference values for the thiols, normalized to creatinine. 3-Mercaptolactic acid, 2-mercaptopropionic acid, and mercaptoethanol were not present in the urine analyzed.