Determination of urinary glycolate by ion chromatography: clinical and experimental implication

Abstract

Small changes in the oxalate concentration exert a greater impact on the urinary calcium oxalate saturation than those in the calcium concentration. Urinary oxalate arises from ascorbate in 30-40% and glycolate in 50-60%. Glycolic acid plays and intermediate role in glyoxylate metabolism being involved in both its synthesis and degradation. A large variation in urinary glycolate values reported in the literature and a potential key role in calcium oxalate urolithiasis prompted us to measure accurately urinary glycolate. The ion chromatography used was a Model IC-100 lon Chromatoanalyzer (Yokokawa Electric Co., Ltd.), connected to an autosampler, Model KSST-601 (Kyowa Seimitsu Co., Ltd.). The separator column was TSK-gel IC-anion-PW (Tosoh Co., Ltd.). As the eluent, 0.01 mM phthalate and, as the scavenger, 50 mM dodecylbenzenesulfonic acid were used at a flow rate of 2 ml/min. 24-hour urine samples were obtained from 30 normal healthy males, aged from 20 to 57 years, and 20 male Wistar-strain rats (approximately 160 gm.). These samples were treated with charcoal and diluted 100-fold with distilled water for the determination of glycolate. The minimum detectable limit for glycolate was 0.4 mumol/l in a standard solution, and the regression line for the standard curve from 0.4 mumol/l to 2.0 mmol/l glycolate had a significant correlation coefficient (In Y = 0.882 x In X-2.304, r = 0.996, p < 0.01). The intra-run coefficient of variation was 3.44%. The overall intra-run and inter-run coefficient of variation, including the sampling and dilution of urine, were 3.92% and 3.38% respectively. The recovery from the addition of a known amount of glycolate to each 10 urine samples was 100.2 +/- 7.41% (mean +/- S.D.). A comparison of our method with the enzymatic method yielded a significant correlation between the results (r = 0.997, p < 0.01). The 24-hour urinary glycolate excretion in 30 normal men was 0.205 to 1.372 mmol/day (0.746 +/- 0.344 mmol/day, mean +/- S.D.), while that in male Wistar rats was 4.868 to 7.347 mumol/day (6.077 +/- 2.289 mumol/day, mean +/- S.D.), as measured in 20 consecutive rat urine samples. The urinary glycolate determination by ion chromatography is simple and not time consuming, requiring a 100-fold dilution of charcoal treated urine and 20 minutes at most for analysis. This method has been proven to be sensitive, specific and reproducible.