An inexpensive method for sensitive enzymatic determination of oxalate in urine and plasma

In this simple, sensitive, and rapid enzymatic method for the determination of oxalate in urine or plasma, oxalate oxidase (EC 1.2.3.4) prepared from barley seedlings is used to convert oxalate to carbon dioxide and hydrogen peroxide, which is determined photometrically at 600 nm, with use of horseradish peroxidase, by oxidative coupling of 3-methyl-2-benzothiazoline hydrazine with N,N-dimethylaniline. Plasma is pre-treated by ultrafiltration and co-precipitation of oxalate with calcium sulfate and ethanol, urine by dilution and reversed-phase chromatography on C18 columns. Analytical recovery for urine is 99 +/- 2%, for plasma 92 +/- 3%. The normal range for urinary excretion is 0.10 to 0.56 mmol/24 h, and for the concentration in plasma 0.4 to 3.7 mumol/L. There were no significant sex-related differences in urinary excretion or plasma concentration. Our within- and between-assay coefficients of variation were, respectively, less than 3.4% and less than 6.0% for urine, and less than 1.5% and less than 4.3% for plasma.