DETERMINATION OF URINARY OXALIC-ACID AND OF OXALATE POOL SIZE BY STABLE ISOTOPE-DILUTION

An isotope dilution procedure for oxalate based upon [1,2-13C2]oxalic acid is described. For routine determinations of urinary concentration, a known quantity of sodium [1,2-13C]oxalate is admixed with the sample, total oxalate precipitated as the Ca salt, and converted by BF3 catalysis to di-n-propyl esters for mass-spectrometric analysis. Selective ion monitoring provides 12C:13C ratios directly, thus precluding the necessity for quantitative recovery at any step of the rapid, single-tube assay. Following a bolus injection of sodium [1,2-13C]oxalate [in beagles], whole body oxalate pools and their turnover rates can be determined by sequential sampling of urine. Biosynthetic rates calculated from the product of pool size and turnover are in excellent agreement with urinary excretion rates, confirming directly that urinary oxalate is a quantitative index of biosynthesis.