Sodium oxalate corrects calcium interference in Lowry protein assay.

We have previously noted that high levels (10 mM) of calcium interfere with protein determination in the Lowry assay. While there are numerous reports in the literature of substances interfering with the Lowry assay, none of these interfering substances are as ubiquitous as calcium. We hypothesized that physiologic levels of calcium may interfere with the protein determination as measured by the Lowry assay. To test this hypothesis, bovine serum albumin standards from 12.5 to 1600 micrograms/ml were tested in the Lowry assay in the presence of calcium chloride concentrations from 0 to 5 mM. In low protein concentrations (12.5-50 micrograms/ml) 1 to 2 mM calcium induced errors of up to 384%. In samples containing 100 to 400 micrograms/ml protein, 1 to 2 mM calcium induced errors of up to 40%. The addition of sodium oxalate to calcium-spiked protein samples significantly reduced the errors in protein determination at all calcium concentrations tested. This effect also is seen throughout the range of protein concentrations tested (50-1600 micrograms/ml). In most cases sodium oxalate pretreatment of samples used in the Lowry assay reduces errors by 70-95%. Physiologic levels of calcium induce significant errors in the estimation of protein concentration using the Lowry method. Routine pretreatment of Lowry samples with sodium oxalate will provide a more accurate estimation of the protein concentration in biological systems.