Enzymatic method for assaying calcium in serum and urine with porcine pancreatic α-amylase

We developed a kinetic assay for calcium in serum and urine, based on the activation of porcine pancreatic α-amylase (EC 3.2.1.1) with 2-chloro-4- nitrophenyl-α-maltotrioside as substrate. The kinetic generation of 2- chloro-4-nitrophenol, monitored at 405 nm, was proportional to the concentration of calcium in serum and urine. The assay was developed and evaluated with the Cobas Bio centrifugal analyzer. The average within-run and between-day imprecision (CV) was 0.96/1.26% and 1.07/1.63%, respectively, for serum calcium and 1.50/2.54% and 1.70/2.64%, respectively, for urine calcium. Results of the proposed method (y) correlated well with those determined by atomic absorption spectrophotometry (x): y(serum) = 1.005x + 0.028 mmol/L (S(y/x) = 0.058, r = 0.974, n = 50), and y(urine) = 1.017x - 0.115 mmol/L (S(y/x) = 0.30, r = 0.981, n = 25). We also present data showing that the method is highly sensitive, rapid, relatively free of interference, and amenable to automation.