We developed an assay for serum magnesium (Mg) by coupling phosphoglucomutase (EC 184.108.40.206) with glucose-6-phosphate dehydrogenase (EC 220.127.116.11). The kinetic generation of NADPH by the action of the above two enzymes upon glucose-1-phosphate (G-1-P) and glucose-1,6-diphosphate (G-1,6-P) was proportional to the concentration of Mg in serum, and was monitored at 340 nm. The average within-run and day-to-day imprecision (% CVs), as determined from 10 replicate analyses for three sera with different Mg concentrations, were 0.8 to 2.1% and 1.9 to 2.7%, respectively. For 60 clinical samples, including several with lipemia and hemolysis, our method showed good agreement with atomic absorption spectrophotometry and the Xylidyl Blue method. We also present data showing that the method is highly sensitive, rapid, relatively free of interference, and amenable to automation.
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