ASSAY OF MAGNESIUM IN SERUM AND URINE WITH USE OF ONLY ONE ENZYME, ISOCITRATE DEHYDROGENASE (NADP(+))

We report a method for assaying magnesium in serum and urine involving only one enzyme, isocitrate dehydrogenase (NADP(+))(EC 1.1.1.42), whi ch requires magnesium ion for activity. The enzymatic reduction of NAD P(+) by isocitrate increases in rate linearly up to at least 20 mmol/L magnesium in the presence of appropriate concentrations of the two me tal-chelating reagents, EDTA and glycol ether diamine-N,N,N',N'-tetraa cetate. Within-run (n = 20) CVs and day-to-day (n = 10) CVs for sera a re less than or equal to 1.5% and less than or equal to 2.6%, respecti vely. Analytical recovery of magnesium in sera averages 96-100%. This method is not affected by bilirubin, hemoglobin, or lipemia. The metho d (y) gives the following results correlating with atomic absorption s pectrophotometry (x): y = 1.03x + 0.06 mmol/L (n = 62, r = 0.995, S-y\ x = 0.03) for sera, and y = 1.03x - 0.10 mmol/L (n = 62, r = 0.989, S- y\x = 0.19) for urines; with the calmagite method (x): y = 0.99x + 0.0 4 mmol/L (n = 62, r = 0.991, S-y\x = 0.03) for sera, and y = 0.98x + 0 .03 mmol/L (0 = 62, r = 0.999, S-y\x = 0.02) for urines.