A Simple Enzymic Method for the Measurement of Magnesium in Serum and Urine on a Centrifugal Analyser

Abstract

SUMMARY.A method is described based on a reaction that requires magnesium-ATP as a co-factor for the activity of hexokinase (HK), coupled with glucose 6-phosphate dehydrogenase (G6PDH). Glucose and NADP are converted to D-gluconolactone 6-phosphate and NADPH. respectively. The rate of increase in absorbance at 340 nm due to the formation of NADPH is proportional to the magnesium concentration in the sample. Magnesium levels in serum and urine measured by the enzymic method compared well with those obtained by atomic absorpton spectrometry.The within-batch precisions were 1·4% and 1·5% for the enzymic method and the atomic absorption method. respectively for a quality assurance sample with a magnesium concentration of 2 mmoVL. The enzymic method is accurate (recoveries of added magnesium to serum samples are 101-102%), reproducible (between batch CV 2·8%). and rapid (23 samples may be measured in to min). Data on accuracy, precision and correlation for the enzymic and atomic absorption methods are presented. concentration. This method is. however. also time-consuming and not well suited for use in a routine laboratory. The end point assay is a three-step manual procedure, measured at 30"C. involving a 5 min pre-incubation. a 15 min incubation in the presence of the enzyme mixture and the addition of EDTA to stop the reaction. We describe the automation of the enzymic method to give a single step kinetic assay at 37°C using a centrifugal analyser. The method simplifies batch analysis and gives savings on reagent costs. An annual supply of all the reagents would be less than £100 for 25 mL of reagent mixture per week which is sufficient for 46 samples plus the standards for calibration.The effect of different concentrations of reagent components on the linearity of the reaction rate at 3JOC has been examined; this is particularly important as the reaction is measured kinetically rather than as an endpoint assay. Good correlation of the enzymic method with the atomic absorption method has been demonstrated using serum and urine samples. Analytical recovery and interference 'studies are reported.

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“…Urinary Mg concentrations were measured with the Cobas-Bio centrifugal analyser using an enzymic method (Tsang et al 1988). Reagents were purchased from Sigma Chemicals Ltd and the Mg(NO,), standard from BDH Ltd, Poole, Dorset.…”

Study of Relation between Serum Magnesium Levels and Vascular Complications in Patients of Type 2 Diabetes Mellitus

“…Coupling hexokinase glucose-6-phosphatedehydrogenase allows the reaction to be monitored at 340 nm with the formation of NADPH. 35 Today photometric methods are more commonly used by clinical laboratories, although AAS considered the reference method is also used by some laboratories. 36…”