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Magnesium Research

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Comparison between measured and calculated ionised concentrations in Mg 2+/ATP, Mg 2+/EDTA and Ca 2+/EGTA buffers; influence of changes in temperature, pH and pipetting errors on the ionised concentrations Volume 20, numéro 1, March 2007

Auteurs
Institute of Biomedical and Life Sciences, West Medical Building, University of Glasgow, G12 8QQ, UK, Department of Statistics, Mathematics Building, University of Glasgow, G12 8QW, UK, Institute of Physiology, B±hlplatz 5, 3012 Bern, Switzerland

The apparent dissociation constants (K app) and total ligand concentrations ([Ligand] T) from extensive published and unpublished macroelectrode measurements for Mg 2+/ATP, Mg 2+/EDTA and Ca 2+/EGTA buffers have been recalculated. These calculations were made feasible by the introduction of an Excel program which reduced the time of calculation for K app and [Ligand] T from over an hour to under five minutes. These estimations of K app and [Ligand] T allowed, not only a comparison between measured and calculated ionised magnesium and calcium concentrations ([Mg 2+] and [Ca 2+]) for Mg 2+/ATP, Mg 2+/EDTA and Ca 2+/EGTA buffers but also a comparison amongst calculated values. Calculated [X 2+] values always differed from measured, and calculated values differed amongst themselves by factors of at least 2. These variations cast doubts on the published absolute values for intracellular [Mg 2+] estimated by 31P-NMR and the resting values for [Ca 2+] in cells.The allowable range for [X 2+] in the buffers and consequently for K app and [Ligand] T has not been defined, which introduces uncertainties into published absolute values for [X 2+]. This paper shows that an upper limit of ± 10% deviation from the mean value for [X 2+] is attainable. This requires the temperature to be maintained within ± 0.5°C, pH within ± 0.01 units and pipetting errors of less than 0.25%. Until internationally defined buffer standards are available, the lack of correlation between measured and calculated [X 2+] means that measurement of K app and [Ligand] T and hence [X 2+] is more reliable than calculation.