Magnesium Research
MENUComparison 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
Illustrations
- Mots-clés : Mg 2+ buffers, Ca 2+ buffers, apparent dissociation constant, ATP, EDTA, EGTA, measured/calculated concentrations, imprecision
- DOI : 10.1684/mrh.2007.0092
- Page(s) : 72-81
- Année de parution : 2007
The apparent dissociation constants (K app) and total ligand concentrations ([Ligand] T) from extensive published and unpublished macroelectrode measurements for Mg 2+/ATP, Mg 2+/EDTA and Ca 2+/EGTA buffers have been recalculated.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.