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 Printable version |
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 |
Magnesium Research. Volume 20, Number 1, 72-81, March 2007, Original article
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 Free Article
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Author(s) : John AS McGuigan, James W Kay, Hugh Y Elder, Daniel LÜthi |
Summary : 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T) from extensive published and unpublished macroelectrode measurements for Mg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. |
Keywords : Mg
2+ buffers, Ca
2+ buffers, apparent dissociation constant, ATP, EDTA, EGTA, measured/calculated concentrations, imprecision |
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