Comparison between measured and calculated ionised concentrations in Mg2+/ATP,Mg2+/EDTA and Ca2+/EGTA buffers;influence of changes in temperature, pH and pipetting errors on the ionised concentrations

McGuigan, J., Kay, J.W. , Elder, H.Y. and Luthi, D. (2007) Comparison between measured and calculated ionised concentrations in Mg2+/ATP,Mg2+/EDTA and Ca2+/EGTA buffers;influence of changes in temperature, pH and pipetting errors on the ionised concentrations. Magnesium Research, 20(1), pp. 72-81. (doi: 10.1684/mrh.2007.0092) (PMID:17536492)

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Abstract

The apparent dissociation constants (K app) and total ligand concentrations ([Ligand] T) from extensive published and unpublished macroelectrode measurements for Mg 2+/ATP, Mg 2+/EDTA and Ca 2+/EGTA buffers have been recalculated.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.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:McGuigan, Prof John and Elder, Dr Hugh and Kay, Dr James
Authors: McGuigan, J., Kay, J.W., Elder, H.Y., and Luthi, D.
College/School:College of Medical Veterinary and Life Sciences
College of Science and Engineering > School of Mathematics and Statistics > Statistics
Journal Name:Magnesium Research
Publisher:John Libbey Eurotext
ISSN:0953-1424
ISSN (Online):1952-4021

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