Calculation of free-magnesium concentration in adenosine 5'-triphosphate containing solutions in vitro and in vivo

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Abstract
An attempt was made to resolve the differences between the levels of free Mg2+ in muscle calculated by Wu et al. (1981) (2.5 mM in guinea pig heart) and by Gupta and Moore (1980) (0.6 mM in frog skeletal muscle) on the basis of substantially identical measurements by 31P NMR of the phosphate peaks in the spectrum of MgATP2-. The differences depend on the method of calculation, including which reactions in which multiple equilibria are being considered. Biochemists and physical chemists customarily use different working definitions of the stability constant for MgATP2- in particular. Wu et al. used in their calculations, without reconciliation, methods involving 3 different operational definitions of the chelation equilibria involved. An algorithm for calculating Mg2+ and total ATP, which can be carried out with a hand calculator, is described here. With it, Mg2+ levels were calculated that agree with those determined by Gupta et al. (1978) with their in vitro systems. Therefore, the finding of Gupta and Moore that the Mg2+ level in skeletal and cardiac muscle is 0.6 mM is probably correct.