Potential pitfall in the determination of free [Mg2+] by 31p NMR when using the β/α-ATP peak height ratio method

Recently, Clarke et al. (Clarke K, Kashiwaya Y, King MT, Gates D, Keon CA, Cross HR, Radda GK, Veech RL. The beta/alpha peak height ratio of ATP. A measure of free [Mg(free)(2+)] using (31)p NMR, J. Biol. Chem. 1996;271:21142-21150.) reported a new method to noninvasively determine the concentration of intracellular free magnesium ([Mg(free)(2+)]) based on the measurement of the peak height ratio h(beta/alpha) of the beta- and alpha-ATP signals in (31)p NMR spectra, h(beta/alpha) varies with [Mg(free)(2+)], however, the study presented here shows that h(beta/alpha) also strongly depends on the homogeneity of the static magnetic field. For this reason, we performed at a magnetic field strength of 1.5 T (31)p NMR measurements of solutions that mimic intracellular medium. The magnetic field homogeneity was varied by changing the currents in the shim coils, and the effect on h(beta/alpha) is demonstrated with and without proton decoupling. In both cases, h(beta/alpha) strongly depends on the magnetic field homogeneity and can therefore lead to a pitfall in the determination of [Mg(free)(2+)]. (C) 1999 Elsevier Science B.V. All rights reserved.