Simultaneous determination of intracellular magnesium and pH from the three 31P NMR Chemical shifts of ATP.

The simultaneous determination of intracellular [MgT]/[ATPT] and pH from the three 31P NMR chemical shifts of ATP has been demonstrated using two-dimensional calibrations. The resulting pH will more accurately represent that of healthy tissue than by using the standard NMR technique. As a result of other possible complexes of MgATP and uncertainties in intracellular pH, errors in the values of KMgATPD (pH), the MgATP NMR shift limits, and thus intracellular magnesium levels are reduced by using this self-consistent analytical method. Direct determination of free magnesium from the (gamma-beta) shifts of ATP may be more sensitive in the alkaline pH range than with the commonly used (alpha-beta) shifts. In addition, the calibration data sets allow for a graphical representation of the uncertainty in magnesium due to the uncertainty in the measured chemical shifts and pH. Our results indicate that KMgATPD is consistent with the literature and favors a value near 50 microM at pH 7.2, but that free magnesium levels will be lower than most prior estimates.