ATP, ADP, and magnesium mixed solutions: in vitro 31P NMR characterization and in vivo application to lens

The difference between the 31P NMR resonance position of the beta-peak of ADP and the gamma-peak of ATP at a fixed monovalent ion concentration level and pH is shown in vitro to depend only on free magnesium concentration. This difference can vary by more than 1 ppm depending on the pH of the solution and free magnesium concentration. Using 31P NMR and ion-selective electrodes we have constructed experimental curves to show how calibration of experimental results can be achieved. Theoretical calculations can be used to predict the functional dependence of the chemical shift difference on the above parameters. However, using the reported ion dissociation constants the fit was not exact and the discrepancy increased at higher pH values. To demonstrate that this technique can yield valid in vivo results and to analyze a previously unreported system where free magnesium levels vary, sample spectra from lenses and enucleated eyes are given. The data show a disagreement between the present methods and the more conventional ATP method. The reason for this difference is not known although some possible reasons are suggested.