COMPETITION BETWEEN LI+ AND MG2+ FOR ATP AND ADP IN AQUEOUS-SOLUTION - A MULTINUCLEAR NMR-STUDY

We used Li-7 NMR spin-lattice relaxation times and P-31 NMR chemical shifts to study the binding of Li+ and Mg2+ to the phosphate moieties of ATP and ADP. To examine the binding of Li+ and Mg2+ to the base and ribose moieties, we used H-1 and C-13 NMR chemical shifts. The Li-7 NMR relaxation times of Li+/Mg2+ mixtures of ATP or ADP increased with increasing concentrations of Mg2+, suggesting competition between the two ions for adenine nucleotides. No significant binding of Li+ and Mg2+ to the base and ribose moieties occurred. At the pH and ionic strength used, 2:1 and 1:1 species of the Li+-ATP and Li+-ADP complexes were present, with the 2:1 species predominating. In contrast, 1:1 species predominated for the Mg2+-ADP and Mg2+-ATP complexes. We calculated the Li+-nucleotide binding constants in the presence and absence of Mg2+ and found them to be somewhat greater in the presence of Mg2+. Although competition between Li+ and Mg2+ for ATP and ADP phosphate binding sites in solution is consistent with the P-31 chemical shift data, the possibility that the Li+ and Mg2+ form mixed complexes with the phosphate groups of ATP or ADP cannot be ruled out.