AL3+ BINDING BY ADENOSINE 5'-PHOSPHATES - AMP, ADP, AND ATP

To assess quantitatively the ability of Al3+ to complete with Mg2+ in many vital processes, we have determined the stability constants for Al3+ binding to adenosine 5'-phosphates. With ATP, Al3+ forms an equimolar complex 4000 times stronger than does Mg2+, and stronger than most metal ions including Cu2+. In addition to an equimolar complex, Al3+ also forms a complex with 2 mol of ligand that dominates in slightly acidic and neutral solutions, from which a deprotonation occurs to form a mixed hydroxo complex. ADP forms only slightly weaker complexes than ATP. Because Al3+ binding occurs primarily at the basic, terminal phosphate, the results also apply to other nucleotides such as GTP. Binding by 10 mM nucleoside triphosphate at pH 6.6 limits the free Al3+ to 3 x 10(-9) times the total aluminum concentration. ATP stands as the predominant binder of Al3+ in many cells. In neutral solutions, citrate withdraws Al3+ from ATP. In a low citrate medium, catecholamines extract Al3+ from ATP. The 10(7.5) times greater effectiveness of Al3+ over Mg2+ in promoting tubulin polymerization to microtubules derives from the 10(3.6) times greater binding strength of Al3+ over Mg2+ to GTP coupled with the 10(3.9) times greater binding strength of GTP-Al3+ over GTP-Mg2+ to tubulin.