The effect of chelators on Mg2+, Na+-dependent phosphorylation of (Na+ + K+)-activated ATPase

1. The effect of free Mg2+, MgEDTA and MgCDTA on the phofphorylation of the (Na+ + K+)-activated ATPase (ATP phosphohydrolase, EC 3.6.1.3) has been studied. 2. 10 mM trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA) added simultaneously with [gamma-32P]ATP to a solution containing the enzyme, 1 mM Mg2+ and 150 mM Na+ does not prevent formation of phospho-enzyme. When [gamma-32P]ATP is added after CDTA the level of phospho-enzyme obtained decreases with increase in the time interval between addition of CDTA and ATP. The inability of CDTA to prevent the formation of phospho-enzyme becomes more pronounced when the medium contains MgEDTA. In the presence of CDTA the maximum amount of phospho-enzyme formed increases with the MgEDTA concentration. 3. Without CDTA the steady-state level of phospho-enzyme is directly proportional to the logarithm of free Mg2+ concentration. Neither with suboptimal nor with optimal concentrations of free Mg2+ does MgEDTA have an effect on the level of phospho-enzyme formed. 4. Using the phospho-enzyme level as a measure of free Mg2+ the experiments show that CDTA reacts slower with Mg2+ than does EDTA, but the stability constant of MgCDTA complex is higher than of MgCDTA, complex. 5. Due to the higher stability constant, of MgCDTA, as compared to MgEDTA, addition of CDTA to a medium containing free Mg2+ and MgEDTA will not only chelate the free Mg2+, but it will also shift the equilibrium from MgEDTA towards MgCDTA, i.e. MgEDTA acts as a source of free Mg2+ which is then chelated by CDTA. The experiments show that it takes minutes before Mg2+, EDTA and CDTA come to equilibrium. Provided the dissociation of MgEDTA is faster than the formation of the MgCDTA complex, the medium will contain a concentration of free Mg2+ which at any given instant is near in equilibrium with a slowly decreasing concentration of MgEDTA; this free Mg2+ can support phosphorylation. This can explain why the rate with which CDTA stops phosphorylation decreases with an increase in the MgEDTA concentration. 6. When phosphorylation is stopped by addition of unlabelled ATP, the rate of dephosphorylation is faster than when it is stopped by addition of CDTA both with and without EDTA in the medium. CDTA reacts too slowly with Mg2+ to be used as a chelator in studies where a fast removal of Mg2+ is required. 7. A previous finding has been verified, namely that the rate of spontaneous, of K+-stimulated and of ADP-stimulated dephosphorylation is independent of the Mg2+ concentration during formation of phospho-enzyme.