EQUILIBRATION OF ADENYLATES BY MAIZE LEAF ADENYLATE KINASE - EFFECTS OF MAGNESIUM ON APPARENT AND TRUE EQUILIBRIA

In this study, the effect of magnesium on equilibration of adenylates by purified maize leaf adenylate kinase (AK) was investigated. The equilibration was expressed in terms of either apparent equilibrium constant, defined as K(app) = (ATP(total))(AMP(total))/(ADP(total))2, or true equilibrium constant, defined as K(true) = (Mg-ATP)(AMP(free))/(Mg-ADP)(ADP(free)). At a fixed concentration of free magnesium (1.8-1.9 mM), the K(app) and K(true) were constant at 0.76 +/- 0.10 and 6.02 +/- 0.75, respectively. On the other hand, at the free magnesium range of 0.0014 to 8.3 mM, the K(app) varied from 0.30 to 1.27, while K(true) remained constant at 5.93 +/- 0.31. The data indicate that, contrary to previous speculations, leaf AK does not maintain an equilibrium of total adenylates. Rather, the enzyme governs an equilibrium of Mg-ADP, free ADP, Mg-ATP, and free AMP, which are the true substrates/products of the AK reaction. Some implications of this finding for studies on energy metabolism in plant tissues are discussed.