Relationships between the rate of synthesis of ATP and the concentrations of reactants and products of ATP hydrolysis in maize root tips, determined by ³¹P nuclear magnetic resonance

Using ³¹P NMR spectroscopy, we have measured the rate of ATP synthesis, and the free concentrations of ATP, ADP, cytoplasmic P_i, and H⁺ in maize root tips under a wide range of conditions. We show that the ratio | [ATP] [ADP] in normoxic root tips is >25. We found no simple relationship between the concentration of ATP and the rate of ATP synthesis: when the rate of ATP synthesis decreases in response to different treatments, the concentration of ATP can increase, decrease, or remain unchanged. Clear relationships were obtained, however, when the rate of synthesis of ATP was plotted against the logarithm of the ratio ψ, defined as [ATP] [ADP][P_i][H⁺]. Two curves were obtained, depending on which of two situations pertained. First, if mitochondrial ATP synthesis was inhibited, e.g., by KCN or hypoxia, ln ψ decreased monotonically as rates of ATP synthesis decreased. The decrease in ln ψ may account for decreases in the rates of biosynthetic reactions dependent on ATP, such as protein synthesis, as they approach equilibrium. Second, if consumption of ATP for biosynthetic reactions was inhibited, by treatment with succinate, ln ψ increased as rates of ATP synthesis decreased. The increase in ln ψ may account for decreases in the rate of ATP synthesis, as oxidative phosphorylation approaches equilibrium.