Molecular Pharmacology, Vol 1, 60-65, Copyright © 1965 by the American Society for Pharmacology and Experimental Therapeutics

The Free Energy of Hydrolysis of Diethylphosphoryl Acetylcholinesterase

¹ Departments of Biochemistry and Neurology, Columbia University College of Physicians and Surgeons, New York, New York

The equilibrium constant for the reaction of diethylphosphofluoridate with acetylcholinesterase was evaluated by measuring the second-order rate constant for the inhibition of the enzyme and the second-order rate constant for the restoration of enzyme activity, starting with inhibited enzyme and using fluoride as a reactivator. The value at pH 7.0, 25° in terms of total concentrations of reactants is 2.3 x 10⁴.

The equilibrium constant for the hydrolysis of the diethylphosphoryl enzyme is estimated to be 2 x 10⁵ in terms of total concentrations of reactants at pH 7.0; if diethylphosphoric acid is written in its acidic form, the equilibrium constant is 0.5. This last value is typical of low energy bonds. Assuming that a diethylphosphorylserine side chain is formed, the energy change in the reaction at the active site has the normal value for the bonds involved.

Note:
ACKNOWLEDGMENTS This work was supported by the Division of Research Grants and Fellowships of the National Institutes of Health, Grant NB00573-17, by United States Public Health Service and Research Career Award GM-K3-15,012, and by National Science Foundation Grant GB 2817.