The equilibrium constants for the reactions of diethyl phosphofluoridate and eel acetylcholinesterase, bovine red cell acetylcholinesterase, horse serum butyrylcholinesterase, and bovine α-chymotrypsin were measured. The values obtained were 2.3 x l04, 3 x 105, 6.4 x 106, and 6 X 108, respectively, in terms of analytical concentrations at pH 7.0 and 25°, with water activity as unity. These were converted to equilibrium constants for the hydrolysis of the diethylphosphoryl-enzyme derivative: 5.2 x l010, 4 x 109, 2 x 108, and 2 x 106, respectively. These values enable one to calculate the equilibrium constant for the reaction of the enzymes with any diethyl phosphate ester whose equilibrium constant for hydrolysis is known. If the rate constant for inhibition is also known, the rate constant for reactivation of the inhibited enzyme can be calculated. These calculations were made for diethyl p-nitrophenyl phosphate and the conjugate reactivator p-nitrophenol and checked experimentally for the eel enzyme.
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