VOL. 4 (1950) 



ACYLATIONS BY ACETYLCHOLINE ESTERASE II 



313 



moderation of the action of esterase inhibitors by way of regulation of p^ might be 

 a matter of some practical as well as theoretical interest, further study of the pn- 

 dependence of esterase-inhibitor inter- 

 actions appears desirable. 



Fig. 2. Acetylcholine hydrolysis as a function o, 

 of ph- Curve i : Hydrolysis of acetylcholine i 

 in the presence of enzyme. Relative initial ^ 

 reaction rates corrected for enzymatic hydro- 

 lysis are plotted on the ordinate. The curve 

 is a composite of data given in Tables I and 

 II. Values for pn 7-8 in phosphate, and pn 

 8.1 and 8.5 in borate are taken equal to 10. 

 Curve 2 : Hydrolysis of acetylcholine in ab- 

 sence of enzyme. Acetylcholine concentration 

 4 //M/ml. Ph was regulated with borate buffer. 

 Initial reaction rates are plotted on the ordi- 

 nate. The value for pn 10.6 is taken equal 

 to 10. The temperature was 21° C. 



TABLE III 



INFLUENCE OF CHOLINE ON ACETYLCHOLINE HYDROLYSIS AT DIFFERENT pH VALUES 



Reaction mixtures contained a constant amount of enzyme, acetylcholine chloride 4 //M/ml, choline 

 chloride (or sodium chloride) 12.5 /^M/ml, potassium phosphate o.i M, sodium chloride 0.05 M, magne- 

 sium chloride 0.02 M, gelatin 0.07% and different amounts of sodium hydroxide. Temperature 37° C. 



B. SYNTHESIS OF ACETYL- AND PROPIONYLCHOLINE BY THE ACTION OF PURIFIED 



ACETYLCHOLINE ESTERASE 



The equilibrium constant of esterification reactions favours strongly the reaction 

 direction of hydrolysis^^. Earlier investigators^^ observed that the pharmacological 

 activity of choline is enhanced by incubation with acetate in the presence of crude 

 tissue preparations of esterase. Demonstration of this synthesis and measurement of 

 the equilibrium was greatly facilitated in the present work by the availability of the 

 hydroxylamine method which could be applied to the determination of the ester in the 

 presence of a large excess of the products of the hydrolysis. 



Figs 3 and 4 analyse the effect of pn on the equilibrium position of the hydrolysis 

 of acetylcholine and propionylcholine respectively by the purified esterase. The approach 

 to equilibrium at three selected p^was realized in each case from both reaction directions. 

 References p. 321. 



