SPECIFICITY IN CHOLINESTERASE REACTIONS 185 



normally acetylated, Fig. 7 (Wilson, 1951). But whereas the acetyl enzyme 

 reacts with water in microseconds, the phosphoryl enzyme requires many hours 

 and the enzyme is therefore inhibited. 



If we want to reactivate such an inhibited enzyme, we have to use a 

 nucleophilic agent, something which will displace the enzyme by making a 

 nucleophilic attack on the phosphoryl phosphorus atom. Hydroxylamine is an 

 active nucleophilic agent and it is interesting that hydroxylamine will reacti- 

 vate enzyme inhibited by this material. Nucleophilic reagents will in general 

 reactivate any enzyme which is inhibited by such materials. When I say "any", 

 I take some liberty because I have tested only four, chymotrypsin, serum 

 esterase, cholinesterase and liver esterase. 



However, the anionic site is not affected, and since we have seen that it can 

 contribute to the activity of this enzyme one would think that if we could 

 combine a quaternary structure with a nucleophilic functional group, as for 

 example in pyridine-2-aldoxime methiodide (Wilson and Ginsburg, 1955), a 

 very active compound might result. This compound is, in fact, quite active 

 and is on the order of 50,000 times as active as hydroxylamine. It will reacti- 

 vate at 10-^ M in one minute. It is really a very potent reactivation agent, and 

 interestingly enough it is good only for this enzyme. It is not good for liver 

 esterase; it is not much good for serum esterase; nor for chymotrypsin. It just 

 fits the special nature of this enzyme. It illustrates the promoting effect of the 

 anionic site. 



References 



Adams, D. H. and V. P. Whittaker. 1950. The cholinesterases of human blood. II. 



The forces acting between enzyme and substrate. Biochim. et Biophvs. Acta 4: 



543-558. 

 Augustinsson, K. B. and D. Nachmansohn. 1949. Distinction between acetylcholine 



esterase and other choline ester-splitting enzymes. Science 110: 98-99. 

 Bergmann, F., I. B. Wilson, and D. Nachmansohn. 1950. Acetylcholinesterase. IX. 



Structural features determining the inhibition bv amino acids and related com- 

 pounds. J. Biol. Chem. 186: 693-703. 

 Wilson, I. B. 1951. Acetylcholinesterase. XL Reversibility of tetraethyl pyrophosphate 



inhibition. J. Biol. Chcm. 190: 111-117. 

 Wilson, I. B. 1952. Acetylcholinesterase. XII. Further studies of binding forces. J. 



Biol. Chem. 197: 215-225. 

 Wilson, I. B. and F. Bergmann. 1950. Cholinesterase. VII. Active surface of acetyl- 



chohne esterase derived from effects of pH on inhibitors. J. Biol. Chem. 185: 



479-489. 

 Wilson, I. B. and F. Bergmann. 1950. Acetylcholinesterase. VIII. Dissociation con- 

 stants of the active groups. J. Biol. Chem. 186: 683-692. 

 Wilson, I. B., F. Bergmann, and D. Nachmansohn. 1950. Acetylcholinesterase. X. 



Mechanism of the catalysis of acylation reactions. J. Biol. Chem. 186: 781-790. 

 Wilson, I. B. and E. Cabib. 1956. Acetylcholinesterase: Enthalpies and entrophies of 



activation. J. Am. Chem. Soc. 78: 202-207. 

 Wilson, I. B. and S. Ginsburg. 1955. A powerful reactivator of alkylphosphate-in- 



hibited acetylcholinesterase. Biochim. et Biophvs. Acta 18: 168-170. 



