320 



R. BERMAN, I. B. WILSON, D. NACHMANSOHN 



VOL. 12 (1953) 



uM ester 

 formed 



centration is about the Michaelis-Menten constant for acetyl CoA. The immsasurably 

 low reactivity of butyryl CoA is not, however, due to poor binding. This is illustrated b}- 



the fact that butyryl CoA in equal concentration 

 with acetyl CoA inhibits the rate of acylation of 

 choline by approximately 80%. 



Fig. 3. Effect of the acyl group of CoA derivatives on the 

 acylation of choHne by choHne acetylase. The reaction 

 mixture contained 140 ^g enzyme and the following com- 

 ponents in fiM per ml : choline 20, phosphate buffer, pH 7, 

 90, acyl CoA either 4 or 8. Total volume i ml, t = 31° C. 



h 4 /iiM acetyl CoA 



+ 8 /nM acetyl CoA 



O O 4 juM propionyl CoA 



# # 8 fiM propionyl CoA 



D 



*-mm 



D 4 and 8 /nM butyryl CoA resp. 



£ /[juM acetyl Co A + 4 ^Af butyryl Co A . 



DISCUSSION 



The specificity patters of choline acetylase assumes enhanced biological interest when 

 the acetylase is recognized as part of a physiological system, the acetylcholine system, 

 which is directly involved in conduction. It is important to compare this enzyme with 

 the other members ot the system to ascertain if any underlying unity exists. 



It is consequently pertinent to describe, albeit very briefly, the mechanism of the 

 system as it is presently envisaged^-^. Acetylcholine is in a bound state during rest; it 

 is released in activity to react with a receptor and is finally destroyed by acetylcholine- 

 esterase, a step essential for the rapid short range recovery of function. Complete re- 

 covery in a long range sense is accomplished by the resynthesis of acetylcholine mediated 

 by choline acetylase. 



The receptor, presumably a protein or a conjugated protein, is assumed to determine 

 the membrane characteristics associated with such phenomena as ion permeability and 

 electrical potential. The combination of ACh with the receptor alters its properties per- 

 haps through change in configuration which brings about the characteristic changes in 

 electrical potential, membrane resistance, and ionic flow observed during conduction 

 of the nerve impulse and other types of conduction. 



Four functions which appear to reside in four discrete proteins, have thus been 

 delineated: storage, receptor, destroying and synthesizing functions. Of these proteins 

 only the two enzymes, the acetylase and the esterase, have been obtained and studied 

 in solution. The receptor has been studied by the response of intact structures so that the 

 observations are affected by secondary features, e.g. permeability properties. We have 

 little knowledge concerning the storage protein and therefore it will not be considered 

 further. On the other hand, the esterase has been extensively studied with regard to its 

 specificity pattern, forces of interaction and hydrolytic mechanism--^^-^^. This infor- 

 mation may be of considerable value in understanding the interaction of ACh with the 

 other members of the system and aid in the design and interpretation of experiments 

 particularly in the case of the receptor and storage proteins where direct experiments 

 have not been carried out as yet. This follows from the fact that a small molecule, such 



References p. 324. 



