DAVID NACHMANSOHN 



Choline Esterase 



Time Factor. One of the essential results of these enzyme 

 studies is the evidence of the high rate of acetylcholine metabolism in 

 nerve tissue. Significant amounts may be split in milliseconds, that 

 is, a period of time sufficient for the passage of the impulse. Conse- 

 quently, the potential rate of acetylcholine metabolism is sufficiently 

 high to justify the assumption that it parallels the rate of the electric 

 changes and may therefore be directly connected with the nerve action 

 potential. 



The frog sartorius muscle is a special case in which this problem 

 of the time factor has been studied and has received a satisfactory 

 answer. A small fraction of this muscle is free of nerve endings. By 

 determining the concentration of choline esterase in this part of the 

 muscle, in the part containing nerve endings, and in the nerve fibers, 

 it is possible to calculate the concentration of choline esterase at the 

 motor end plates. Since the number of end plates in a frog's sartorius 

 is known, the amount of acetylcholine which may be split during one 

 millisecond can be calculated. This turns out to be 1.6 X 10^ mole- 

 cules of the ester. About ^/s of the enzyme at the motor end plate 

 is localized inside the nerve ending. On the assumption that one 



o 



molecule of acetylcholine covers about 20 to 50 sq. A., the amount 

 which may be hydrolyzed during one millisecond at one end plate 

 would cover a surface of 100 to 250 sq. p.. 



Localization at the Neuronal Surface. A high concentration of 

 choline esterase, of similar order of magnitude to that at motor end 

 plates, exists at all synapses, whether central or peripheral. But the 

 difference between axon and synaptic region is only quantitative. 

 Fibers with a very thin myelin sheath, until recently considered as 

 nonmyelinated, like the sympathetic chain of mammals or the abdom- 

 inal chain of lobster, offer a favorable material for demonstrating this 

 distribution. Hence, the rate of acetylcholine metabolism may be 

 high everywhere in nerves. 



Experiments on the superior cervical ganglion of cats indicate 

 that the enzyme might be concentrated at or near the neuronal surface. 

 The giant axon of squid, another special case, was used for testing such 

 an assumption. The axoplasm may be extruded and thus separated 

 from the sheath. Most of the sheath is connective tissue to which are 



