DAVID NACHMANSOHN 



esterase at the neuronal surface, making possible a rate of acetylcholine 

 metabolism sufficiently high to parallel the electrical changes. In 

 electric tissue, the rate may be at least 100,000 times, but is probably 

 close to 1,000,000 times, as high as that of respiration. We must dis- 

 tinguish, however, between the possible rate and the absolute amounts 

 metabolized. Acetylcholine is released and hydrolyzed within a very 

 short period. The actual duration of one thousand discharges is about 

 three seconds. The recovery may require one to two hours during 

 which the rate of respiration may be increased. If the absolute 

 amounts of acetylcholine possibly metabolized are compared with those 

 of the phosphorylated compounds actually metabolized and the 

 observed rate of respiration, a satisfactory pictui-e is obtained. Since 

 this whole chain of reactions connected with the nerve action potential 

 is initiated by the release of acetylcholine, it has been called the 

 "acetylcholine cycle." 



Choline Acetylase 



It appeared essential to test whether or not phosphate bonds are 

 really the energy source of acetylcholine formation as these investiga- 

 tions suggest. Evidence for the correctness of this conclusion would 

 show that the energy of the primary recovery process is really used for 

 the resynthesis of the compound which, by its release, supposedly ini- 

 tiates the nerve impulse. It would, therefore, at the same time, 

 constitute a new support for the assumption that the "excitatory disturb- 

 ance" (Keith Lucas) which produces a propagated impulse may be 

 indeed the release of the ester. Also, we would have another example 

 of the "reconstruction of the chemical events in living cells," to use an 

 expression of Green (8). 



In accordance with the assumption made, a new enzyme, 

 choline acetylase, could be extracted from brain which, in cell-free 

 solution, under strictly anaerobic conditions in the presence of adeno- 

 sine triphosphate, forms acetylcholine (16). 



The enzyme may be extracted from homogenized brain. From 

 one gram of fresh rat or guinea pig brain an enzyme solution may be 

 prepared which forms 120 to 150 ng. of acetylcholine per hour. The 

 presence of eserine and fluoride is necessary to inhibit the action of 

 choline esterase and adenosine triphosphatase, respectively. The 

 enzyme may also be extracted from tlie powder of acetone-dried brain. 



