420 ANNALS NEW YORK ACADEMY OF SCIENCES 



as high as that of synthesis. It is doubtful whether these figures in- 

 dicate the real difference of the possible rates of the two enzymes. 

 Cholinesterase is an extremely stable enzyme. Its activity is deter- 

 mined in a well ground and homogenized suspension of the tissue. It 

 appears probable that the maximal possible activity is actually meas- 

 ured in vitro. This is almost certainly not the case with choline 

 acetylase. The enzyme is an extremely labile and a rather complex 

 system which has to be extracted from the tissue. During the prepara- 

 tion, part of the activity may have been lost. We do not know 

 whether the conditions used at present are optimal or even close to 

 optimal. Although the enzyme was discovered three years ago, the 

 rates of formation obtained are still continuously increasing, since more 

 and more factors are becoming known which activate the enzyme 

 (Nachmansohn and Berman, unpublished experiments). In such a 

 case, it is possible and, in fact, probable, that the activity in vivo may 

 be considerably higher than that observed in the solution. A sharp 

 distinction has, moreover, to be made between the potential and the 

 actual rate. Rates of enzymes measured in vitro are potential rates. 

 The actual rates in the living cell may be entirely different. Many 

 enzymes are present in excess in the cell. An excess of 3 to 5 times 

 above the actual requirement is nothing unusual. A 5-fold excess of 

 cholinesterase above that necessary for function has been recently 

 observed in the case of lobster nerve (Bullock et aL*^) . Other enzymes 

 are in much greater excess, whereas, in some cases, the excess activity 

 appears to be relatively small. Nothing is known, at present, as to 

 whether or not choline acetylase is present in excess. Even if this 

 is the case, it may be much smaller than that of cholinesterase. 



For an understanding of the problem, the decisive difference which 

 has to be considered is not the difference of rates, but the difference of 

 function. There is a fundamental difference between the function of 

 cholinesterase and that of choline acetylase. If the release of ACh 

 is an essential event in the alterations of the membrane during the 

 passage of the impulse, then the active ester has to be destroyed within 

 a millisecond or less, so that the resting condition may be restored. 

 Therefore, the enzyme which removes the active ester, cholinesterase, 

 has to be very active, but only during this brief period, and may then 

 be inactive until the passage of the next impulse. The formation of 

 ACh, on the other hand, need not be such a rapid process. It is gen- 

 erally assumed that the active ester is released from an inactive form. 

 This is supported by the fact that the primary energy released during 

 recovery is used for the synthesis of ACh, thus implying that the syn- 



