STRUCTURE OF NERVE CELL MEMBRANES 143 



Since we are going to examine the consequences of having ACh-ase activity 

 localized in some of the interspaces of the membrane, it is worthwhile to state 

 just what these are. With respect to the inhibition of the enzyme, it must be 

 possible to accomplish this without filling more than a fraction of the inter- 

 spaces with inhibitor. Because the membrane is concerned with the transfer of 

 materials associated with the operation of cellular metabolic processes, the 

 total inhibition of such transport may be expected to be incompatible with 

 function even over short intervals of time. The inhibitors that are spoken of 

 as 'competitive' are, according to this interpretation, incorrectly so classified 

 because their principal contribution to ACh-ase inhibition is the decrease in 

 mean interspace size that they bring about. This action effectively shuts off 

 access of substrate to enzyme without the requirement that an inhibitor mole- 

 cule must occupy every active site. Similarly, the phenomenon of substrate 

 inhibition shown so strongly by ACh (Augustinsson and Nachmansohn, 1949) 

 may be brought about in part by a mechanism similar to that suggested for 

 inhibitors, rather than by the formation of a 'super complex'. Interestingly, 

 ethyl chloroacetate which lacks the two points of attachment generally re- 

 quired for complex formation, shows substrate inhibition while the tertiary 

 analog of ACh, dimethylaminoethyl acetate which binds almost as well as 

 ACh and has two points of attachment, does not (Berman et al, 1953), sug- 

 gesting that both the quaternary structure and a CI atom of chloroacetate 

 distort the interspace distribution because of their non-conforming shapes, 

 while the tertiary structure is a better fit. 



The apparent concentration of enzyme in vitro and in vivo is of importance 

 for the present discussion because it cannot be claimed that the greatest amount 

 of ACh-ase can be extracted from tissues containing the largest amounts. 

 Quite aside from the large losses caused by adsorption to cellular components, 

 if, as has been suggested, enzyme activity depends upon some supramolecular 

 organization, a premium is placed upon those methods of extraction that 

 favor molecular aggregation of various specified sorts. In the membrane, 

 enzyme activity may be related to the number of interspaces greater in size 

 than a limit fixed by the size of ACh in a required orientation, but t N he binding 

 of substrate will be greatest for those interspaces that correspond most closely 

 to the size of ACh and so will the barrier to penetration of ACh. Maximum 

 rate of reaction will presumably be obtained with interspaces somewhat larger 

 than those for optimal binding because of the diminished penetration barrier 

 for such interspaces. For extracted enzyme, similar considerations may be 

 expected to apply, if activity is taking place by virtue of some organization of 

 macromolecules. For a cylindrical molecule of radius 20 A. and length 100 A. 

 a molecular weight of about 100,000 can be calculated (assuming a density of 

 1.2). Since even purified ACh-ase is poorly characterized by present-day stand- 

 ards for proteins, it is possible that the reported molecular weight of 3,000,000 



