144 L. J. MULLINS 



represents an aggregate of 30 molecules or a dispersion of aggregates of different 

 size. There is no certainty that the interspace distribution in micelles of mem- 

 brane molecules in solution (ACh-ase) corresponds to interspace distributions 

 in the cell, hence attempts have been made to assay ACh-ase in intact tissue 

 (Schleyer, 1955). Penetration barriers to ACh have necessitated the use of 

 other substrates, the latest of which is ethyl chloroacetate. The results of these 

 studies are not entirely satisfactory as only 60% of the extractable ACh-ase 

 is reached by the substrate — suggesting that perhaps the extracted micelles 

 are a more open structure than the membrane. In addition the presence of 

 another enzyme in large amounts was noted. This enzyme was not inhibited 

 by ACh-ase inhibitors and the results might be explained if the substrate were 

 decomposed by some Cl-membrane-C=0 interaction in addition to the cation- 

 membrane C=0 interaction. 



The possibility exists that the actions of a variety of drugs that apparently 

 selectively affect either receptor or enzyme, can be explained as a consequence 

 of the change in mean interspace size that they induce. It has been noted by 

 Friess and McCarville (1954) and by Bernhard (1955) that in eserine the center 

 of positive charge and the carbonyl group are 1.1 to 1.4 A (depending upon 

 model parameters) further apart than in ACh at full extension. This is a quite 

 remarkable situation since the presumed effectiveness of eserine as an inhibitor 

 of the enzyme might be expected to depend upon a precise correspondence 

 between reactive groupings. The observation is understandable if it is assumed 

 that sites fitting ACh are small when compared with the mean interspace size, 

 while eserine interacts with sites of a larger size. Fig. 10 has been constructed 

 to represent the relationship of ACh receptor to enzyme sites assuming that 

 the action of an inhibitor is to shift the distribution of site sizes to lower mean 

 values. The proper amount of enzyme inhibition may be expected to increase 

 the number of receptor sites, while larger inhibitions, or larger doses of inhibitor 

 will eliminate receptor sites. The change in the profile size of the molecule as 

 CH 2 groups are added to ACh is shown on the diagram; it is not clear whether 

 or not the receptor is able to operate by a cation-CH 2 group interaction, but 

 the fact that enzyme action fails (because of the screening of CH 2 groups) at 

 butyryl choline, while the receptor is active, is understandable from the dia- 

 gram. A functional role for ACh-ase is also apparent; it is concerned with the 

 removal of ACh from those interspaces that are not of receptor size, and thus 

 of preventing narcosis. Butyryl choline has less need of such an enzyme because 

 of the smaller number of sites larger than this substance. 



Evidence has been presented that ACh-ase activity depends upon the exist- 

 ence of anionic and esteratic sites (Wilson, 1951a), hence within the interspace 

 we must assign such specific binding points. Unfortunately there are several 

 possibilities in this regard and any decision between them must rest on the 

 ability of such a model to correlate a variety of observations on both receptors 



