STRUCTURE OF NERVE CELL MEMBRANES 



137 



Diameter of Site 

 Fig. 6. The number of sites in a distribution is plotted against the size of a site in 

 (A) top curve all sites vacant; middle curve, 25% of all sites occupied; lowest curve, 

 50% of all sites occupied. The vertical bar indicates DDT sites. In (B), curves are as 

 above, but the molecule filling the membrane occupies large sites only and hence 

 shifts the mean interspace size as well as filling sites; DDT sites indicated as above. 



specificity of this enzyme, makes it possible to compare substrate with receptor 

 specificity. The reaction of DDT with alkali represents a dehydrohalogenation 

 that may be written as below. 



CI 



+ OH 



C— CI 



\ 



CI 



c- 



+ HOH 



C 



c 



CI 



CI 



+ ci- 



Cl CI 



CI CI 



CI CI 



The hydrogen on the 2 carbon of ethane is very dependent for its reactivity 

 on the p- substituent of the benzene rings; CH 3 0— makes the hydrogen least, 

 and iodo- the most reactive. Since DDT, in resistant flies, is transformed by 

 enzymatic action into the non-toxic dichlorethylene derivative (DDE), it is 

 reasonable to expect that a dehydrohalogenation, similar to that taking place 

 with alkali, has occurred. The relationship between p- substituents and enzyma- 

 tic reactivity is completely at variance with this reaction mechanism as the 

 data in Fig. 7 show. Aside from DDT itself, the most reactive compound is 

 p-CH a O— which ought to be the least, and one of the least reactive is iodo- 

 which should be one of the best. It is also clear that the enzyme shows precisely 



