296 P. Alexander, S. F. Cousens and K. A. Stagey 



and found that it very readily alkylated amino groups in pro- 

 teins, but Fahmy and Fahmy (unpublished) were not able to 

 produce any mutations in Drosophila with it. Similarly 

 specific thiol reagents which inactivate sulphydryl enzymes 

 in the cell (cf. Ross, 1953) do not bring about the highly 

 specific forms of genetic damage associated with radiation 

 and the alkylating agents belonging to the mustard class. 

 These experiments indicate that reaction in vivo with amino 

 or sulphydryl groups does not produce radiomimetic effects 

 and the biologically significant reaction of the alkylating 

 agents considered in this paper would appear to be the 

 esterification of acid groups. This type of reaction can only 

 be brought about under physiological conditions by sub- 

 stances most of which are mutagenic, and cannot be pro- 

 duced with the inactive substances. 



The in vitro reaction with nucleic acids and proteins lends 

 powerful support to the arguments just advanced that 

 combination with acid groups is the biologically important 

 reaction. That is not to say that this is the only or even 

 the predominant reaction which takes place with cellular 

 constituents, and a substantial proportion of the reagents are 

 probably wasted in useless reactions. The important process 

 can therefore not be discovered merely by following the in vivo 

 fate of an active compound. The experiments described in 

 this paper were designed to throw light on the mechanism by 

 which these reagents exert their biological activity, although 

 general considerations strongly suggest that the relevant 

 reactions of chemical mutagens must be with the nucleic acid 

 moiety of nucleoprotein. A summary of our results with 

 proteins is given for the sake of completeness. 



Reaction with Proteins 



A number of papers have been published on the reaction 

 of proteins with mustard gas (see review by Alexander, 1954) 

 but the results do not provide information about the relative 

 reactivities of different groups in the proteins since in almost 



