ACTION OF DRUGS UPON ENZYME SYSTEMS 89 



It therefore seems clear that not all the substances 

 having a common physiological effect exercise that effect 

 by an action upon a single enzyme system. 



The Classification of Drugs in Terms of Enzyme 

 Systems upon Which They Act 



Whilst very little valuable information comes from classi- 

 fying drugs according to the physiological effect they 

 produce, since it soon becomes apparent that the sub- 

 stances producing a common physiological effect must 

 do so by acting on a diversity of receptor systems, it 

 still remains possible that substances acting specifically 

 upon a certain enzyme system may produce a standard 

 physiological effect. As examples we may take substances 

 which act particularly upon SH enzymes, substances 

 which are inhibitors of choline esterase, and substances 

 which are inhibitors of hexokinase. 



It seems likely, from the work of DixON and his 

 colleagues, that substances acting specifically on SH 

 groups, such as those of triose phosphate dehydrogenase, 

 are always lachrymators. In particular substances con- 

 taining the groupings 



R.CO.CH2X, where X = halogen, and R.CO.CH=CH2 



are often rather specific agents for SH groups. They 

 include many of the substances which have been found 

 useful in chemical warfare as lachrymators. On the other 



