84 ENZYMES AND DRUG ACTION 



by the diagram, there is a strong tendency for each drug 

 to act rather selectively on a particular enzyme system. 

 But this is no more than a tendency, and with the enzyme 

 systems with which we are concerned here some of the 

 drugs act at several points. For example, HCN can act 

 both on the cytochrome system and also by combining 

 with phosphoglyceraldehyde : lewisite can act on hexokin- 

 ase, on triose phosphate dehydrogenase and on the 

 pyruvic oxidase system: mustard gas can also act at more 

 than one point. From these observations it is clear 

 that a drug may have the potentiality of acting at several 

 stages, even in one chain of biochemical events. Conse- 

 quently the analysis of the action of a drug on the intra- 

 cellular enzymes is extremely complicated. In the sys- 

 tem which we have been considering there are about 

 twenty enzyme systems only. In the living cell there 

 must be thousands of enzyme systems. To decide which 

 of these enzyme systems is of importance in the medi- 

 ation of the effect of any particular drug is obviously a 

 task of the first magnitude. In a few cases, where the 

 action of the drug is very rapid, as is the case with HCN, 

 the position is simplified because it is clear that the 

 physiological effects of the drug must be produced on 

 those enzyme systems, such as those involved in respi- 

 ration, damage to which can produce an immediate re- 

 sult. But where there is a serious delay between admin- 

 istration of a drug and the emergence of its action, no 

 such simplification is possible and the whole enzyme 



