FUNCTIONS OF DRUGS 77 



no examples of such activity have yet come to light^. 

 8. Action as substrate removers. In the living cell the 

 course of metabolism of a particular substance is in part 

 determined by the presence of a suitable chain of en- 

 zyme systems. The functioning of such a chain of en- 

 zyme systems is dependent upon the product of the 

 action of one enzyme passing on to another enzyme for 

 which it is a specific substrate. Serious interference may 

 occur by modification of the structure of an intermediate, 

 in the course of its passage by diffusion from one enzyme 

 to another. For example, the chain of enzymes con- 

 cerned in the anaerobic metabolism of glucose may have 

 their action disrupted in this way by HCN, or by 

 H3ASO4. The action of HCN is to form a cyanhydrin 

 with phosphoglyceraldehyde, thus removing the sub- 

 strate for phosphoglyceraldehyde dehydrogenase. 

 H3ASO4 excercises its toxicity in part by combining 

 with phosphoglyceraldehyde under the action of triose 

 phosphate dehydrogenase, so that as a result of the 

 activity of this enzyme a phosphoarsenoglyceric acid 

 results instead of diphosphoglyceric acid. The phospho- 



^ Cosubstrate activity is best understood by considering an example. 

 When trypsin is added to a solution of glycyl-leucine, the dipeptide is 

 not split by the enzyme. But if, now, to the solution is added a little 

 acetyl-phenyl-alanyl-glycine, synthesis occurs of a little of the substance 

 acetyl-phenyl-alanyl-glycyl-glycyl-leucine. Following this synthesis, 

 trypsin splits off first leucine and then glycine, leaving acetyl-phenyl- 

 alanyl-glycine as a residue which is not attacked. The compound acetyl- 

 phenyl-alanyl-glycine is said to have cosubstrate activity in the splitting 

 of glycyl-leucine by trypsin. 



