76 ENZYMES AND DRUG ACTION 



Stances such as the arsenoxides, the nitrogen mustards, 

 iodoacetate, fluoride and iodine, are able to inhibit the 

 action of enzymes by forming a chemical compound 

 with chemical groups which are essential for the mainten- 

 ance of enzyme activity. 



4. Action as physical (competitive) inhibitors. In this 

 category we may mention malonate, which acts as an 

 inhibitor for succinic dehydrogenase; glyceraldehyde, 

 which acts as an inhibitor for triose phosphate dehydro- 

 genase, and the sulphonamides, which are believed to 

 compete with ^-aminobenzoic acid for enzyme systems 

 concerned in the metabolism of the latter. 



5. Action as prosthetic groups. Certain substances are 

 able to act as the prosthetic groups of enzymes, thus 

 activating previously existing apoenzyme molecules. 

 Examples are vitamin Bj, which is concerned as a 

 prosthetic group in the pyruvic oxidase system, pyri- 

 doxal, which is a prosthetic group for some decarboxy- 

 lases, and vitamin Bg, which is a constituent of flavo- 

 protein systems. 



6. Action as coenzymes. As an example of this may be 

 mentioned nicotinic acid, which is incorporated into the 

 molecule of coenzyme II. 



7. Action as cosuhstrates. No examples of this are 

 known, but abnormal cosubstrate activity could clearly 

 be a serious source of trouble to a cell. It is but com- 

 paratively recently that Bergman introduced the con- 

 ception of cosuhstrates, so that it is not surprising that 



