114 growth: synthesis of bacterial protoplasm 



Fildes and Woods proposed that ^-amino-benzoic acid is 

 an " essential metabolite " for bacteria, and that if its meta- 

 bolism is in any way prevented, then growth ceases. Since 

 sulphanilamide and ;p-amino-benzoic acid have similar 

 chemical structures, it was suggested that sulphanilamide 

 acts as a competitive inhibitor of the enzyme carrying out the 

 essential metabolism of jo-amino-benzoic acid. 



H^N-^-^-SO^NH^ H2N-:^~\-C00H 

 Sulphanilamide ^-amino-benzoic acid 



The fact that folic acid contains jo-amino-benzoic acid as 

 part of its structure, indicates that the latter substance must 

 undergo some metabolism within the organism in order to 

 become incorporated in the larger molecule. 



According to this theory, ^-amino-benzoic acid is an 

 " essential metabolite," and if the organism has no power to 

 synthesise this metabolite, then it becomes a growth factor 

 for that organism. Sulphanilamide acts by preventing the 

 utilisation of p-amino-benzoic acid. If this is the case, then 

 it should be possible to inhibit the growth of other exacting 

 organisms by presenting them with substances of structure 

 similar to that of their specific growth factors (" metabolite 

 analogues ") which will compete with the growth factor for 

 an enzyme surface, will block the metabolism of the growth 

 factor, and, consequently, prevent growth. For example, 

 Pr. vulgaris is exacting towards nicotinic acid; if we add 

 pyridine-3-sulphonic acid to the medium, we find that growth 

 is prevented by competition between the growth factor, nicotinic 

 acid, and its antagonistic analogue, pyridine-3-sulphonic acid. 



_COOH ^SOgH 



Nicotinic acid Pyridine-3-sulphonic acid 



In this case it is interesting that pyridine-3-sulphonic acid 

 acts as an antagonist towards nicotinic acid, but not towards 

 nicotinic amide, so presumably the antagonist prevents the 



