236 PHYSIOLOGY OF THE FUNGI 



organisms that antimetabolites are active by virtue of interfering with 

 various enzymatic processes. It is also interesting to note that com- 

 petitive inhibition has been demonstrated with isolated enzyme systems, 

 Schopfer and Grob (1949) found the action of urease to be inhibited by 

 2-chloro-l,4-naphthoquinone. Most of the activity was restored by the 

 addition of 2-methyl-l,4-naphthoquinone (vitamin K3). 



Table 41. The Reversal of Inhibition Caused by 2,3-Dichloro-1,4-naphtho- 

 QuiNONE by 2-Methyl-1,4-naphthoquinone 

 Test fungus, Saccharomyces cerevisiae. Concentration of 2,3-dichloro-l,4-naphtho- 

 quinone, 0.005 jug/ml. (Woolley, Proc. Soc. Exptl. Biol. Med. 60, 1945. Published 

 by permission of the Society for Experimental Biology and Medicine.) 



2-methyl-l,4- Turbidity 

 naphthoquinone, ^g/ml. (100 = no growth) 



0.0 93 



0.04 60 



0.02 68 



0.01 77 



0.005 85 



Other vitamin antagonists. The sulfonic-acid analogue of nicotinic 

 acid inhibits the growth of certain bacteria (Mcllwain, 1940). Appar- 

 ently this analogue has not been tested in nicotinic acid-deficient fungi. 

 Woolley (194Ga) has reported maize to contain a "pellagragenic" agent 

 which may tentatively be considered as a naturally occurring anti- 

 nicotinic-acid factor. 



Among the recently developed insecticides, 7-hexachlorocyclohexane 

 is of considerable value. Kirkwood and Phillips (1946) have shown that 

 the growth of Saccharomyces cerevisiae is inhibited by this compound, and 

 that the inhibition is overcome by meso-inositol. The other isomers of 

 hexachlorocyclohexane were not very effective inhibitors of yeast growth ; 

 neither are they of much value as insecticides. These observations point 

 to competitive inhibition as a possible mechanism of insecticidal action 

 of this compound. 



AMINO-ACID ANTAGONISTS 



Organisms must either synthesize or obtain from exogenous sources 

 the different amino acids they require for the synthesis of protein. Anti- 

 metabolites which antagonize the synthesis or utilization of essential 

 amino acids would have a profound effect upon growth or other functions 

 of organisms. The role of amino acids is not confined to the synthesis 

 of proteins but extends to the synthesis of other essential metabolites. 

 An amino-acid antagonist may act in two ways, (1) by inhibiting protein 

 synthesis and (2) by inhibiting the synthesis of essential metabolites 

 which are derived from amino acids, either directly or indirectly. If 

 an amino a^id functions in more than one way, the action of an amino- 



