50 



PHYSIOLOGY OF THE FUNGI 



not others. There appears to be a close relation between the chemical 

 constitution of the prosthetic group of the enzyme and the inhibitors 

 which inactivate it. We may postulate that inactivation results from a 

 chemical reaction between the inhibitor and the prosthetic group of an 

 enzyme. 



One characteristic of an oxidase is inhibition by cyanide and hydrogen 

 sulfide. This points to some common moiety in these enzymes which is 



/Vo cyanide 



0.95 X 10'^ M NaCN 



o — '-' — o- 

 2.5xlO'^M NaCN 



U P — o — o- — o — o, 



l2.4xlO'^MNaCN 



50 75 100 



Oxygen tension (mm Hg) 



Fig. 10. The effect of cyanide on yeast respiration. (Courtesy of Winzler, Jour. 

 Cellular Comp. Physiol. 21: 238, 1943. Published by permission of Wistar Institute 

 of Anatomy and Biology.) 



able to react with cyanide. The oxidases are metalloproteins, and in 

 view of the property of cyanides of reacting with metals to form complexes, 

 it would appear likely that cyanide reacts with the metal to form inactive 

 little-ionized compounds. The typical properties of ferrous and ferric 

 ions are masked by cyanide. Tyrosinase, a copper-containing enzyme, is 

 inactivated by cyanide. Hydrogen sulfide acts on many of the same 

 enzymes which are inhibited by cyanide ; the action may be assumed to be 

 due to the formation of insoluble metal compounds rather than the forma- 

 tion of non-ionized complexes. 



Winzler (1943) studied the effect of different concentrations of cyanide 

 upon the respiration of yeast maintained under different oxygen tensions. 

 The effect of cyanide on yeast respiration is shown in Fig. 10. It may be 



