OXIDATIVE MECHANISMS IN ANIMAL TISSUES 31 



begin at concentrations higher than those required for cyanide 

 poisoning. As the azide concentration is increased above this initial 

 value, inhibition progresses as rapidly as with cyanide until a 

 concentration of about 10"^ molar is reached. Further increase in 

 azide concentration produces no further effect, and inhibition of 

 respiration remains incomplete, never reaching the maximum value 

 obtained with cyanide. How then can this diflPerence in the be- 

 havior of cyanide and azide be explained if they both act on cyto- 

 chrome oxidase? Is it necessary to postulate that azide and cyanide 

 inactivate separate systems? 



An explanation of the difference in the behavior of cyanide and 

 azide may be given in terms of their diflFerent effects upon the poten- 

 tial of the cytochrome oxidase system. Azide, like cyanide, is as- 

 sumed to combine with the ferric form of cytochrome oxidase. Un- 

 like cyanide, it is also assumed to combine with the ferrous form. 

 In Figure 4 the effect of azide upon the potential of the cytochrome 

 oxidase system is plotted on the basis of these assumptions. Since 

 azide inhibition first manifests itself at concentrations higher than 

 those for cyanide, its affinity for the ferric form of the oxidase is 

 assumed to be less than that of cyanide. Depression of the potential 

 of the oxidase system is therefore portrayed as starting when the 

 concentration of azide reaches a value between 10 * and 10 "' molar. 

 As the concentration of azide is increased above this value, the 

 potential is assumed to be lowered along the same slope as for the 

 cyanide system. Now since inhibition with azide reaches a maximum 

 at a concentration of 10"^ molar, it is assumed that at this concentra- 

 tion the azide begins to combine with the ferrous form of cyto- 

 chrome oxidase. According to Clark et al. (10), the effect of such a 

 combination on tlie potential of the system will be to alter the 

 slope to a 0.0 value, and thus no further change in potential occurs 

 as more azide is added. On the basis of this assumption it can be 

 seen that, as depicted in Figure 4 the potential of the cytochrome 

 oxidase system in the presence of azide can be depressed only to a 

 level corresponding to that for the cytochrome a system. At this level 

 the cytochrome oxidase may still function to oxidize the cytochromes, 

 but its efficiency will be at least 50 per cent impaired. It is thus 

 possible to conceive of the respiration of resting muscle as being 

 unimpaired by azide, since even at a lower level of efficiency the 

 cytochrome oxidase may still be capable of supplying the oxygen 

 needs of the resting muscle. If, however, increased demands for 

 oxygen are made upon the muscle by stimulation, the impaired 



