STUDY OF RESPIRATION BY INHIBITORS 881 



(745), who originally observed this phenomenon in frog muscles, also 

 found it in skeletal and heart muscle of the rat. So far, their experi- 

 ments in this species have not been repeated, but in man recovery of 

 radioactive carbon monoxide after inhalation is within experimental 

 error (2811). 



The inhibition l)y carbon monoxide may be also negligible if there 

 is a great excess of respiratory enzyme over the dehydrogenase 

 systems, i.e., if the respiratory enzyme is not saturated with substrate 

 or if the reaction of the oxidase with the cytochrome c is not the 

 slowest, rate-determining, step. Finally the particular tissue may 

 contain a diflFerent respiratory enzyme of hematin nature with much 

 lower affinity for carbon monoxide. 



Cyanide and azide inhibition. The assumption that an alternative 

 pathway of respiration exists in which the respiratory enzyme does 

 not function is supported by other evidence, although this evidence 

 is again not considered to be entirely conclusive. 



The respiration of resting frog muscle is not inhibited by azide 

 {2609), which inhibits the respiration of the muscle stimulated by 

 electricity, potassium, acetylcholine, or caffeine. Cyanide inhibits 

 both the respiration of the resting as well as of the stimulated muscle, 

 but in a somewhat different manner. The respiration of the stimu- 

 lated salivary mammalian gland is inhibited by cyanide, that of the 

 resting gland is not {1572). Similarly the respiration of the unfer- 

 tilized sea urchin egg is not inhibited by cyanide, while that of the 

 fertilized egg is inhibited {1572,1576,2393,239^).* 



Ball (i~4) suggested that cyanide, in contrast to azide, may decrease the 

 oxidation-reduction potential of the respiratory enzyme sufficiently to prevent 

 it from interacting witli cytochrome c (potential -i- 0.25 v.), to which it is 

 geared in fertilized eggs. The decrease of potential of the oxidase by com- 

 bination with cyanide may. however, in the unfertilized egg, still permit its 

 interaction with flavoproteins, which have a lower potential (about — 0.07 v.). 

 The cyanide-insensitive respiration in the unfertilized egg is assumed to be 

 due to this direct reaction of cyanide-combined ferric oxidase with flavo- 

 proteins, while the great increase of respiration on fertilization in the absence 

 of cyanide together with the cyanide sensitivity is assumed to be due to the 

 "gearing" of the oxidase to the cytochrome svstem. Azide, however, is 

 assumed to inhibit incompletely, combining with both ferric and ferrous 

 enzyme. There is, however, as yet no evidence for the direct reaction of 

 oxidase with flavo-proteins. The explanation of Ball fails to account for 



* While azide certainly does not inhibit the respiration of unfertilized sea urchin 

 eggs (c/. Fisher, Henrv, and Low, VO-Ja), the noninhibition l>v cyanide has become 

 doubtful (Robbie, ,',?7ya). 



