30 A SYMPOSIUM ON RESPIRATORY ENZYMES 



cytochrome oxidase will be depressed below that of the cytochrome 

 a and c systems. In such a tissue as muscle, then, we might expect 

 100 per cent inhibition of the cytochrome-mediated respiration. It 

 is at such a concentration that cyanide exerts its maximum effect 

 upon respiration. 



Let us now return to the case of the unfertilized arbacia egg, 

 where the cytochromes appear to be absent. Here cytochrome 

 oxidase may be considered to react directly with a flavoprotein. The 

 addition of cyanide may be pictured as depressing the potential 

 of the cytochrome oxidase in the same manner. In this case, how- 

 ever, the potential of the cytochrome oxidase still remains above that 

 of the flavoprotein with which it normally reacts. Oxidations can 

 therefore still proceed, and no cyanide inhibition is observed. In 

 fact, a stimulation of respiration by cyanide such as was observed 

 by Marsh and Goddard (20) in the fully mature leaf might be 

 encountered if such a set of conditions exists. Barron (7) has shown 

 that the rate of reaction of oxygen with reduced dyestuffs increases 

 as the oxidation-reduction potential of the system decreases. A 

 similar change may occur in the case of the cytochrome oxidase 

 system as its potential is lowered by cyanide. 



Fertilization of the arbacia egg may then be looked upon as the 

 gearing of the cytochrome oxidase system to another system of 

 higher potential than the flavoprotein, which, though serving to in- 

 crease the respiratory rate, also causes the system to become 

 cyanide-sensitive. The fact that arbacia sperms are rich in cyto- 

 chromes a, b, and c is of interest in this connection. For a further dis- 

 cussion of this aspect see Krahl et al. (19). 



One other example of an inhibitor study which may be employed 

 is furnished by the work of Stannard (23). This investigator has re- 

 cently presented data to show that resting and stimulated frog 

 muscle respond differently to cyanide and azide. Whereas cyanide 

 inhibits the oxygen consumption of both resting and stimulated 

 muscle, azide inhibits only the oxygen consumption of stimulated 

 muscle. The results have been interpreted to indicate that the path- 

 ways of oxidation are different in resting and in stimulated muscle. 

 As Stannard points out, the cyanide sensitivity of the resting respira- 

 tion seems to preclude the possibility that it represents an independ- 

 ent functioning of a flavoprotein system. 



Now azide is believed to inhibit respiration also by reacting with 

 cytochrome oxidase. Quantitatively, however, its action is different 

 from that of cyanide. Inhibition of respiration by azide appears to 



