170 A SYMPOSIUM ON RESPIRATORY ENZYMES 



pancreas by secretin, and myometrium by oxytocin all show that 

 large increases of respiration attend the change from a state of 

 rest to one of activity. The finding of azide-stable and relatively 

 cyanide-stable respiration, combined with spectroscopic observa- 

 tion of the cytochrome bands, indicated that the resting metabolism 

 was not proceeding through the cytochrome system, but that the 

 extra metabolism that followed stimulation was mediated through 

 this system. It is important to note that in the resting state the 

 cytochrome system was nevertheless "available" for the oxidation of 

 p-phenylenediamine. It was postulated that the cytochrome system 

 becomes "geared" or "linked" to the substrate-dehydrogenase system 

 by an agent capable of reducing the cytochrome that is made avail- 

 able upon stimulation of the cells. Again there is no clue to the 

 nature of the oxidizing enzymes that replace the cytochrome sys- 

 tem in the resting cells. 



Perhaps the most serious obstacle to the immediate acceptance of 

 Stannard's and Korr's conclusions is the possibility that at the low 

 levels of oxygen consumption the oxidase system is only partially 

 saturated with substrate and hence considerable amounts could be 

 blocked by inhibitors without affecting the oxygen consumption of 

 the tissue. Such a view has been expressed by Warburg (86), and 

 Commoner (87) has actually demonstrated that cyanide inhibition 

 of yeast respiration is dependent on substrate respiration. 



The early work of Keilin showed that whereas in the resting 

 muscle the cytochromes were oxidized, when it became active the 

 bands of the reduced cytochromes appeared. Thus during activity 

 the ratio of reduced to oxidized cytochrome c is higher. It is known 

 that reduced cytochrome c forms a complex with the oxidase (74, 

 75). Since under conditions of activity the oxidase is relatively more 

 saturated with its substrate (reduced cytochrome c), the respiration 

 of this system should, according to the "under-saturation" concept, 

 be more sensitive to the same concentration of oxidase inhibitor. 

 The "activity" respiration should then be totally sensitive to azide 

 if azide only affects the rate of oxidation of the cytochrome. But 

 Stannard's experiment demonstrated that during activity only the 

 extra oxygen consumption caused by the activity was azide- 

 sensitive. 



The experiments of Stotz, Altschul, and Hogness (75) on the rela- 

 tions of the oxidase and of cytochrome c on hydroquinone oxidation 

 showed that the rate of oxidation was a function of both compo- 

 nents. Thus at a fixed concentration of reduced cytochrome c (oxi- 



