22 A SYMPOSIUM ON RESPIRATORY ENZYMES 



proteins seems rather small. At any rate, the gap between these two 

 systems in terms of the energy that would be released upon their 

 interaction is only a small fraction of the overall. There seems little 

 doubt, however, that cytochrome c and the flavoprotein systems are 

 necessary links in the chain. No flavoprotein has yet been obtained 

 from animal tissues that will react directly with cytochrome c. This 

 implies that there is some link in the chain between these two con- 

 stituents. Until cytochrome h can be isolated its claim to this posi- 

 tion must remain in dispute. Another system that we have not yet 

 mentioned must also be considered in this connection. Szent- 

 Gyorgyi has suggested that the succinate-fumarate system links the 

 flavoprotein systems to the cytochromes. It will be seen that the 

 potential of the succinate-fumarate system is such that it could play 

 this role. Here again, however, clean-cut proof is lacking, for suc- 

 cinic dehydrogenase and the cytochrome system appear to be 

 intimately tied together and have so far defied separation. Pennit 

 me in passing to call your attention to the position of the methylene 

 blue system. Situated as it is at this crossroad, it is well adapted to 

 react with the flavoproteins on the one hand and on the other to 

 bypass the cytochrome system in reacting with oxygen. 



The chief pathway, then, by which energy is released in the living 

 cell, so far as we can tell today, appears to be that shown here. The 

 energy liberated when substrates undergo air oxidation is not 

 liberated in one large burst, as was once thought, but is released in 

 stepwise fashion. At least six separate steps seem to be involved. 

 The process is not unlike that of locks in a canal. As each lock is 

 passed in the ascent from a lower to a higher level a certain amount 

 of energy is expended. Similarly, the total energy resulting from the 

 oxidation of foodstuffs is released in small units or parcels, step by 

 step. The amount of free energy released at each step is propor- 

 tional to the difference in potential of the systems comprising the 

 several steps. As indicated in this diagram, the steps involving the 

 cytochromes account for more than two-thirds of the total energy 

 released by this chain. 



Now also, just as each lock in a canal must be passed in sequence, 

 so here each link in the chain appears to be indispensable. Each 

 component of the chain seems to react readily only with that com- 

 ponent lying immediately above or below it. This marked specificity 

 of interaction is most extraordinary in view of the fact that these 

 substances may react with oxidizing and reducing agents foreign to 

 the living cell. Methylene blue has already been given as an example 



