ENZYME-SUBSTRATE COMPOUNDS 



The members of the reaction chains written one over the 

 other are those that react to give the products following the 

 arrows. A discussion of this chain appears elsewhere (29,30). 



OTHER RESPIRATORY CHAINS 



The detailed experiments of Keilin and Warburg afford 

 strong evidence that the CO-sensitive cytochrome a^ is a ter- 

 minal oxidase of cellular respiration. And the evidence for cy- 

 tochrome c participation in respiration of intact mitochondria 

 is adequately supported by the cessation of respiration and the 

 oxidation of cytochromes c, a, and a^ upon addition of anti- 

 mycin-A (29,31). Nevertheless, there are three variations of 

 the respiratory chain that require consideration. 



The first variation is the antimycin-A-insensitive cyto- 

 chrome b(, pathway for DPNH oxidation by oxygen (27,46,57, 

 65). This chain is inoperative in intact mitochondria because 

 the intramitochondrial DPNH is not accessible to the micro- 

 somes which contain cytochrome b^ (31). But upon hypotonic 

 treatment of the mitochondria and addition of cytochrome c, 

 considerable antimycin-A-insensitive respiration is obtained via 

 this pathway (46,59) : 



O2 < oxidase < c < microsomes < DPNH < substrate 



(65) 



With intact mitochondria, DPNH must also be added to activate 

 the external pathway. 



The possiblity of this pathway illuminates an open question 

 with regard to the physiological function of mitochondria: 

 how do they oxidize cytoplasmic DPNH to which the isolated 

 mitochondria are impermeable? Whether or not the cytochrome 

 ^5 pathway solves this problem is equally dubious, because the 

 ^5 pathway involves the oxidation of cytoplasmic cytochrome c, 

 which may not exist and to which the mitochondria are relatively 

 inactive. Perhaps the only solution to such a dilemma is that 

 the mitochondria as prepared have a diminished permeability 

 compared to that of their state in nature. The only direct 

 evidence bearing on this point is that when DPN is reduced in 



333 



