g6 SUBCELLULAR PARTICLES 



whereas that of DPNH does not (12). These observations clearly point to basic 

 differences in the succinic and DPNH chains, but they do not necessarily exclude 

 a part of the chain which is common to both. Thus several formulations have to 

 be considered, of which two obvious ones are shown below: 



/ fs-bc*-a 



I I I , two separate intercommunicating chains, where c* is 

 fi)-bc*-a 

 either c or Ci, or 



// fs\ 



(b)o (ci + c) — (a)>, a single chain into which both flavoproteins 



funnel electrons. The weakness of the second formulation is that all the differences 

 between the two chains would have to be localized between the two flavoproteins 

 and the bc^ cytochromes. One of the weaknesses of the first formulation is that 

 there is insufficient Ci for two chains and c and c^ are not equivalent. The other 

 weakness is that electronic intercommunication of two separate chains would re- 

 quire implementation by a special set of components, and no components are as 

 yet known which fulfill that function. 



In mitochondria both cytochromes b and c-i are rapidly reducible by either suc- 

 cinate or DPNH, according to the measurements of Chance and Williams (8). 

 In nonphosphorylating ETP the reduction of cytochrome Ci is far more rapid 

 than that of cytochrome i>, and in several derivative particles prepared by frag- 

 mentation of ETP the disparity in the two rates of reduction is so great that for 

 practical purposes cytochrome b can be considered no longer capable of reduc- 

 tion (29). Whatever the position of cytochrome b in the chain, it cannot be the 

 electron donor for cytochrome c^. The reduction of cytochrome Ci is antimycin- 

 sensitive, whereas that of cytochrome b is not. Thus cytochrome Ci cannot be the 

 electron donor for cytochrome b. These observations appear to exclude a common 

 chain for cytochromes b and c^ and they suggest that electrons from succinate or 

 DPNH reach cytochrome a either by way of cytochrome b or by way of cyto- 

 chrome Ci. The path by way of cytochrome b is more labile than that by way of 

 cytochrome r,, and it is only in the intact mitochondrion of ETP, with phos- 

 phorylating potentialities preserved, that the two pathways are equivalent. Thus 

 formula I for the respiratory chain of ETP has to be modified as shown below: 



la 



