MITOCHONDRIAL STRUCTURE AND FUNCTION 



95 



ETP 



Succinic 



dehydrogenase 

 complex 



Succinic-DPNH 

 dehydrogenase 

 complex 



Green particle 



M^^ (a)2 



ps (^)2^ri 



(b)2C-L 



D 



Fig. 8. Elementary composition 

 of ETP and derivative i)articles. 



^D ^^^2=1 (^^2 



fg = succinic dehydrogenase 



f = DPNH dehydrogenase 

 D 



is, correspondingly, only DPNH flavoprotein and no succinic flavoprotein in this 

 particle. Particles which lack cytochrome a no longer are capable of reacting with 

 molecular oxygen. However, red particles appropriately prepared will react readily 

 with cytochrome c, and the bound cytochromes of these particles are rapidly re- 

 ducible by succinate or DPNH. 



At present there is insufficient knowledge to permit writing a definitive formula 

 for the electron transfer chain. We still do not know all the components of the 

 chain and, in fact, it is still impossible to say with confidence which two com- 

 ponents of the chain react directly with one another. But at least we are now in a 

 much better position to state the problems to be resolved even if the solution still 

 eludes us. 



When succinate or DPNH is added to the electron transfer particle, all the 

 hemes, cytochromes b, c^, c and a, are rapidly reduced. Thus regardless of the way 

 by which electrons flow into the chain — whether by way of succinate of DPNH — 

 all the cytochromes in the particle become reduced. Taken at face value this ob- 

 servation would suggest one respiratory chain with two entry points. However, 

 ETP can be fragmented into daughter particles which contain either a succinic or 

 DPNH chain, and this fragmentation cannot be accounted for in terms of the 

 liberation of free flavoprotein. 



This type of evidence suggests separate chains for oxidation of succinate and 

 DPNH. When ETP is treated in a variety of ways the requirements for oxidation 

 of succinate and DPNH are often entirely different. The oxidation of DPNH may 

 depend upon the addition of cytochrome c whereas that of succinate docs not, or 

 vice versa. The oxidation of succinate requires the addition of coenzyme Q 



