88 



SUBCELLULAR PARTICLES 



Pyrldlnoproteins 



(DPNH Dehyoxogenase) 

 Flavoprotein 



(Cytochromes) 

 Hemoproteins 



Substrates 



of > EPN 



Citric Cycle 



T^ 



•^ Flavin 



Hemes 



Pyridinoprotein Enzyme Complex 

 with Boimd Pyridine Nucleotide 



T 



ETP 



Fig. 5. Schematic representation of electron transfer chain. 



strates and, leaving succinate out of the picture, all these electrons are funneled 

 through the electron transfer chain by way of bound DPN. In the electron trans- 

 port particle electrons can only enter the chain by way of externally added 

 DPNH or succinate. These particles have no bound pyridine nucleotide and 

 lack most of the pyridinoprotein enzymes of the citric acid cycle. 



Now why the pyridinoprotein enzymes and bound DPN are lost during frag- 

 mentation of mitchondria by sonication is an extremely intriguing problem, 

 in which we have been deeply interested. There is a good deal of information 

 now available on this point, but I fear that this is not the appropriate occasion 

 to do more than raise the problem. 



When the fragmentation of mitochondria is carried out under other conditions, 

 an electron transport particle can be obtained which can oxidize both succinate 

 and DPNH, but these oxidations are no longer coupled to phosphorylation (10). 

 From the standpoint of chemical composition and the components of the electron 

 transfer chain the phosphorylating and nonphosphorylating electron transport 

 particles are indistinguishable. The electron microscope has shed light on the 

 basic difference between phosphorylating and nonphosphorylating forms of the 

 electron transport particle (figs. 4 and 6). The nonphosphorylating particle has 

 a vesicular structure, whereas the phosphorylating particle has the double mem- 

 brane structure characteristic of cristae and envelope. There is thus a very pre- 

 cise relation between double membrane structure and the capacity for oxidative 

 phosphorylation. In our laboratory Dr. A. W. Linnane has discovered a com- 

 ponent which determines whether a particular particle will phosphorylate or 

 not (24). When the component is made available, the particles are coupled; when 

 it is absent the particles are uncoupled. 



Our studies on mitochondrial fragmentation have led us to the conclusion that 

 the mitochondrion is a polymer, as it were, of a repeating unit which we believe 

 to be the electron transport particle. There are probably many thousand such 

 units in a single mitochondrion. This hypothesis is represented diagrammatically 

 in figure 7. 



