Reversal of Electron Transfer in the Respiratory Chain* 



Brittox Chanxe 



The Eldridge Reeves Johnson Foundation for Medical Physics, University of 

 Pennsylvania, Philadelphia, Pa., U.S.A. 



I. Energy-linked DPN reduction 



GENERAL FEATURES OF THE REACTION 



It was obsened some time ago in collaboration with Dr. G. R. Williams 

 [i] that addition of succinate to mitochondria oxidizing a DPNH-linked 

 substrate caused a significant increase of pyridine-nucleotide reduction. 

 This phenomenon was especially clear in guinea-pig-kidney and rat-heart 

 mitochondria studied with Dr. G. R. Hollunger [2, 3] and was most recently 



DPNH 



Succinate 



"Switch"' hypothesis 



Fig. I. Diagram illustrating how competition between DPNH and succinate 

 for oxidizing equivalents from cytochrome chain could lead to increased pyridine- 

 nucleotide reduction ; the number of arrowheads indicates the proportion of electron 

 transfer which might flow in the chain and its two branches. (MD 102). 



obseryed in pigeon-heart mitochondria with Dr. B. Hagihara [4]. The 

 result has been confirmed by chemical assays in a number of laboratories 

 [5-10]. 



Such a phenomenon might readily haye been ascribed to a competition 

 between succinate and DPNH for oxidizing equiyalents in the cytochrome 

 portion of the chain (Fig. i).-\- On this basis one would haye expected that 



* This research was supported in part by the National Science Foundation. 



f The "switch" hypothesis is discussed in some detail by Birt and Bartley [21] 

 although kinetic studies were not possible with their analytical methods. This 

 hypothesis received only a short discussion previously [3] where the general case 

 of a "simple kinetic explanation" based upon a greater speed of pyridine-nucleo- 

 tide reduction by succinate appeared to be inadequate. Here we elaborate our 

 views and present additional evidence. 



