REVERSAL OF ELECTRON TRANSFER IN THE RESPIRATORY CHAIN 1 35 



Discussion 



Arnon : Is it a fair inference from your talk that the mechanism of this reaction 

 (the reduction of DPX by succinate with the aid of ATP) offers a way to study the 

 niechanism of electron transfer but that the reaction as such is of no physiological 

 significance ? Do you ascribe any physiological significance to this type of reaction 

 at the cellular level ? 



Chance : I suppose that photosynthesis may be a physiological event, probably 

 the one to which you were referring, and might be of some importance here, and 

 have some comments to make on that tomorrow, particularly on the possibility 

 that light-induced cytochrome responses observed in anaerobic photosynthetic 

 bacteria may be due to photo-produced ATP. Professor Lindberg referred this 

 morning to the reversal of electron transfer into pyridine nucleotide that has a 

 probable physiological implication. Active transport by reversed electron transfer 

 has been considered for nearly a decade. 



Arnon : I should add that I was specifically excluding photosynthesis from my 

 question. 



AzzoNE : What type of mitochondria did you use in your oligomycin experi- 

 ments ? 



Ch.ance: Pigeon-heart mitochondria. 



Azzone: Why do you think addition of oligomycin inhibits the succinate- 

 induced pyridine nucleotide reduction ? Is it not possible to generate the energy 

 necessary for DPN reduction merely by succinate oxidation ? My second question 

 is: have you tested the effect of dinitrophenol in your system where you get the 

 cytochrome oxidation after addition of ATP in anaerobiosis ? 



Chance: The answer to your first cjuestion is no. Our reaction involves the 

 oligomycin-sensitive steps of Fig. 1 1. Suitably prepared pigeon-heart mitochondria 

 have an absolute requirement for ATP for succinate-linked reduction of DPX. The 

 answer to the second question is that we have tested 2,4-dinitrophenol and find it 

 to block the ATP-activated reduction of DPN. 



Azzone: As Dr. Ernster will report later oligomycin, at least in liver mitochon- 

 dria, does not inhibit the succinate-induced pyridine nucleotide reduction. This 

 in our opinion means that, in the presence of oligomycin, liver mitochondria can 

 still synthesize high energy intermediate(s) and that these intermediate(s) can 

 provide the energy required for DPX reduction. Thus in this system there is no 

 requirement for externally added ATP. 



With regard to the second question : you know that we have done some experi- 

 ments in collaboration with Dr. Klingenberg (Xature, Loud., 188, 552 (i960)) where 

 we have found that dinitrophenol does not inhibit the ATP-induced pyridine 

 nucleotide reduction. 



Obviously it was more difficult to observe the ATP effect in our system because 

 the ATP-induced reduction was counter-balanced by the dinitrophenol-induced 

 oxidation of the mitochondrial pyridine nucleotide. Thus your system is perhaps 

 more suited for testing the effect of uncouplers since in anaerobiosis the stimulating 

 effect of dinitrophenol on electron transport is abolished. 



Chance: We may use oligomycin and 2,4-dinitrophenol to block the ATP- 



