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DISCUSSION 



Dr. Lipmann: I think Dr. Smith, in summarizing, has said about what I 

 want to say. namely, that niitochondrial phosphorylation and photophos- 

 phorylation are very similar. I do not think that the term reductive phos- 

 phorylation should i^e used for photophosphorylation. Dr. Hill has given 

 us earlier in his talk a scheme in which the YO is aJDOut at the potential of 

 oxygen, that is, +0.81 and the XH is at about the potential of hydrogen, 

 that is, —0.41, and all that light does is make 70 and XH. I am a little less 

 prejudiced with o.xygen because when Dr. Frenkel found Rhodospirillum 

 photophosphorylation in my laboratory, it was quite obvious that it had no 

 relation to oxygen evolution. It was fortunate, I believe, that we got our 

 first glimpse at it in this bacterial system. It became very soon evident that 

 no substrate was involved or needed in this system of photophosphorylation; 

 the light in this system was furnishing both oxidant and reductant. Photo- 

 phosphorylation was coupled, as far as we could see, to the recombination 

 of the oxidant and the reductant producing the same gradient in potential 

 differences as is set up in the mitochondrial system by oxygen against sub- 

 strate. There certainly are differences between photophosphorylation and 

 oxidative phosphorylation. One of them is that photophosphorylation is 

 not as sensitive to the classical uncouplers such as dinitrophenol. In his 

 early experiments. Dr. Frenkel thought that nitrophenol was entirely inactive. 

 However, Geller later could show that nitrophenol in higher concentration 

 interrupts photophosphorylation. Dicumerol, which is a good uncoupler in 

 oxidative phosphorylation, uncouples in many systems of photophosphoryla- 

 tion; and a substance which we have been using as an uncoupler and which 

 is related to thyroxin (the compound is the butyl ester of diiodo-para-hydroxy- 

 benzoic acid) uncouples photophosphorylation in all cases where we have 

 tried it in very similar concentrations as it imcouples oxidative phosphoryla- 

 tion. So I agree with Dr. Smith that photophosphorylation and mitochondrial 

 phosphorylation are really very similar, although some of the components, 

 e.g., the cytochromes, are of a different type. 



Dr. .Arson : I would like to say a word about this matter of nomenclature. 

 This may seem at first unimportant, but really is important, because it adds 

 to clarity of thinking. I cannot agree more with what Dr. Lipmann said, that 



