LIGHT-INDUCED PHOSPHORYLATION 441 



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Discussion 



Vernon : I shovild like to discuss briefly the effect of ascorbate on photo- 

 phosphorylation. There is an alternative explanation which also agrees with the 

 earlier experiments of Geller in which he observed an inhibition of photophos- 

 phorylation with high concentrations of succinate or with completely reduced 

 phenazine methosulphate. The experiments of Kamen and Newton with Chro- 

 matium also indicate that if conditions are excessively reducing, photophos- 

 phorylation is inhibited. In our laboratory we have found that addition of ascorbate 

 or succinate under anaerobic conditions produces an inhibition of photophos- 

 phorylation with chromatophores of R. rubriim. This implies that in the presence 

 of air, succinate and ascorbate partly reduce and thus poise the electron transfer 

 agents at an appropriate level, and if an excess of ascorbate or succinate is added 

 the medium becomes over-reducing and photophosphorylation is inhibited. I 

 think the ascorbate data can be better interpreted from the point of view of poising 

 of the electron transport agents. 



Baltscheffsky : Well, the difference between your point of view and our point 

 of view is not one of principle but one of specificity ; you say that electron transport 

 agents need to be poised and we go one step further and say that cytochrome c^ is 

 the electron carrier which needs to be poised, that is, adequately reduced in order 

 for the system to function under aerobic conditions. Not only ascorbate but also 

 succinate and reduced diphosphopyridine nucleotide act, we feel, by poising 

 cytochrome c.,. 



Jagendorf : I think it is very interesting that both you and Dr. Arnon find a 

 higher quantum efficiency with low light intensities using a system other than 

 phenazine methosulphate. However, I worry about coming to the conclusion that 

 this necessarily means there is one site for phosphorylation when phenazine 

 methosulphate is used and two sites in the other system. I think there are still 

 some alternative interpretations open ; for instance, maybe with phenazine metho- 

 sulphate you waste some quanta because there are two electron transport paths 

 operating, one not phosphorylating at all and the other one involving the same two 

 phosphorylation sites as with FMN, etc. The point is that a direct measurement 



