PHOTOSYNTHESIS BY ISOLATED CHLOROPLASTS 301 



the vitamin K is lost, or do you think that it is in some way inactivated? It is 

 yen' insoluble in water. 



Amon : I think it may be partly inactivated. We have some recent data based 

 on the response to total vitamin K, that is, vitamin K present in the chloroplast 

 plus vitamin K added, and there is good agreement there. We think that perhaps 

 part of the reactive surface may be inactivated in the preparation. That is a pos- 

 sibility. 



Clendenning : In j'our scheme there were two sources for oxygen. Do you find 

 an inhibition of oxygen evolution when ATP is put in the system? 



Amon : We can inhibit CO2 fixation quite appreciably by putting in the com- 

 ponents of the phosphorylating system. 



Clendenning : How about oxygen evolution? 



Amon: Which would correspond to the same thing in our scheme of things. 

 We obtain oxj'gen evolution only when CO^ fixation takes place. 



Whatley: However, in the experiment in question only CO2 fixation and not 

 oxygen evolution was measured. 



Clendenning: What is the importance of the isolation procedure versus the 

 reaction conditions in the oxidation convection system that you have? As far as I 

 can see, the actual isolation procedure has not changed so very much since way 

 back in the nineteenth century. Engelmann and many others in that period knew 

 very well how to isolate whole chloroplasts. It is a very simple matter. 



Amon: Engelmann did not measure photosynthetic phosphorylation. 



Clendenning : He knew how to isolate whole chloroplasts. WTiat is added after 

 you isolate the whole chloroplast? 



Amon: Engelmann did not discover the Hill reaction, although he observed 

 oxygen evolution, because he did not add the hydrogen acceptor. His chloro- 

 plasts were capable of doing the Hill reaction and probably also capable of doing 

 photosynthetic phosphorylation and CO2 fixation if he had known what cofactors 



to add. 



Frenkel : Dr. Arnon and Dr. \Miatley presented data with regard to the vitamiji 

 K requirement of photosynthetic phosphorylation by chloroplast fragments in 

 the presence of ascorbate. I was told by Mr. David Geller at Dr. Lipmann's 

 laboratory in Boston that in the partially purified photophosphorylating system 

 from Rhodospirilluni rubrum vitamin K apparently is not required for full activity 

 when succinate is added. However, when succinate is replaced by ascorbate the 

 addition of vitamin K has a pronounced effect in increasing the rate of photo- 

 phosphorylation of bacterial preparations over the rate with ascorbate alone. 

 This observation makes me wonder whether vitamin K is required for the phos- 

 phorylating system per se, or whether its sole function is to help transfer electrons 

 from ascorbate to the phosphorylating system. It might be of interest to see 

 whether other substances than ascorbate will give rise to active photophosphoryla- 

 tion in the chloroplast system and whether such substances if found will also re- 

 place the vitamin K requirement. 



References 



1. Hill, R., Symposia Soc. Expll. Biol, 5, 223 (1951); Advances in EnzymoL, 12, 

 1 (1951). 



