PHOSPHORYLATION OF R. Tuhrum 309 



The relationship of these two systems is not clear at present and will 

 require further elucidation. 



Discussion 



Whittingham : Which of the sediments has the highest activity? 



Frenkel: The material containing the highest activity roughly has the same 

 sedimentation behavior as the "chromatophores" isolated from Rhodospirillum 

 rubrum by Schachman, Pardee, and Stanier. 



Lucile Smith : Is oxidized DPX just as effective as reduced DPN? 



Frenkel: No, oxidized DPN does not work in the light system studied. 



Vishniac : Have you tried the effect of some of the chelating agents which have 

 been shown to enhance oxidative phosphorylation by removal of calcium ions? 



Frenkel : I have tested Versene only with the crude preparations and have not 

 observed any significant effects. 



Vishniac : The fraction which you believe to be active myokinase— could it be 

 replaced by nucleotides? The reason I ask is because, in the study of oxidative 

 phosphorylation, Pinchot discovered a heat-stable factor which at first he be- 

 lieved to be myokinase, just as you indicated. However, it turned out to be 

 nucleotides. 



Frenkel: The heat-stable factor described here is nondialyzable, precipitates 

 with perchloric acid (8%), will convert ADP into AMP + ATP, and also in other 

 ways resembles the myokinase described by Colowick and Kalckar. 



Tolbert : Can you estimate whether the chlorophyll/protein ratio in your small 

 particles is the same as that in the whole cell? 



Frenkel : No, I have no such analyses. However, I would like to mention an 

 interesting observation. As indicated in ref. 2, when crude preparations are 

 washed by repeated high-speed centrifugations, the photophosphorjdating activity 

 based on chlorophyll content remains constant. Thus it appears that in the frac- 

 tionation procedures employed thus far, the photophosphorylating system has not 

 been separated from the photochemical system and most likely is on one and the 

 same particle. 



Newton: We have some observations that I think bear on your hypothesis. 

 Dr. Kamen and I have been studying a similar system in purple sulfur bacteria 

 which I will discuss later, but pertinent here is the fact that the phosphorylation 

 which we observe is inhibited by small amounts of substances like thiosulfate which 

 can serve as hydrogen donors for photosynthesis in these organisms, and it might 

 be that what we are doing is siphoning out the photochemical oxidant with the 

 added hydrogen donor. 



Frenkel : I would like to make a comment about the demonstration of a vitamin 

 K requirement. Mr. D. Geller at Dr. Lipmann's laboratory has studied the vita- 

 min K requirement of the photophosphorylating system from R. rubrum. As far 

 as I know he has not observed such a requirement for washed preparations in the 

 presence of Succinate. However, in the presence of ascorbate the washed system is 

 not activated unless vitamin K is added, and in this way it behaves very much like 

 the chloroplast preparations of Dr. Anion and co-workers. It may be that, at 



