262 H. M. HABERMANN AND A. H. BROAVX 



quinone all used up and plenty of oxygen present. Mehler and I have published 

 data showing with tracer oxygen that, as long as quinone is present, you observe 

 no oxygen consumption; tracer oxygen uptake begins only at the end of the 

 quinone reaction. 



Frenkel : Has vitamin K worked? 



Brown : It does not improve the situation. It was used in Kamen's system. He 

 may have some comments about that. We could not confirm the ameliorating 

 effect of the vitamin K. It was not necessary to add it to our system in order to 

 get the maximum ascorbic acid effect. So in the work reported it was not included. 



Kamen : Our experience relates only to Chromaiium. Dr. Newton and I have 

 not been able to get any effect of increased efficiency with menadione in carrjing 

 out phosphorylation in light. Moreover, we cannot find any evidence whatever 

 for any vitamin K either in Chromatium or in the chromatophore. I don't know 

 about Rhodospirillum rubrum. 



As far as the reduction of oxygen is concerned. Dr. Vernon and I did a large 

 amount of work on that and our experience was something, it seems to me, very 

 close to that of Wessels', in that we found that we could not oxidize ascorbic acid 

 unless we added the hydroquinone or the quinone as a carrier. 



Duysens : Wessels recently gave an alternative explanation of the effect of 

 ascorbate and quinone. To support his explanation he did the following experi- 

 ment: He used a chlorophyll solution in ethanol and added 2,6-dichlorophenol- 

 indophenol and ascorbic acid and he got an uptake of oxygen and an oxidation of 

 ascorbic acid. This experiment indicates that the light-induced oxidation of 

 ascorbic acid in chloroplast suspensions is quite different from the "normal" Hill 

 reaction. 



Amon : It seems to me that the point made by Dr. Duysens that you can get a 

 similar effect with chlorophyll solution as distinguished from chloroplasts raises 

 a question about the physiological significance of this effect. 



Duysens : The ascorbic acid oxidation in chloroplast suspensions is not affected 

 by various metal poisons which are inhibitors of the "normal" Hill reaction which 

 indicates that it is a nonenzymatic photochemical reaction between chlorophyll 

 and oxygen. 



Brown : But the mechanism of the ascorbic acid reaction which Mehler tenta- 

 tively proposed is such as to predict that the process should be less sensitive than 

 the Hill reaction to inhibitors of oxygen evolution. In other words, this result is 

 to be expected on the basis of Wessels' photosensitized oxidation explanation, 

 which you favor, as well as from the standpoint that oxygen is consumed via 

 Mehler's mechanism, which I prefer. We seem to be using the same data to sup- 

 port different mechanisms. 



Vernon: Wliat Miss Habermann and Dr. Brown have shown is the coupled 

 reaction. The ascorbic acid stimulates both parts — both the uptake and evolution 

 of oxygen — which means that it is not strictly a photochemical oxidation of the 

 ascorbic acid. You are stimulating two parts of the system, and I think that it is a 

 distinction between chlorophyll in solution and chloroplasts. 



Brown: Ascorbic acid does stimulate both oxygen production and consumption 

 in the case of an unstimulated Mehler reaction. However, in the case of the quinone- 

 stimulated Mehler system, ascorbic acid also increases uptake but decreases pro- 

 duction of oxygen. 



