FAST LIGHT REACTIOxX 199 



Rhodosjyirillum rubrum, so could not use DPNH as an indicator of intracellular 

 pH changes. 



Strehler: Do you get any evidence that a substance other than that from p- 

 phenyl mercuric acetate treated cells is being reduced in the light? 



Chance : No, not to any appreciable extent in R. rubrum. The spectroscopic 

 effects of Fig. 1 (p. 193) observed in the aerobic cells don't appear until an ap- 

 preciable portion of the cytochrome has been reduced under anaerobiosis. 



Frenkel: Have you observed any spectral changes in cell-free preparations? 



Chance: Yes, but they are very different from those observed in whole cells. 

 A photooxidation reaction is observed when you reduce the cell-free preparation 

 with DPNH. Also the cell-free preparations show an aerobic light effect similar to 

 that observed in phenyl mercuric acetate treated whole cells. 



Frenkel : I just ran an experiment which somewhat reminds me of your reduc- 

 tion in the dark. I refer to the reduction of cytochrome added to a cell-free prepara- 

 tion after the hght period. The reaction appears to be almost completely anaerobic, 

 but there must be a small amount of oxygen present because on illumination the 

 cytochrome is oxidized. I don't know whether it goes as you, or as Vernon and 

 Kamen, postulate. The interesting thing is that, after the light is turned off, the 

 cytochrome is reduced at about twice the initial rate. So, this effect resembles the 

 one you obtained. 



Chance: Yes, our records show a rapid reduction of the intracellular cjrto- 

 chromes when the light is turned off, and our results with the DPNH-reduced 

 extract are in agreement, too. 



Frenkel: I just wondered whether this was evidence for the formation of a 

 reducing power in the light. 



Chance : Reducing power is formed, but our data do not say where. Remember 

 that the dark oxidation of DPNH is not slowed by illumination (Fig. 5, p. 195). 



Amon : Did I understand you to say that, in cell-free preparations, you observed 

 reduction rather than oxidation? Is that correct? 



Chance : A change at 430 mp in cell-free preparations is also observed in whole 

 bacteria inhibited by phenyl mercuric acetate. This effect may be due either to a 

 reduction of cytochrome or to an oxidation of bacterial chlorophyll. On the other 

 hand, add a reductant like DPNH to the extract and you will still observe this 430 

 m/z effect. In this case it wiA be followed by a slow oxidation reaction. Thus one 

 may question whether a reduction occurs. 



Amon: Will you just define what you mean by 430 effect? 



Chance: There are two hypotheses: (1) an oxidation of chlorophjdl or (2) a 

 reduction of hemoprotein. 



Amon : Is the reducing effect the last one? 



Chance : Yes, although this is a less plausible hypothesis. 



Frank Allen: Is it possible to calculate in any way the extent of the change 

 that one would expect in chlorophyll under normal conditions and to compare 

 this with the changes that are observed whose nature is undetermined to see if 

 we are at least in the right ball park? 



Rabinowitch: What you mean by "under normal conditions"? 



Frank Allen : Any conditions you can do it under. Given an illuminated plant, 

 can we not assume that the changes we observe are related to the number of 



