182 L. SMITH 



at 550 vcifi on illumination of the mixture are not due to the oxidation 

 of cytochrome c. Similar results were obtained with mammaUan and 

 ruhrum cytochrome c and with either powdered glass or sonic extracts 

 of the bacteria. 



The data of Fig. 1 indicate that a soluble cytochrome can be either 

 oxidized or reduced on illumination in the presence of aerobic ex- 

 tracts of Rhodo spirillum ruhrum, the predominant reaction depend- 

 ing upon the state of oxidation of the cytochrome. There is some 

 evidence that the oxidizing and reducing substances can accumulate 

 in solution. For example, there is observed a more rapid rate of re- 

 duction of cytochrome c in the presence of succinate in the dark 

 after a period of illumination. This view is strengthened by the ob- 

 servation that cytochrome c can be oxidized or reduced when the 

 cytochrome in solution is illuminated in the presence of whole cells 

 of the bacteria. It has been shown that cytochrome c does not pene- 

 trate whole cells. 



When the extract of Rhodo spirillum ruhrum was heated at 70 °C. 

 for 30 minutes and the mixture suspended in 1.5 M sucrose to prevent 

 settling of the precipitate, it was found that cytochrome c could still be 

 oxidized on illumination of the mixture. Since this amount of heating 

 will destroy even the most heat-stable enzymes, this means that the 

 cytochrome c is oxidized or reduced on illumination in the presence of 

 ruhrum extract by a nonenzymatic reaction. This eliminates the need 

 for postulating the presence of special '^cytochrome c photo-oxidase" 

 in the bacteria. 



Discussion 



Vernon: I should like to ask Dr. Smith if she has taken into account, in this 

 rate constant, the fact that in this organism there is a relatively high cytochrome 

 reductase activity. 



Lucile Smith : Not in washed particles. They have no substrate and are com- 

 pletely free of reductase activity. 



Vernon: Another point I would like to make is that the effect of heating is 

 similar to what was observed earlier by Kamen and me. 



Lucile Smith: Heating to 70°C. for 30 minutes is much more effective than 

 boiling. This will inactivate even horse-radish peroxidase, which is particularly 

 heat-stable. If you heat it this way, you get a fine precipitate, which you can then 

 suspend in strong sucrose solution. 



