184 METABOLISM AND PHYSIOLOGY 



TABLE 7 

 Reactivation of Autiuiyciii A -Inhibited Cyclic Photophosphorylatioii by Dyes 



^ . ^ m 4. ^ Per cent of 



Experiment Treatment 



phosphorylating activity 



The phosphorylating activity of a similar system without antimycin A was 

 designated as 100 percent. 10 jugoi antimycin A was included with all the treat- 

 ments shown above. Where the dyes were added their respective concentrations 

 were: phenazine methosulfate, 6.7 x 10-5 M; methyl viologen, 10-4 M; 2,6- 

 dichlorophenol indophenol (DPIP), 6.7 x lO'^ M. Fresh chromatophores were 

 used in Exper. B, 4-day old chromatophores in Exper. A and 30-day old chro- 

 matophores in Exper. C. Other conditions and components of the phosphorylation 

 mixture were the same as given in Table 1. 



II. PYRIDINE NUCLEOTIDE REDUCING 

 SYSTEM IN CHROMATOPHORES 



Frenkel (19) and Vernon and Ash (20,21) have shown that chroma- 

 tophores of R. rubrum can photoreduce DPN by succinate, reduced 

 FMN or the ascorbate-DPIP couple. We have confirmed the photo- 

 reduction of DPN, using succinate or the ascorbate-DPIP couple as the 

 electron donor (22), The reduction of DPN by chromatophores is a 

 photochemical reaction; our attempts to replace light with ATP were 

 unsuccessful. DPNH2 is more reducing than the electron donors used, 

 and photons absorbed by the photosynthetic pigments supply the addi- 

 tional energy needed to drive the electron transfer against the thermo- 

 dynamic gradient, by what appears to be a light-induced noncyclic 

 electron flow. 



Washed chromatophores were found to be highly specific in their 

 ability to photoreduce DPN; TPN was not reduced. The addition of 

 ferredoxin from R. nihruyn, Chromatium, or spinach (27) did not 

 change the rate or specificity of DPN photoreduction. However, we 

 found that washed chromatophores were able to photoreduce TPN as 

 rapidly as DPN when the reaction mixture included a water-soluble 

 extract of R. rubnim cells (the supernatant solution from the chroma- 

 tophore preparation). 



