CYCLIC AND NONCYCLIC PHOTO PHOSPHORYLATION 183 



TABLE 6 

 IitJiibitioii of Cyclic Pliotophosphorylation by Dyes under Reducing Conditions 



„ . , rr. . . « moles ATP formed /mg 



Expei'iment Treatment ^ r hi /h 



A 3.3 X 10-4 M DPNH2 137 



10-4 M methyl viologen 154 



DPNH2 + methyl viologen 7 



B 6.7 X 10-3 jYI ascorbate 239 



6.7 X 10-5 M 2,6-dichlorophenol indo- 



phenol (DPIP) + 6.7 x 10-3 ^ 



ascorbate 9 



ascorbate, DPIP, air 240 



C 10-4 jvi phenazine methosulfate (PMS) 314 



10-4 M PMS + 6.7 X 10-3 m ascorbate 9 



PMS, ascorbate, air 240 



D H2 146 



H2 + hydrogenase 55 

 H2 + hydrogenase + 10-4 m methyl 



viologen 2 



A 16-day old chromatophore preparation was used in Experiments B and C. 

 Other conditions and components of the phosphorylation mixture were the same 

 as given in Table 1. Anaerobic conditions were employed except as otherwise 

 indicated. 



antimycin A (10,23), It seems likely that DPIP, like phenazine metho- 

 sulfate (10), acts as a b3rpass agent around an antimycin A- sensitive 

 site in cyclic photophosphorylation (cf, 23), 



The ability of these dyes to act as bypass agents for the antimycin 

 A- sensitive site of cyclic photophosphorylation is influenced by the 

 redox status of the system. As shown in Table 7, phenazine metho- 

 sulfate did not bypass antimycin A inhibition in the presence of 

 5 X 10-3 M ascorbate. Likewise, DPIP failed to catalyze an antimycin 

 A-insensitive phosphorylation in the presence of 10-3 m ascorbate. 



The influence of ascorbate concentration on the bypass effect of 

 DPIP is of special relevance to our subsequent discussion. As shown 

 in Table 7, DPIP catalyzed an appreciable antimycin A-insensitive 

 cyclic photophosphorylation in the presence of lO"'^ M ascorbate. 

 However, this cyclic photophosphorylation was abolished at concentra- 

 tions of ascorbate of 10-3 m or higher. Attention is called to these 

 experimental conditions, since, as will be shown later, an antimycin 

 A-insensitive noncyclic photophosphorylation was measured in the 

 presence of DPIP but at concentrations of ascorbate at which cyclic 

 photophosphorylation is excluded (Table 7), 



