182 METABOLISM AND PHYSIOLOGY 



Table 5 shows that the addition of DPNH2 fully restored the phos- 

 phorylating activity of aged chromatophores. Although DPNH2 is a 

 stronger reducing agent than ascorbate, it appears that the addition 

 of DPNH2 alone, even at a concentration as high as 1,7 x 10-2 M, 

 cannot result in a pronounced overreduction of the phosphorylating 

 system. 



TABLE 5 



Effect of DPNH9 oil Cyclic Photophosphorylation 

 by Fresh and Aged Chromatophores 



_, . , TAT^-NTTT 4- + " //moles ATP formed/mg 



Chromatophores DPNH2 concentration ^ r hl/h 



The reaction mixture for fresh chromatophores lacked the hexokinase sys- 

 tem but contained instead 10 ^ moles A DP. Other conditions and components of 

 the phosphorylation reaction mixture were the same as given in Table 1. 



Overreduction with DPNH2, at the relatively low concentration of 

 3,3 X 10-3 y[^ ^a^s observed, however, in the presence of methyl 

 viologen (Table 6), Similar results were obtained in the presence of 

 benzyl viologen, but DPIP was not effective. Table 6 also shows that 

 the inhibitory effect on photophosphorylation by overreduction with 

 ascorbate was markedly enhanced by the presence of DFIP or 

 phenazine methosulfate. Overreduction by H2 plus hydrogenase was 

 greatly enhanced by the addition of methyl (or benzyl) viologen (Table 

 6); in this system, as with DPNH2, DPIP was much less effective. That 

 these dyes have indeed enhanced overreduction is indicated by the re- 

 versibility of their effects on admission of air to the system (Experi- 

 ments B and C in Table 6), 



These results suggest that these dyes have a strong affinity for the 

 phosphorylating system of chromatophores into which they promote 

 an electron flow not only from a moderately reactive reductant such 

 as DPNH2 but also from the more reactive ascorbate. 



The affinity of phenazine methosulfate and DPIP for the phosphory- 

 lating system of chromatophores is so strong that, under certain con- 

 ditions, they catalyze cyclic photophosphorylation in the presence of 



