190 METABOLISM AND PHYSIOLOGY 



acceptor for noncyclic photophosphorylation. After this quantity of 

 DPN was completely reduced, the rate of ATP formation greatly de- 

 creased. However, when substrate amounts of DPN or a DPN- 

 regenerating system was then added to the reaction mixture, the rate 

 of ATP formation became greatly accelerated. 



Effect of aging 



Another line of evidence which supports the existence of both cyclic 

 and noncyclic photophosphorylation in chromatophores is the differ- 

 ential stability of these two systems to aging. As was already shown in 

 Table 5 and is again demonstrated in Table 10, chromatophores re- 

 tained a capacity for cyclic photophosphorylation after at least 30 

 days of storage; such decrease in activity as occurred during this 

 period was fully restored by the addition of ascorbate. By contrast, 

 Table 10 shows that chromatophores stored for 30 days lost completely 

 the capacity for noncyclic photophosphorylation and that this loss was 

 irreversible. 



TABLE 10 

 Loss of Noncyclic Photophosphorylatioi in Aged Chronatophores 



/ymoles of ATP formed/mg Bchl/hr 



fresh aged 



system i^^^^^^.^^^.^ chromatophores chromatophores 



Photophosphorylating AHHf fresh aged 



Noncyclic ascorbate, DPIP, 43 2 



DPN, antimycin 



A 



Cyclic none 133 41 



Cyclic ascorbate (5 X 10-3 _ I35 



M) 



Inhibited cyclic antimycin A (10 ^g) 9 5 



Noncyclic photophosphorylation was carried out under the same conditions as 

 given in Table 8. Experimental conditions and reaction mLxture for cyclic photo- 

 phosphorylation are given in Table 1. 



We have suggested elsewhere (4,5) that cyclic and noncyclic photo- 

 phosphorylation in chloroplasts and, by extension, in chromatophores, 

 share a common site for ATP formation. If this hypothesis is correct, 

 it would follow that the loss of noncyclic photophosphorylation on stor- 

 age is the result of the inactivation of the DPN- reducing system without 

 which ATP formation could not occur by the noncyclic pathway, but 

 which would not affect ATP formation by the cyclic pathway. This 

 interpretation is consistent with the experimental findings, Chromato- 

 phores kept at ca 4°C for a week lost more than 90 percent of their 



