PHOTOSYNTHETIC PHOSPHORYLATION III 



each of the component redox systems is at an optimal oxidation level. We may 

 suppose that alteration of the steady-slate condition tor optimal coupling will 

 result in a certain amount of inactivation of the system. In the extreme cases 

 of complete oxidation or reduction of all components, there would he minimal 

 photophosphorylation. It is reasonable to assume that isolation ot the chromalo- 

 phores from the cell upsets one or more of the redox systems in its relation to the 

 others so that suboptimal conditions for coupling arise. Compounds which can 

 enter into redox equilibrium with the chain can activate by 'pushing" or 'pull- 

 ing' the afifected redox systems back to a better electrochemical range tor coupling, 

 that is, a range in which the redox system operates during chromatophore func- 

 tion in the intact cell. The dramatic repression of activation by phenazine metho- 

 sulfate, or ascorbate, when excess reducing agent is added (20), could be ex- 

 plained on the basis of this mechanism; i.e., if phenazine methosulfate is 

 converted to a more reduced state, it may no longer be effective in restoring the 

 electrochemical potential of the redox system postulated to be altered by the 

 isolation procedure. 



Excessive oxidation of the phenazine methosulfate should also create a sub- 

 optimal redox state. This, in fact, is noted when phenazine methosulfate is photo- 

 oxidized; i.e., the products formed are inhibitory (20). Geller (12) has observed 

 that a slight reduction of phenazine methosulfate is required for optimal activa- 

 tion. 



The remarkable effect of 2,6-dichlorophenolindophenol in inhibiting photophos- 

 phorylation activation by ascorbate or phenazine methosulfate in Chromatium 

 preparations (12) cannot be ascribed to direct reactions with the activators be- 

 cause the indophenol dye exerts an effect when added in catalytic concentrations. 

 A possible explanation is that a redox couple between the activator and the dye 

 is set up which effectively bypasses a portion of the natural chain; if the potential 

 drop left along the chain is insufficient to supply the requisite energy for forma- 

 tion of ATP when one electron is transferred, phosphorylation would not occur. 

 This 'bypass' mechanism has also been proposed to account for the fact that 

 while antimycin inhibits succinate-catalyzed photophosphorylation, it does not 

 affect the phenazine methosulfate-activated system (12). It is remarkable that the 

 condition — presence of a redox couple such as ascorbate +- 2,6-dichlorophenolindo- 

 phenol — which is optimal for photo-oxidation in air (31) is also the same condi- 

 tion which under anaerobic conditions leads to inhibition of photophosphoryla- 

 tion. This suggests that the redox couple is setting up a competitive interaction 

 with the portion of the electron transport chain involved in photophosphorylation 

 and effectively bypassing a part of the chain. The involvement of vitamin K in 

 the chloroplast photophosphorylation, inferred from effects of addition of vitamin 

 K and ascorbate (i), may possibly be explained as owing to a bypass mechanism 



