Reduction of Dinitrophenol by Chloroplasts 



J. S. C. Wessels 



Philips Research Laboratories, 



N. V. Philips' G/oeilampenfahrieken, 



Eindhoven^ Netherlands 



Isolated chloroplasts are capable of synthesizing ATP from inorganic 

 phosphate and ADP in light, provided they are supplemented with 

 catalytic amounts of an electron carrier such as FMN, vitamin K, or 

 phenazine methosulphate. This ATP formation was named cyclic photo- 

 phosphorylation by Arnon to distinguish it from the phosphorylation 

 associated with the reduction of substrate amounts of TPN or K3 Fe(CN)6. 



In contrast with the oxidative phosphorylation of mitochondria, which 

 is completely uncoupled by concentrations of DNP (2,4-dinitrophenol) as 

 low as io~^-io^* molar, cyclic photophosphorylation of chloroplasts 

 proved to be rather insensitive to DNP. Inhibition was observed only at 

 concentrations at which the Hill reaction was also blocked, that is at about 

 io~^ molar. 



In the course of investigations on the effect of DNP on cyclic photo- 

 phosphorylation it was found that DNP is even capable of catalyzing the 

 generation of ATP by illuminated chloroplasts. As shown in Table I, the 

 optimal concentration of DNP under anaerobic conditions is about o-6 

 /xmole per 3 ml. of reaction mixture, which is of the same order of magni- 

 tude as the optimal concentrations of vitamin K3 or FMN. 



It has been shown earlier that under anaerobic conditions vitamin K3 

 and FMN are involved in separate pathways for cyclic photophosphoryla- 

 tion. The FMN-pathway proved to be more sensitive to the poisons 

 NHoOH, NaN3, KCN, and o-phenanthroline than the vitamin K3- 

 pathway. Cyclic photophosphorylation catalyzed by DNP is similar to 

 phosphorylation in the presence of vitamin K3 as regards the insensitivity 

 to KCN and NaNg and the inhibition by dicoumarol, /)-chloromercuri- 

 benzoate, and CMU (3-(4-chlorophenyl)-i,i-dimethylurea). NH.,OH and 

 o-phenanthroline, however, are more inhibitory to cyclic photophos- 

 phorylation catalyzed by DNP than to phosphorvlation catalyzed by 



