612 



Mordhay Avron and Noun Shavit 



approximat-ely doubles the rate of ferricyanide reduction. A further addition of an un- 

 coupler increases the rate to an even higher value. While bringing about this further in- 

 crease in the rate of electron flow the rate of the accompanying phosphorylation drops to 

 zero. Thus, the P/2e" ratio of about 1 obtained in the absence of the uncoupler gradually 

 decreases to zero as the concentration of the uncoupler is Increased. The low rate of ferri- 

 cyanide reduction, obtained In the absence of the phosphorylatlon-components is increased 

 by the uncoupler to the same maximal rate obtained in the system containing the phospho- 

 rylatlon-components. This results in a tripling cf the rate of electron flow. Increasing the 

 concentration of the uncoupler beyond that required for maximal rate results in a sharp 

 decrease of the rate of reduction. 



Potent new uncouplers of photophosphorylation 



Until recently ammonium salts were the most potent uncouplers of photophosphorylation 

 known. Attempts to find uncoupling activity in the chloroplast system with the commom 

 potent uncouplers of oxidative phosphorylation have resultedin failure. Thus, dinitrophenoP ' 

 pentachlorophenol(°), and valinomycInC^)^ have no effect at the concentration in which 

 they completely uncouple oxidative phosphorylation. 



Recently several more potent and unrelated uncouplers of photophosphorylation have 

 been discovered. Figure 2 describes the uncoupling effects of atebrin (quinacrlne, atabrine) 

 and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (P-CF3O-CCP). Atebrin is 

 a rather poor uncoupler of oxidative phosphorylation(13), || ^/qs reported by Baltscheffskyv''*^ 

 to Inhibit photophosphorylation between 10~5 - 10"^ M. Also, ArnonC^) mentioned that 

 atebrin is an uncoupler of photophosphorylation.As seen in Fig. 2 atebrin acted as a classical 

 uncoupler. 50% inhibition of photophosphorylation was attained at about 1x1 0"-* M. 



F 300 

 I 



i 



I 



j.oo 



I 



-^ 



r^eRRiCYAMioe 



300- 



p-CFjO-CCP 



200^ 



^PHOSPHOftTUVTlON 



10- 10- 



PHOSPHORYLATION 



Fig. 2. Uncoupling of the ferricyanide system by atebrin or carbonyl 

 cyanide p-trlfluoromethoxyphenylhydrazene (p-CFsO-CCP). 

 Details as described under Fig. 1. 



Several derivatives of carbonyl cyanide phenylhydrazone were shown by Heytler('°) to be 

 potent uncouplers of oxidative phosphorylation and Inhibitors of photophosphorylation with 

 phenazine methosulfate as cofactor. Bamberger et al.v'7) has recently demonstrated their 

 uncoupling effect on photophosphorylation coupled to NADP reduction. Fig. 2 illustrates 

 the uncoupling effect of the most potent of the derivatives described by Heytler, carbonyl 

 cyanide p-trlfluoromethoxyphenylhydrazone, on ferricyanide reduction and the accompa- 

 nying phosphorylation. 50% inhibition of photophoJphorylatlon was attained at about 

 IxlO"^ M. Thus, P-CF3O-CCP is 1-he most potent uncoupler of photophosphorylation 



