THE EFFECT OF OXYGEN ON PHOTOPHOSPHORYLATION 



AND THE EFFECT OF CARBONATE ON THE 



HILL REACTION^ 



BiRGiT Vennesland, T. Nakamoto, and Babette Stern 



Department of Biochemistry, University of Chicago 

 Chicago, Illinois 



The effect of oxygen on pliotophospliorylation by spinach chloro- 

 plasts with FMN as a cofactor has been described previously (4) . At 

 low cofactor concentrations, the photophosphorylation is completely 

 dependent on oxygen, but ^vith increasing cofactor concentrations, 

 the reaction appears to proceed as well anaerobically as aerobically. 

 A parallel study with menadione (Kg) has shown similar results. 

 At about 2 X 10-*^ M K3, optimal photophosphorylation occurs 

 aerobically, but the "anaerobic" rate is quite small. With 10 to 40 

 times as much Kg, the rate in nitrogen approaches that in air. With 

 K3, air sometimes becomes inhibitory at higher cofactor concentra- 

 tions. Similar studies Avith phenazine methosulfate and pyocyanine 

 also show that at low cofactor concentrations, oxygen has a stimulatory 

 effect. 



The stimulatory effect of oxygen suggested that the photophos- 

 phorylation was accompanying a cyclic process in which the added 

 cofactor is photoreduced, with O2 evolution, and is then reoxidized 

 by molecular oxygen (Equations 1 and 2) . 



cofactor + (H2O) » reduced cofactor -+-3/^02 (1) 



hv 

 reduced cofactor -j- 32 C>2 ' cofactor + H2O {2) 



This was confirmed by the demonstration that both FMN and K3 

 elicit a rapid oxygen exchange reaction, as measured with O^^. Some 

 representative data of Nakamoto, Krogmann, and Mayne (3) are 

 shown in Table 1. 



It is our opinion that chloroplasts which have not suffered damage 

 to their oxygen-evolving mechanism prefer a cycling process in which 



' Supported bv a grant from the National Science Foundation. 



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