SOME EFFECTS OF OXYGEN IN PHOTOSYNTHESIS 

 BY CHLOROPLAST PREPARATIONS 



F. R. Whatley 



It has been suggested that the presence of oxygen is neces- 

 sary for photosynthetic phosphorylation. Thus, Nakamoto et al. 

 (1) showed that the cyclic photophosphorylation catalyzed by 

 flavin mononucleotide (FMN) was greatly stimulated by oxygen 

 when very low concentrations of the cofactor were supplied. 

 The formation of ATP in this system was found to be accompanied 

 by an oxygen exchange (2,3), suggesting that, under the experi- 

 mental conditions employed (compare also ref. h) , the mechanism 

 of the electron transport involved a reaction of reduced FMN 

 with oxygen. But this mechanism cannot be a general one for 

 cyclic photophosphorylation since, as is generally accepted, 

 phenazine methosulfate (PMS) catalyzes a true cyclic photophos- 

 phorylation not involving an oxygen exchange under either aero- 

 bic or anaerobic conditions (2,3>^)' However, even in such a 

 truly cyclic system oxygen has been found to play a role, 

 apparently adjusting the redox balance of the system under some 

 conditions . 



Another system in which a part of the photosynthetic appara- 

 tus of chloroplasts has been linked with oxygen consumption is 

 the "cytochrome £ photooxidase" activity observed by Nieman and 

 Vennesland (5) in digitonin extracts of chloroplasts. 



This article discusses two aspects of the participation of 

 oxygen in photosynthetic reactions of chloroplasts: (1) in the 

 photooxidation of ferrocy tochrome c by digitonin extracts of 

 chloroplasts and (2) as a "poising agent" to regulate electron 

 flow in the true cyclic photophosphorylation catalyzed by PMS 

 in broken chloroplasts. 



( 1 ) Photooxidation of reduced cytochrome c . 



Confirming the results reported by Nieman and Vennesland (5), 

 it was found that digitonin extracts of chloroplasts (prepared 

 by continued extraction of spinach chloroplasts by L'fo digitonin, 

 followed by the centrifugation procedure described by Nieman 



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