431 



Birgit Vennesland 



scaftered light is shown by the dashed line. The curves show how the fluor- 

 escence diminishes during illumination, but reappears after a dark interval. 



We can also use visible light for the photoreduction, and follow the dark 

 reoxidation by taking readings at intervals with a very small amount of 

 fluorescence-exciting light. The measurements made in Fig. 4 were ob- 



100 r 



2 



2 4 

 Minutes 



Fig. 4 Effect of Cyanide and Azide on Photoreduction and Dark Reoxi- 

 dation of FMN 



tained in this way. This experiment was done to show how reagents which 

 inhibit catalase delay the unnecessary back reaction. Three different cu- 

 vettes were set up under argon with identical chloroplast plus flavin reac- 

 tion mixtures, except that one contained cyanide in addition, one azide, 

 and one, no inhibitor. All cuvettes were exposed to the same amount of 

 light during the period indicated by the dashed line. Conditions used were 

 selected to show how both cyanide and azide cause an apparent enhance- 

 ment of the photoreduction of flavin. They assuredly do this by inhibiting 

 the unnecessary back reaction. That is, the dismutation of flavin to re- 

 duced flavin and H^O^ appears to occur faster than the simple Hill re- 

 action, because 0_ reoxidizes reduced flavin faster than does H_0„. 



