432 



Birgit Vennesland 



To convince oneself that the reoxidation is inhibited by azide, one must 

 compare rates starting from the same oxidation-reduction level of flavin. 

 The much greater inhibition seen with cyanide than with azide is due to the 

 fact that cyanide has a separate effect. With cyanide, the oxidant (O^, 

 H^O^ or oxidized FMN) eventually disappears under illumination, by 

 dation of some reductant furnished by the chloroplasts. Such a consumption 

 of oxidant occurs in the absence of cyanide too, but much more slowly. 

 Azide does not enhance this disappearance of oxidant. 



2' 

 oxi- 



PHOTOPHOSPHORYLATION 



All of the facts mentioned in the preceding section can be correlated 

 with the effect of cyanide and azide on FMN-supported photophosphoryla- 

 tion, to prove that it is oxidized FMN which is required for the phosphoryl- 

 ation reaction. Thus cyanide causes a striking increase in the inhibitory 

 effect of anaerobiosis on ATP synthesis, whereas azide has no such action. 



If the photophosphorylation is carried out in CO^-HCO„ buffer, the 

 disappearance of acid during ATP synthesis leads to an uptake of CO^ 

 which can be followed manometrically. An experiment done in this way is 

 shown in Fig. 5. In this experiment, two identical reaction mixtures were 



100 

 80 



6 60 



E 

 < 

 ' 40 



1 1 1 



1. 5 /i moles FMN 



5% CO2 -Argon 



0.01 MHCN 



+ l.5yL(.moles 

 FMN 



AP=-I3.4 



AP=-7.9/imoles 



15 



25 



35 



Minutes 



Fig. 5 Reactivation of Photophosphorylation 



