265 



Sakae Katoh and Atusi Takamiya 



0.12 



2 3 



T I M E ( min ) 

 Fig. 1. Time course of photooxidation or reduced 

 plastocyanin with digitonin-treated chloroplasts 



oxygen from the reaction mixture by repeated evacuation and flushing with 

 nitrogen resulted in a complete suppression of the oxidation even when 

 illumination. 



The rate of photooxidation was maximal at about 5, 000 lux. This is 

 considerably lower than the light intensity required for photoreduction of 

 plastocyanin with intact chloroplasts. The pH curve was rather flat with an 

 optimum at pH 8.0-8. 5. 



The reaction was found to be insensitive to various poisons, and there 

 was even a distinct stimulation of the reaction with some of the reagents 

 tested. The stimulation observed in the presence of cyanide, o-phenanthroline 

 and EDTA may be due, in part, to the removal of metal ions from the reaction 

 mixture, since the reaction was highly sensitive to heavy metal ions such as 

 mercury and silver. DCMU at a concentration of 10"6 M was completely with- 

 out effect. PCMB is somewhat inhibitory at a concentration of lO'^ m. 

 Chloroplast fragments, which had been heated at 650 for five minutes were 

 incapable of oxidizing the protein. The reaction was markedly stimulated by 

 a rather high concentration of ammonium sulfate, which was previously shown 

 by Bishop et al. to induce a remarkable acceleration of the photooxidation of 

 reduced cytochrome c'lB), 



