497 



Joseph S. Kahn 



chlorophyll complex was able to photoreduce ferricyanide but not 

 di- or trichlorophenol-indophenol or NADP, the pH optimum for 

 the ferricyanide reduction being 8.0-8.5, similar to that for 

 chloroplasts. The reduction of ferricyanide appeared to be non- 

 enzymatic, since the activity of the complex was not greatly re- 

 duced by boiling for 5 minutes. No photophosphorylation could be 

 detected, but a slow photohydro lysis of ATP, amounting to 2-3 

 pmole/mg chlorophyll/hr , could be observed. Plastoquinone could 

 not be detected in the protein-chlorophyll, and the addition of 

 plastoquinone to protein-chlorophyll extracted with iso-octane 

 had no effect on ferricyanide reduction (Table I). Menadione 

 gave a small but consistent stimulation, the reason for which 

 will become apparent later. 



Table I 



The effect of plastoquinone and menadione on ferricy- 

 anide reduction by the protein-chlorophyll. The 

 plastoquinone and menadione were dissolved in ethanol to 

 give a final concentration of 5% ethanol in the reaction 

 mixture. 



jumole ferricyanide reduced % of 

 mg chlorophyll x hr control 



Control 23.1 100 



Extracted with isooctane 22.1 96 



+ 5 X 10~ M plastoquinone 21.4 93 



+ 5 X lO' M menadione 27.7 120 



+ 57o ethanol 23.4 101 



Polarographic measurements failed to show any evolution of 

 oxygen during ferricyanide reduction. In agreement with this, 

 CMU (p-chlorophenyl-1, 1-dimethyl urea), which inhibits oxygen 

 evolution in whole chloroplasts and thus inhibits their ferri- 

 cyanide reduction, failed to decrease the activity of the 

 protein-chlorophyll. Although there was a substantial bleaching 

 of the chlorophyll during illumination, there was no stoichi- 

 ometry between the chlorophyll bleached and the ferricyanide re- 

 duced. The nature of the electron donor for the reduction is as 

 yet unknown. 



