498 



Joseph S. Kahn 



When the protein-chlorophyll was treated with reduced thiols 

 and then dialyzed under a nitrogen atmosphere, ferricyanide re- 

 duction was strongly inhibited. The inhibition appeared to be 

 due to the formation of -SH groups rather than the disruption of 

 disulfide bonds, since the inhibition could be reversed by 

 p-chloromercuribenzoate (Table II). Arsenite or thorough 

 aeration also reversed the inhibition by reduced thiols. 



Table II 



The effect of reduced thiols and of p-chloromercuri- 

 benzoate (PCMB) on the rate of ferricyanide reduction 

 by the protein-chlorophyll. 



The thiols were dialyzed out before assay, PCMB was 

 added to the reaction mixture. 



jumole ferricyanide reduced °/o of 

 mg chlorophyll x hr control 



control 23.1 100 



5 X 10"^ M PCMB 20.0 87 



5 X lO" M glutathione (red.) 7.8 34 



5 X 10 M cysteine 7.1 31 



5 X 10 M/3 -mercaptoethanol 7.3 32 



glutathione + PCMB 19.6 85 



cysteine + PCMB 22.7 98 



;3-mercaptoethanol + PCMB 18.4 80 



The rate of ferricyanide reduction by the protein-chlorophyll 

 as a function of light intensity is given in Figure I. Maximal 

 activity appears to be reached at 7000-8000 foot-candle, but 

 measurements at these light intensities were uncertain because 

 of the rapid bleaching of the chlorophyll. The rate of ferri- 

 cyanide reduction was linear only up to 10 pgm chlorophyll/ml 

 of the reaction mixture; moreover, the rate decreased during 

 illumination even at low light intensities, becoming non-linear 

 after 5-10 minutes. As a consequence, the activity below 30 



