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Donald L. Keister 



the indophenol prior to illumination. The observations can all 

 be explained by cyclic phosphorylation. 



Fig, H illustrates a time course of the reduction of TCI and 

 ATP formation under aerobic conditions. There was no ATP forma- 

 tion in this experiment until the reduction of dye had almost 

 reached completion. The rate of phosphorylation was the greatest 

 Just after the completion of dye reduction and was proportional 

 to time thereafter. The ATP formation in Krogmann and Vennesland's 

 experiments was measured only at a single time interval, and thus 

 the inhibition of ATP formation before the dye was reduced was 

 overlooked. 



The concentration dependence of phosphorylation with DCI and 

 TCI is shown in Fig. 5. Approximately a 3-fold higher concentra- 

 tion was required for optimum ATP formation under argon than with 

 an air atmosphere. It is to be noted that concentrations optimal 

 for aerobic phosphorylation catalyze very little anaerobic 

 phosphorylation and thus would appear to be inhibited by removing 

 oxygen. One explanation for the different optimal concentrations 

 is that the reoxidation of the dye by the chloroplast is the rate 

 limiting step in the anaerobic system and the active site for the 

 oxidation has a lower affinity for the dye than does the active 

 site for the reduction. Since the autooxidation of the indophenols 

 by oxygen is rather marked at alkaline pH's, this source of 

 oxidized dye would enhance the turnover of dye in the aerobic 

 system thus shifting the optimal concentration toward the lower 

 optimal concentration of the reducing system. Therefore, a lower 

 concentration would be required for the aerobic phosphorylation. 



The effect of light intensity on ATP formation with reduced 

 indophenols as compared with pyocyanln and FMN is illustrated in 

 Fig. 6. The FMN and pyocyanin curves are identical for both the 

 aerobic and anaerobic systems, therefore, only one curve was 

 drawn for each. The FMN system became saturated at fairly low 

 light intensities as has been previously reported for fmN^^"^ and 

 TPNH^^9; which catalyze non-cyclic phosphorylation, whereas the 

 pyocyanin which catalyzes cyclic phosphorylation did not saturate 

 even at very high light intensities^^^ ) . Fig. 6 demonstrates 

 that under aerobic conditions with reduced DCI as catalyst of 

 photophosphorylation the reaction became saturated at low light 

 intensities similar to that of FMN, whereas under argon the 

 curve is similar to that of pyocyanin which was not saturated 

 with light. These results are indicative that anaerobically the 

 indophenols catalyze a cyclic phosphorylation similar to that of 

 pyocyanin, whereas with oxygen present the reaction appears to 

 be of the non-cyclic type. However, in view of other results it 

 is more probable that it is a combination of cyclic and non- 

 cyclic phosphorylation. 



