BENT LEY (iLASS 889 



the process is to be repeatctl, and il X and } are catalysts, not sub- 

 strates, in amount, then there nuist be a How ol electrons from XHj 

 and to YO. In reaction 2, XH2 is oxidized by some electron acceptor 

 such as a redox dye or a pyridine nucleotide. The reduction of YO 

 to }' may take place either by reaction 3 (as in the Hill reaction) or 

 by reaction -/ (as in cyclic electron transport for which phenazine 

 methosulfate and pyocyanine are cofactors) . When AH.^ is oxidized 

 by molecidar oxygen (reaction 5) , reaction -/ is precluded, and YO 

 must be reduced by reaction 3. The participation of oxygen in the 

 sequence of reactions 1,2,3,5 may be detected by means of O^^ as an 

 oxygen exchange reaction between the water and the atmosphere. 

 [Ed. This corresponds in Arnon's terminology to the "oxygen- 

 catalyzed cycle."] In the present Symposium Birgit Vennesland and her 

 associates have discussed this reaction, and have shown that both 

 FMN and vitamin K3 elicit a rapid oxygen exchange in isolated 

 chloroplasts. Actually, reaction 3 fails to provide sufficient oxygen 

 for reaction 5, so that additional oxygen is required to stimulate 

 phosphorylation with low concentrations of FMN as cofactor. 



Jagendorf and Forti have worked recently with the j^hotophosphory- 

 lation inhibitor p-chlorophenyldimethylurea (CMU) , and have come 

 to the conclusion that it inhibits specifically reaction 3, the oxygen- 

 evolving step. For CMU strongly inhibits the Hill reaction (steps 

 1,2,3) and the FMN-catalyzed phosphorylation (steps 1,2,3,5) , but 

 it does not strongly inhibit an anaerobic cyclic electron flow with 

 PMS or pyocyanine as cofactor (steps 1,2,4) . A number of complica- 

 tions appeared with PMS or pyocyanine as cofactors. Thus a small 

 amount of oxygen present in the system makes pyocyanine-catalyzed 

 electron flow sensitive to CMU. This was explained as an oxidation 

 of pyocyanine by O2 (reaction 5) , thereby using up ^Hg (reduced 

 pyocyanine) and blocking reaction 4. With reaction 3 simultaneously 

 blocked by the inhibitor CMU, there would be no way open for 

 YO to be reduced to Y, and so electron flow would stop. But PMS 

 does not behave like pyocyanine, in the presence of oxygen and CMU, 

 because in white light or upon the addition of ascorbate it is reduced 

 non-enzymatically, thereby restoring reaction 4. Jagendorf and Forti 

 point out the similarity of CMU-poisoned chloroplasts to bacterial 

 chromatophores, the cyclic electron flow and phosphorylation of which 

 are also poisoned by small amounts of oxygen but relieved by electron- 

 donating substrates. 



Having some assurance from these studies that CMU inhibits re- 

 action 3, workers may use this inhibitor to aid in distinguishing cyclic 



