439 



H. Gaffron, W. Wiessner and P. Homann 



OXYGEN EVOLUTION 



Some time ago Bishop in our laboratory showed that the her- 

 bicidal action of DCMU and other substituted ureas is due to 

 their ability to inhibit specifically the oxygen evolving process in 

 green plants. Oxygen evolution is completely inhibited by DCMU 

 concentrations as low as 10"^ M. The same concentration of 

 this poison also stops the anaerobic acetate assimilation of 

 Chlamydobotrys. This observation furnishes one important support 

 for our assumption that an intact oxygen-evolving system is 

 needed for the acetate photometabolism when no external oxygen 

 is available. 



Recent re-investigations on the light-induced evolution of 

 hydrogen by adapted algae, first observed by Rubin and Gaffron, 

 have led to the discovery that this hydrogen evolution in the 

 presence of uncouplers of phosphorylation is nearly always 

 accompanied by a simultaneous evolution of some oxygen. As 

 Bishop reports elsewhere in this volume, the knallgas evolution 

 by adapted algae (first seen in adapted Chlorella by Spruit) 

 depends on an intact oxygen evolving system. 



While the absorption of hydrogen during photoreduction is 

 completely unaffected by DCMU, its evolution was found to be as 

 sensitive to this poison as photosynthesis itself. Without going 

 into details, we may say that a simultaneous production of hydro- 

 gen and oxygen, often in a ratio 2:1, under conditions when 

 synthetic processes are severely curtailed, strongly suggest that 

 a photolysis of water is the very first result of the cooperation 

 between the primary processes in pigment systems I and II. 



Where DCMU and the other substituted ureas interfere with 

 the free flow of electrons within the presently accepted two pig- 

 ment system is not known. There is unanimity that the inhibition 

 disturbs the path of oxygen near the end point, where an oxidized 

 photoproduct dismutates to free oxygen gas. Manganous ions 

 apparently act as a part of the enzymatic system which releases 

 oxygen, as Kessler has shown in our laboratory. Therefore 

 Homann began to investigate the influence of the substituted 

 ureas on an artificial photoreactive system where manganous ions 

 play the role of very specific catalysts. In the reaction mixture 

 under study, 2, 3 diketogulonic acid is photoxidized in the 

 presence of light-excited flavin molecules, manganous ions and 

 oxygen. The observed effect of the substituted ureas on this 

 photoreaction was unexpected. Instead of reacting with 



