REDUCTION OF DINITROPHENOL BY CHLOROPLASTS 447 



Discussion 



Vennesland : I would like to comment on the fact that you find that hydro- 

 quinone and catechol are not co-factors for cyclic phosphorylation. We have been 

 working with a variety of diphenolic compounds and we find that if we put in a 

 high enough concentration of hydroquinone or catechol or any compound which 

 has this type of structure, they will support photophosphorylation ; we have to put 

 in what appear to be substrate amounts but, nevertheless, we think that the reaction 

 is cyclic. That is, the hydroquinone and the catechol are oxidized mainly non- 

 enzymically — this is why you need such high concentrations — and then the 

 quinones are reduced with accompanying photophosphorylation. Have you looked 

 to see what happens if you go to high concentrations of the substances you tested ? 

 Let me add that the reason that we are particularly interested in this is that some 

 of the flavonoids, which are di-o-hydroxy compounds, appear to be good cyclic 

 cofactors. These substances occur in leaves, but they have been neglected as 

 possible natural cofactors for photophosphorylation. Perhaps if we are looking 

 for substances that might function in this way physiologically, the flavonoids 

 should be considered. 



Wessels : The compounds which were found to be capable of catalyzing cyclic 

 photophosphorylation showed an optimal activity at a concentration of about 

 10 ~* M. For this reason all phenols which we have examined were added in con- 

 centrations ranging from 10 ~^ to 10^ M. We have not added substrate amounts 

 of any of the substances compiled in the Table. 



