175 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



The table indicates, that in the presence of all quinones ATP is formed, 

 which in most cases is accompanied by oxygen evolution. It seems safe to con- 

 clude, that substituted benzoquinones can be used as Hill reagents and that their 

 reduction is coupled to ATP formation. The non-cyclic type of photophosphoryla- 

 tion(8) can be generalized to probably all Hill reagents. 



However, as seen in table 1, stoichiometry between oxygen evolution and 

 ATP formation is usually not obtained. With falling redoxpotential of the qui- 

 none, oxygen evolution is lagging behind ATP formation; when the redoxpotential 

 is approaching zero V, no oxygen is evolved at all. This is in agreement with 

 the results of Wessels^^^ The reason for this seems easy to understand. The 

 more negative the redoxpotential, the more autoxidizable is the hydroquinone. 

 Therefore the oxygen evolved in the reduction of the quinone is reacting back 

 with the hydroquinone formed. This was already suggested by Wessels^ as 

 possible explanation for his failure to observe reduction of compounds with a 

 redoxpotential below -loo mV. That such backreactions occur, can now be seen 

 by the formation of ATP without any measurable (by manometric techniques) 

 oxygen evolution. The experiments in table 1 have been performed in nitrogen; 

 apparently the small concentration of oxygen formed is sufficient to react with 

 the hydroquinone formed. Therefore oxygen as well as the quinone/hydroquinone 

 couple are then cycling. 



substrate 



^atoms O 

 evolved 



^moles ATP 

 formed 



ferricyanide 



p -b enz oquinone 



toluquinone 



2 , 3 -dimethyl-p-benz oquinone 



2 , 5-dimethyl-p-benz oquinone 



trimethylbenz oquinone 



2 , 6-dimethoxy-p-benz oquinone 



2 , 6 -dimethoxy-methylbenz oquinone 



2-hydroxy-benzoquinone-5-propionate 



2 -methyl-naphthoquinone 



phthiokol 



anthraquinone-2- sulfonic acid 



o.l 



5.0 



5,8 



4.4 



5.1 



3,9 



4.1 



3,3 



2,6 



o,l 



o 



o 



o 



Table 1: Quinones as Hill reagents 



Illumination for 15 min at 15° with 3 5ooo Lux in nitrogen. Each 

 vessel contained in /imoles: trisbuffer p 8,o 8o; MgCl2 5; ADP lo; 

 Pj^ lo; quinone 5 or ferricyanide lo; and broken chloroplasts (Pj^^j^) 

 with o,3 chlorophyll in a total volume of 3 ml. 



