179 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



sum: 



H^O + 1/2 O^ + ADP + P.- 

 2 2 1 



►H^O^ + ATP 



The ratio of ATP and HO formation to O uptake is 1 



1 : o,5 



(14,22) 



The Hill reaction with a quinone may therefore be followed not by oxygen 

 evolution, but by oxygen uptake, if the experiment is done in air and lo~-^m 

 KCN is added. Only catalytic amounts of a quinone are required, which is often 

 important, when substrate amounts of a quinone are inhibitory or insoluble. 



H2O2 formation by illuminating chloroplasts and its stimulation by quinones 

 is long known as Mehler reaction''^°'. It seems, however, somewhat misleading, 

 to speak of oxygen as a Hill reagent, when it is only a variant of a Hill reaction 

 with a quinone. 



4. The photooxidation of hydroquinones 



It is proper to assume, that the reaction between a hydroquinone and oxy- 

 gen under formation of H2O2 in aerobic photophosphorylation is an autoxyda- 

 tion, particularly if the hydroquinone has a low redoxpotential. This is not 

 correct, however, in the case of the oxidation of hydroquinones with rather pos- 

 itive redoxpotentials, since these are not, at ppj 8, readily autoxidizable. Still, 

 p-hydroquinone and even better, chlorogenic acid or dopamin are, as shown in 

 table 2, excellent cofactors of aerobic photophosphorylation and these hydro- 

 quinones are therefore rapidly oxidized by chloroplasts. The suggestion, that 

 a phenoloxidase might be responsible' I'*', can be ruled out, since the experi- 

 ment can be done in the presence of KCN''^'^', which would inhibit the phenol- 

 oxydase. 



Table 5 shows, that chlorogenic acid, p-hydroquinone and dopamin (with 

 redoxpotentials above +29o mV) are not substantially oxidized by chloroplasts 

 in the dark, whether KCN is absent or not (there is some phenoloxidase activity, 

 as seen in the dopamin experiment). In the light, however, oxygen is taken up 

 and H2O2 accumulates, even and particularly in the presence of KCN (which 

 again inhibits endogenous catalase). 



in the dark in the light 



/Liatoms O /imoles H2O2 /iatoms O jumoles H2O2 

 taken up 



chlorogenic acid o 



+ lo-^m KCN o 



p-hydroquinone o 

 " + lo-^m KCN 



dopamin 1 , 3 



+ lo"^m KCN o 



Table 5: Photooxidation of hydroquinones (5 ^mol) by broken chloroplasts 



(conditions as in table 1; 15 min). 



