180 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



These hydroqainones therefore seem to be photo oxidized. This photooxi- 

 dation is not a chlorophyll sensitized photooxidation for two reasons. 1. The 

 photooxidation is inhibited by lo""m DCMU and lo'^^m o-phenanthroline^ ' 

 (see also table 11). 2. Treatment of chloroplasts with a detergent and solu- 

 bilizing the chlorophyll destroys the ability to photooxidize hydroquinones^^*^) . 



The inhibition of this photooxidation by DCMU seems to indicate that the 

 oxygen evolution system of photosynthesis somehow participates. It is difficult 

 to give an explanation. It seems conceivable that a hypothetical peroxyde, which 

 gives off oxygen in usual photosynthesis, is oxidizing the hydroquinone, perhaps 

 via a quinolperoxyde. A mechanism like this has been proposed also by Jagendorf 

 as an explanation for the photooxidation of ascorbate by otherwise unsupplemented 

 chloroplasts^-^^). This theory implicates that oxygen is required in order to 

 evolve oxygen, even in normal photosynthesis. Such a hypothesis has been ad- 

 vanced in particular by Schenck on the basis of the behavior of chlorophyll in 

 chemical photoreactions'-^^'. Certainly more experiments are needed, to sup- 

 port such a view. 



5 . Quinone s in the photooxidation of ascorbic acid 



As already naentioned, the photooxidation of o-hydroquinones shows sim- 

 ilarities to the photooxidation of ascorbic acid. There are several possible ways, 

 in which ascorbic acid may be oxidized by chloroplasts, which cannot be dis- 

 cussed here in detail (see Jagendorf^^-^' for a review of the pertaining literature). 

 Stimulation of ascorbic acid oxidation by quinones was studied in several labo- 



ratories(^4-4o, 31) Wessels concluded that the quinone stimulated photooxida- 



• • • • (34) 



tion of ascorbic acid is a chemical, chlorphyll sensitized, photooxidation^ '. 



Others, however, concluded that ascorbic acid photooxidation proceeds via all 

 or part of the electron transport chain of chloroplasts'35 , 37)^ Quinone stim- 

 ulated ascorbic acid photooxidation was inhibited by o-phenanthroline in Ikeda's 

 experiments' ''. Substituted p-benzoquinones with a redoxpotential in the range 

 from o till + Zoo mV cannot be used for the stimulation of ascorbic acid photo- 

 oxidation by chloroplasts, since they catalyze already a rapid dark oxidation. 

 Also vitamin K3 at a concentration of lo~m catalyzes a chemical dark oxidation 

 of ascorbic acid. More interesting is the stimulation of ascorbic acid oxidation 

 by low concentrations of vitamin K3 (lo'rn) and by anthraquinonesulfonic acid, 

 which occurs only by illumination with a chloroplast system. This stimulation 

 is inhibited by DCMU (table 6), which suggests that the electron transport chain 

 of photosynthesis in chloroplasts or part of it is participating, lo" m KCN was 

 not inhibiting and was added to prevent H2O2 decomposition by the endogenous 

 catalase. Coupled ATP formation in a ratio 1 to 1 to H2O2 also supports the 

 view that we are not just dealing with a chlorophyll sensitized photooxidation. 



Two explanations for this stimulation of ascorbate oxidation can be offered 

 (see also Jagendorf^-^'^'): 



1. The quinone is reduced in a Hill reaction and rapidly autoxidized under for- 

 mation of H2O2. The H2O2 (perhaps with the help of a peroxidase) is quickly 



