182 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



Table 7 indicates, that photophosphorylation catalyzed by indigo- sulfonic acids 

 is like the FMN system stimulated by ascorbate. This stimulation is again 

 DCMU sensitive. 



0,2 ymol indigo-sulfonic acid 



" " +2,5 ymol ascorbate 



1' " " " + lo""^ DCMU 



0,2 jimol indigo-disulfonic acid 



" " +2,5 ;imol ascorbate 



" + lo"'* DCMU 



o, 1 ^mol FMN 



" " + 2,5 ^mol ascorbate 



1! <• " " + lo"^ DCMU 



Table 7: Stimulation of cyclic photophosphorylation by ascorbate (conditions 

 as in table 1; 15 min light in N^). 



This might best be explained as a donation of electrons by ascorbate into 

 the electron transport chain before the DCMU block. In the absence of ascorbate, 

 the oxidation of the reduced cofactor by an endogenous oxidizing compound of 

 the chloroplasts is limiting. Ascorbate alone is not a good cofactor, since then 

 the reduction of dehydro- (or monodehydro-) ascorbate is limiting^^ '. Against 

 the explanation that ascorbate substitutes for water is, that a stoichiometry of 

 oxygen evolution and TPNH formation is observed also in the presence of as- 

 corbate. 



o-Quinones (or rather o-hydroquinones) behave quite different from qui- 

 nones with negative redoxpotentials in the ascorbate oxidation by chloroplasts. 

 o-Hydroquinones do not stimulate ascorbic acid photooxidation by chloroplasts. 

 On the contrary, ascorbate inhibits the photooxidation of these hydroquinones 

 (table 8). 



o, 1 jLtniol chlorogenic acid 



0,1 /xmol chlorogenic acid + lo |imol ascorbate 



o, 1 jxmol catechol 



o, 1 ^mol catechol + lo ^imol ascorbate 



o, 1 ^mol anthraquinonesulfonate 



o, 1 ^mol anthraquinonesulfonate 



+ lo ^mol ascorbate 14,1 6,7 



Table 8: Inhibition of o-hydroquinone photooxidation by ascorbate (in compar- 

 ison with anthraquinone) (15 min light in air; lo'^m KCN per vessel). 



