181 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



oxidizing ascorbic acid. The DCMU sensitivity of the Hill reaction would ac- 

 count for the DCMU inhibition of the overall reaction. 



However, at the short exposure time in the experiments of table 6, ascorbic 

 acid is stable in the presence of HO. 



additions to lo ^mol ascorbate 



o, 1 ixmol anthraquinonesulfonate 

 o, 1 ^mol anthraquinonesulfonate 



+ lo-^m DCMU 

 o,o3 /imol vitamin Ko 

 o,o3 jimol vitamin K^ + lo" m DCMU 



Table 6: Inhibition of quinone stimulated ascorbate photooxidation by DCMU 



(conditions as in table 1; 15 min light in air, lo"-^m KCN per vessel). 



Z. Ascorbic acid is donating electrons into the electron transport chain at a 

 point before the DCMU inhibition site. The electron is transported to the qui- 

 none, which is then autoxidized. This is a mechanism, also proposed by 

 Ikeda^ 'and very recently by Chiba'-^"' and which is in agreement with the ex- 

 periments of Habermann(-^^)and Marre^-^^^). In this view, ascorbate (Aa) 

 would substitute for water as electron donor at the same site. 



ADP + P. + H^Aa + oiiliiQH + Aa + ATP 



i 2 ~ ^2 



\ -Q + ^2^2 



QIL + O^ ►Q + H^O^ 



sum: ADP + P. + H_,Aa + O^ ►Aa + H^O. + ATP 



1 Z Z d c 



The ratio of H2O2 and ATP formation to O^ uptake in this reaction type is 

 1 : 1 : 1 as against the ratio of 1 : 1 : o, 5 in aerobic photophosphorylation. 



This quinone stimulated ascorbic acid photooxidation differs in many re- 

 spects from the DCPIP stimulated ascorbate oxidation. The latter is not inhib- 

 ited by DCMU('^°). One major change introduced by the addition of DCPIP is 

 the point of entry of the electrons donated. As the work of Vernon^ and 

 Witt^'^' clearly indicated, ascorbate donates electrons via DCPIP into the 

 cytochrome chain. 



Explanation 2 would also account for the ascorbate stimulation of cyclic 

 photophosphorylation with certain cofactors. Vennesland^ showed, that for 



FMN catalyzed cyclic photophosphorylation catalytic amounts of ascorbate are 

 required and that the ascorbate stimulation is abolished by o-phenanthroline. 



