178 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



should be expected (whereas almost any redox-catalyst will be active in the 

 presence of oxygen). Inhibition studies with low concentrations of DCMU and 

 KCN(^^'^^' ^^) and the separation of a cyclic photophosphorylation system 

 from the oxygen evolution system^^-^), seems to support strongly the view, 

 that true cyclic photophosphorylation exists, at least when vitamin K3 or PMS 

 as cofactors are used. Of course, this does not mean that cyclic photophos- 

 phorylation, as observed in isolated chloroplasts, is also a physiological 

 reaction. 



Very recently Arnon showed, that ferredoxin can act, under certain con- 

 ditions, as a cofactor of cyclic photophosphorylation' '. The stimulation of 

 this cyclic photophosphorylation by DCMU^^^' is reminiscent of the cyclic sys- 

 tem with DC PIP as catalyst{22). As table 4 shows, DC PIP is effective as a co- 

 factor of a photosynthetic ATP formation in the absence of oxygen. This ATP 

 formation is not only insensitive to high concentrations of DCMU, but is actu- 

 ally stimulated by it. Photophosphorylation in the presence of oxygen is, how- 

 ever, inihibited. (Since the DCPIP must first be reduced in a DCMU-sensitive 

 Hill reaction, DCMU was added after 1 min pre -illumination). Cyclic photophos- 

 phorylation with DCPIP as catalyst has recently been confirmed in several 

 laboratories (2 5"^^). 



/imoles ATP formed in 



additions to o,3 fxmol DCPIP nitrogen 



air 



.-- 1.5 6,3 



+ lo" m DCMU 5,3 o, 1 



Table 4: DCPIP as cofactor of cyclic photophosphorylation (conditions as in 



table 1, 15 min light. DCMU was added after 1 min pre-illumination). 



3. Quinones in HO formation 



Warburg showed that in aerobic photophosphorylation the hydroquinone, 

 acting as a catalyst, is reoxidized by O2 under formation of H2O2' '. Since 

 endogenous catalase of the chloroplasts would decompose most of this H2O2, a 

 catalase inhibitor has to be added in order to observe H2O2 accumulation. KCN, 

 aminotriazole or diethyldithiocarbamate can be used, since these compounds do 

 not interfere with the photosynthetic reactions^^^) . lo"-^m KCN is the most 

 convenient inhibitor. Instead of inhibiting the endogenous catalase, an ethanol/ 

 catalase trap for H2O2 may be added(^°'^^) . The reaction sequence of aerobic 

 photophosphorylation in the presence of KCN is then: 



Q + HO + ADP + P. -QH^ + 1/2 O, + ATP 



2 \ I c. 



QH^ , O^ .Q + H^O^ 



