185 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



photosynthesis . Plastocyanine is inhibited by salicylaldoxime . Its prop- 



erties are not against the site of function we are suggesting. Therefore plasto- 

 cyanine might be identical with the copper enzyme, which is inhibited by sal- 

 cylaldoxime in isolated chloroplasts . The site of inhibition of salicylaldoxime 

 might also be at the compound E, Witt is proposing at that particular point^*''. 

 Further experiments are needed to establish the role of copper and its function 

 at the suggested site in photosynthesis. 



Table 9 shows that rather high concentrations of KCN behave just like 

 salicylaldoxime. We think therefore, that the inhibition of photosynthesis by 

 lo'^m KCN, as observed by Warburg(49), Jagendorf(^^), Schwartz'^°' and 

 Vennesland(51) might be due to the inhibition of the copper enzyme. 



The influence of salicylaldoxime on photosynthetic reactions of chloro- 

 plasts can also be shown in broken chloroplasts, which were treated shortly 

 with 5* lo'rn salicylaldoxime and then washed once to remove most of the 

 salicylaldoxime during the actual photosynthesis experiment. In such "salicyl- 

 aldoxime treated" chloroplasts again only the photosynthetic reduction of TPN 

 with DCPIP/ascorbate as electron donor is possible; cyclic photophosphorylation 

 and O7 evolution are blocked'^^). Broken chloroplasts treated shortly with 

 3- lo-^m KCN behave as salicylaldoxime treated chloroplasts^ '. 



These salicylaldoxime treated chloroplasts show a number of interesting 

 properties besides those, already mentioned. 



7, Photooxidations in salicylaldoxime treated chloroplasts 



It is seen already in table 9 that in the presence of salicylaldoxime ascor- 

 bate alone without the addition of DCPIP is able to restore TPNH formation to 

 a certain extent (in DCMU experiments this is not possible). The same is also 

 true in salicylaldoxime treated chloroplasts^^^). Ascorbate seems to enter the 

 electron transport chain in these experiments not before the DCMU inhibition 

 site (as in the photooxidation experiments in intact chloroplasts, as discussed 

 in chapter 5), but even after the salicylaldoxime inhibition site, possibly at 

 cytochrome f. Loosening of the copper enzyme, usually tightly coupled to cy- 

 tochrome f by salicylaldoxime seems to allow access of the ascorbate to the 

 cytochrome f without the mediation of DCPIP. 



This change of the point of entry of ascorbate into the chain of electron 

 carriers by treating chloroplasts with salicylaldoxinie is supported by the dif- 

 ferent behavior of quinone stimulated ascorbate photooxidation. In untreated 

 broken chloroplasts (Pls3 in table lo) the photooxidation of ascorbate is inhib- 

 ited by DCMU (as discussed in chapter 5), in salicylaldoxime treated chloro- 

 plasts it is not (table lo). 



The behavior of o-hydroquinone photooxidation also changes. In broken 

 chloroplasts the photooxidation of o-hydroquinones is inhibited by DCMU^ °', 

 in salicylaldoxime treated chloroplasts it is not (table 11). This distinguishes 

 treatment of chloroplasts with salicylaldoxime (or KCN) from that with detergents, 



