187 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



8. p-Hydroxylation of salicylaldoxime by chloroplasts 



The salicylaldoxime experiments, described so far, were done at a con- 

 centration of lo" m. In air at a concentration of less than lo~ m, salicylald- 

 oxime behaves quite different. It is then not an inhibitor, but a cofactor of 

 photophosphorylation in chloroplasts^ ', Of a number of substituted phenols 

 tested, only salicylaldehyde and its oxime were active as cofactor s of an aero- 

 bic photophosphorylation (table 12). 



/imoles ATP formed 



-2 

 lo m salicylaldoxime in nitrogen 



lo"^ m " in " 



lo m " in air 



lo" m 



lo"^ m 



lo m salicylaldehyde 



o-cresol 



m-cresol 



p-cresol 



salicylalcohol 



salicylic acid 



gentisinalcohol 



g enti s inald ehyd e 



gentisinic acid 



l,o 

 o,5 

 o, 5 

 o,5 

 5,5 

 6,3 

 3,9 

 1,1 

 1,1 

 o,9 

 1.4 



1,1 

 6,5 

 6,9 

 5,5 



Table 12: Aerobic photophosphorylation with substituted phenols (conditions 

 as in table 1 ). 



Since salicylaldehyde (or its oxime) is not a reversible redoxcatalyst, one 

 has to assume, that it can be converted into one by the chloroplast system. It 

 is, indeed, possible to isolate gentisinaldehyde as its dinitrophenylhydrazone, 

 after salicylaldehyde has been incubated in air with chloroplasts in the light^^^)^ 

 This indicates that salicylaldehyde and its oxime are hydroxylated in the 

 p-position to a compound, which can now be reversibly oxidized and reduced and 

 which is the actual cofactor of the photophosphorylation. As seen in table 12, 

 gentisinaldehyde is quite an active cofactor of photophosphorylation in air. A 

 somewhat more detailed examination of the hydroxylation system suggested, 

 that a peroxydase reaction is responsible for the hydroxylation' '. 



The inhibitor salicylaldoxime is by this hydroxylation converted into a 

 cofactor of photophosphorylation. This is reminiscent of Wessel's experiments 

 with dinitrophenol. This inhibitor of photophosphorylation is, by reduction to an 

 aminophenol by illumination with chloroplasts, also converted into a cofactor'-' '. 



