188 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



9. Benzoquinones in the reactivation of petrolether extracted chloroplasts 



As already mentioned, plastoquinone = dimethyl-solanosyl-p-benzoquinone 

 is an endogenous quinone of chloroplastst'*). Its importance for the photosyn- 

 thetic activity of chloroplasts became apparent, when Bishop showed that the 

 ferricyanide (and DC PIP) reduction in petrolether extracted chloroplasts was 

 plastoquinone dependent(5). A number of groups have now investigated the sys- 

 tem of Bishop i.e. the properties of petrolether extracted chloroplasts in more 

 detail(55"^°). The results are, that all known photosynthetic reactions of chloro- 

 plasts— the Hill reaction with different acceptors (ferricyanide^^ > ^^ ' °°', DCPIP^ ', 

 benzoquinones^^^) and TPN^^^ ' 59))_are inactivated and become plastoquinone 

 dependent, when the chloroplasts were exhaustively extracted with petrolether 

 or acetone. The only exception is the reduction of TPN by DCPIP/ascorbate, 

 which seems to be independent of plastoquinone'^" ' ^^' (see however Crane^ °'). 

 This provides some indication as to the location of endogenous plastoquinone in 

 the electron transport chain of chloroplasts. In agreemient with Witt's interpre- 

 tation of experiments, which gave more direct spectroscopic evidence' ', plas- 

 toquinone is probably the acceptor of the second light reaction and is situated 

 before the cytochrome chain. 



We have pointed out, however, that incomplete extraction of plastoquinone 

 leads to a somewhat different picture'^"^ By extraction of 7o% of the endoge- 

 nous plastoquinone (which can easily be accomplished by an only short treatment 

 of chloroplasts with petrolether), only the reduction of ferricyanide and oi 

 o-quinones is impaired, but not the reduction of TPN and p-benzoquinones^ '. 

 We have concluded from this, that there is a second site of plastoquinone in the 

 electron transport chain^^"'. This second site has to be in a sidepath to the 

 main chain (leading to TPN) after the first light reaction (see scheme) and is 

 participating only in ferricyanide reduction. This second plastoquinone site 

 has recently been confirmed by Witt by direct spectroscopic observations at 

 26o mJ^^K 



Crane recently suggested also several plastoquinone sites in photosyn- 

 thesis(^°). The higher sensitivity of the Hill reaction towards UV-light as com- 

 pared to cyclic photophosphorylation, noticed by Avron'" ', and the stimulation 

 of photosynthetic reactions of chloroplasts by the further addition of plastoqui- 

 none(°^'°^) (surprising in view of the high plastoquinone content of chloroplasts) 

 might also be explained by two plastoquinone sites, only one of which is of phys- 

 iological importance. 



The two plastoquinone sites are different also in the specifity by which 

 they can be reactivated after petrolether extraction of the endogenous plasto- 

 quinone^^^). Table 13 shows that after incomplete extraction of plastoquinone 

 (removal of plastoquinone at site 2), the reduction of ferricyanide, vitamin K3 

 and anthraquinone-sulfonic acid is impaired and shows a stimulation by the ad- 

 dition of plastoquinone-45, whereas this is not the case with the reduction of 

 TPN and the two substituted benzoquinones. 



