L.K.H* Duysens 



(25) 

 ridine nucleotide occvirs^ . 



In algae, pyridine nucleotide reduction could be_mo8t readily- 

 studied in the blue-green alga Anacystia nidulansC^A), As pre- 

 dicted by the scheme of Pig, 1, the action spectrum for the ini- 

 tial rate of pyridine nucleotide reduction was found to be 

 roughly proportional to the action spectrum for C 420 oxidation 

 in the presence of an inhibitor of the cytochrome reducing re- 

 action. Measurements of the quantum requirement for this reduc- 

 tion were consistent with the assumption that one hydrogen ab- 

 sorbed by system 1 was sufficient for the transport of one hy- 

 drogen-equivalent to pyridine nucleotide. Since about 8 quanta 

 (absorbed by both pigment systemsl^are probably necessary for 

 the reduction of 1 C0„ molecule (26), this implies that each 

 photochemical system requires one quantum per transported hy- 

 drogen atom or electron. The quantum requirement for C 420 oxi- 

 dation by system 1 is estimated to be higher/th^n 2 for Porphy- 

 ridium ^ ' and still higher for Anacystis '(24), which suggests 

 that part of the hydrogens transported from OH to P 700 bypasses 

 C 420. For this reason we have put C 420 within a rectangleo 



Vredenberg (unpublished observations) observed a auch higher 

 efficiency for cytochrome oxidation after cooling to 2 C. 



Quinone reactions 



Addition of plastoquinone stimulates the/Hill reaction under 

 certain conditions as first shown by Bishop . This has been 

 taken as a proof of participation of plastoquinone as a redox 

 intermediate in the Hill reaction. It is also conceivable, how- 

 ever, that the quinone acts as a structural factor, which is 

 necessary for optimal rate of the Hill reaction. Also the ob- 

 servation that quinones are reduced or oxidized by chloroplasts 

 does not prove, as was argued in a general way in the introduc- 

 tion, /that quinone is a photosynthetic intermediate. Recently 

 Amesz^ 'obtained, upon illumination of the blue-green alga 

 Anacystis , in the ultraviolet region a difference spectrum which 

 was similar to the difference spectrum of oxidized minus reduced 

 plastoquinone (see Fig. 3). Fig. 3 then indicates that light ab- 

 sorbed by system 1 causes oxidation of quinone, and that light 

 absorbed by system 2 favors its reduction. The reduction of qui- 

 none was inhibited by low concentrations of DCMU. The time 

 courses of the oxidation and reduction indicate thatthe quantum 

 efficiency for these reactions is rather high. The tl^ value of 

 quinone, and the fact that DCMU inhibits quinone reduction in- 

 dicates that quinone is located between *4 and C 420. The amoxait 

 of quinone participating in this light-driven redox reaction is 

 only about 1^ of the amount of chlorophyll ^ present. This is 



