OK THE PARTICIPATION OF CYTOCHROME f IN PHOTOSYNTHETIC 



ELECTRON TRANSPORT 



Giorgio Forti, Maria Luisa Bertole and Bruno Parisi 



Cytochrome f was first isolated by Hill and Scarisbrick( D from higher 

 plants, and obtaine'd in highly purified form from parsley(2). Evidence for the 

 occurrence of this pigment in a large number of photosynthetic tissues has been 

 presented(2), as well as for its being present only in green tissues<2). 



In order to investigate the participation of cytochrome £in the photosyn- 

 thetic reactions of isolated chloroplasts and grana, we have studied a new 

 method for the purification of cytochrome f, utilizing the excellent initial ex- 

 traction procedure reported by Davenport and Hill(2), with only one modifica- 

 tion. This consisted of the addition of the non-ionic detergent Triton X 100 (at 

 the concentration of 1% v/v) to the ethanol-ammonia extraction solvent. The 

 new method, to be published elsewhere<3)^ involves further ammonium sulfate 

 fractionation and column chromatography on Sephadex dextrans. A highly 

 purified preparation was obtained, showing a ratio O. D. at 422 m^ to that at 

 278 mu of 2. 8. The absorption spectrum, as well as the reduced minus oxidized 

 difference spectrum is identical to the one reported by Davenport and Hill^ '. 

 The purity of the present preparation seems slightly higher, as indicated by 

 the higher ratio of absorbancy at 422 m^ to absorbancy at 278 m/i. The most 

 purified preparation still retains catalase activity, as reported by Davenport 

 and Hill(2). Whether this activity is due to contamination or is an intrinsic 

 property of cytochrome f_ is not clearly established at this moment. 



REA CTIONS OF CYTOCHROME f WITH ISOLATED CHLOROPLASTS 



Cytochrome f is reduced by chloroplasts or grana in the light, as pre- 

 viously reported(4J7 and ATP formation is coupled to this reaction with a 

 P/2e- ratio of l(4). 



Recent experiments showed that the photoreduction of cyt.£ is observed 

 in air as well as in N2 atmosphere, contrary to our first report that N2 was 

 needed. Careful washing of the chloroplasts seems to be critical for efficient 

 photoreduction in air. According to current schemes on the function of cyto- 

 chrome f in photosynthesis, this component should be oxidized by the same 

 photoche"mical system which is responsible for TPN reduction (chlorophyll a, 

 or system 1 of Duysens and Coworkers(5)), and should be reduced by the photo- 

 chemical reaction of the "accessory pigments" (system 2(5)). Such a scheme 

 would imply that cyt. f reduction is inhibited by p-chlorophenyldimethylurea (CMU), 



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