289 



Giorgio Forti, Maria Luisa Bertole and Bruno Parisi 



Recent results of Chance et aL^O) indicate that a cytochrome b, rather 

 than cytochrome f, is rapidly oxidized when Chlamydomonas cells are illumi- 

 nated with short flashes of light of 6943 A (absorbed by system 1) obtained by 

 means of an optical maser. The oxidation of cytochrome f, observed upon 

 more prolonged illumination, "bears a remote relationship to the photosynthetic 

 apparatus"^ ^^^. These observations are in agreement with our results on the 

 lack of photooxidation of cyt. f_ by system 1 in isolated chloroplasts. 



In conclusion, our results provide the basis and some evidence for the 

 hypothesis that cytochrome f_ is reduced in the light by chloroplasts by means 

 of electrons coming from system l'^', through the action of PPNR and a 

 TPNH-cytochrome f reductase present in chloroplasts. This enzyme has been 

 extensively purifiecTtogether with the diaphorase-transhydrogenase of Keister 

 et al.C?). 



Some properties of the new enzymatic activity discovered are described, 

 including a preliminary value of Ks for TPNH of 4 x 10-6m. It is to be estab- 

 lished whether the transhydrogenase and cytochrome f_ reductase activities 

 can be separated by further purification. The results so far obtained do not 

 allow any speculation on the path of cyt.£ photooxidation by chloroplasts: it is 

 possible that the same 02-requiring, light-requiring system responsible for 

 the oxidation of ascorbateHl) and of quinones^^^) might be involved, with the 

 participation of other intermediates. 



ACKNOWLEDGMENT 



This work was supported by Grant GM-09861-01 from the U. S. Public 

 Health Service. It was performed at the Laboratory of Plant Physiology of the 

 Instituto di Scienze Botaniche dell'Universita, Via Colombo 60, Milano, Italy. 



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3. Forti, G. et al. unpublished. 



4. Forti, G. , Bertole, M. L. and Parisi, B. , Biochem. Biophys. Res. 

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5. Duysens, L. N. M. and Amesz, J., Biochim. Biophys. Acta 64, 243 

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6. Jagendorf, A. T. , The Photochemical Apparatus: its Structure and 

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