287 



Giorgio Forti, Maria Luisa Bertole and Bruno Parisi 



activities of the enzyme. It can be seen from table 2 that cyt.£ is reduced by 

 the enzyme 3 times faster than DPN, no additions being required. Cytochrome c 

 is not reduced without the addition of an intermediate carrier as FMN or PMS. 

 The enzyme is specific for TPNH as the electron donor, and no activity is ob- 

 served with DPNH (table 2). 



TABLE 2 

 Activities of the Purified Flavoprotein From Spinach 



Substrate 



TPNH 2 X 10-5m 

 id. 

 id. 

 id. 

 id. 



DPNH 2. 7 X 10-5m 

 id. 



Activity 



Fe(CN)g reduction 



cyt. f reduction 

 cyt. c reduction 

 DPN reduction 

 On reduction 



cyt. f reduction 

 Fe(CN)f^ reduction 



Addition 

 None FMN 5x10-6m 



Conditions: Cytochrome f°^ 1.64 x lO'^M; cytochrome c 1.5 mg/ml; 

 Ferricyanide 5 x lO'^M. Tris 0.05 M, pH 8.0. 

 The concentration of TPNH was kept constant by use of 

 Glucose 6-P dehydrogenase system. The values are 

 expressed in ji equivalents /min. x mg protein. 



Preliminary experiments showed that the Ks for TPNH is approximately 

 4 X 10-6. 



The question of whether the three activities, namely diaphorase, cyt.£ 

 reductase and transhydrogenase are due to the same enzyme or to different 

 proteins present in the preparation remains to be established. The cytochrome f_ 

 reductase could possibly be the same protein as the diaphorase of Avron and 

 Jagendorf^S), and the cytochrome c reductase (requiring the addition of FMN) 

 described by Marre and Servettaz^^). These Authors suggested^) that cyto- 

 chrome f could be the natural electron acceptor for the enzyme. The proper- 

 ties of the enzyme are being studied. 



DISCUSSION 



The results obtained suggest an hypothesis on the participation of cyto- 

 chrome f in photosynthetic electron transport, different from the most widely 

 acceptecTscheme according to which cyt. f mediates the electron transfer from 



