369 



Kouchkovsky et al 



That a Mehler reaction can occur during the Hill reaction appears also 

 from the following results. It was found that the decrease in the rate of oxy- 

 gen evolution during the photoreduction of ferricyanide is accompanied by an 

 increase in the rate of oxygen uptake (measured by the extra-absorption, as 

 described above). This is partially shown in Fig. 5 by some segments of 

 curves. Moreover, it was possible to distinguish two components in the value 

 of the steady-state rate of oxygen evolution (Gyi/t). If one assumes that, in 

 the Mehler reaction, there is twice as much oxygen absorbed as oxygen 

 evolved, then it is possible to subtract from the value of Gpj/t (corrected for 

 photo-oxidation) half of F^/t, which represents the rate (Vj^) of oxygen evolu- 

 tion by way of the Mehler reaction. The difference resulting from this sub- 

 traction will correspond to the rate (Vj^) of oxygen evolution by way of the Hill 

 reaction. On plotting the values of Vy[ as a function of V-^, a straight line 

 was obtained'''. 



4 XHOHY ^>- 4XH + O2 + 2H2O + 4 Y (V = I+II) 



4XH + 4FeCy > 4 FeCy red. +4X (VI) 



^M 

 4 XH + 2O2 -^ 2 H2O2 + 4X (VII - m) 



Let us assume that the total amount (a) of the complex XHOHY + the reduc- 

 tant XH is constant. In our experiments, ferricyanide reduction will be 

 second order (Vh = ^n (XH) [FeC^ ), and the Mehler reaction will be first 

 order (Vj^ = k^ [XH] )• From this mechanism, one can derive the follow- 

 ing reaction: 



kM ^M ^L ^ 



^M " - kL +kM ^H + k^ + k^ , 



giving the expected linear dependence of Vj^ on Vh- Moreover, by graphical 

 analysis, the maximum amount (a) of the complex XHOHY could be evaluated; 

 the computed quantity was approximately the same as that determined from 

 direct measurement of the burst (i. e. , from the parameter E, Fig. 5). The 

 numerical value of this quantity was 1 mole oxygen produced per 150 to 300 



(5 9) 

 moles chlorophyll ^ ' '. 



DISCUSSION 



It was possible to show that under certain conditions (in the absence of Hill 

 reagents and in the presence of ferricyanide), induction occurs with isolated 

 chloroplasts. As a result, the cyclic mechanism, previously postulated for 

 whole cell photosynthesis, appears equally valid for at least some chloro- 

 plasts reactions in vitro. The exceptional case, i. e. , the absence of induc- 

 tion with DPIP (at the concentration employed), can be understood assuming 

 that with this reagent only "system 2" functions ^^^'. Photoreduction of the 

 dye would then be faster than oxygen evolution. 



