PHOTOOXIDATION AND PHOTOREDUCTION REACTIONS 257 



phore which are not ordinarily associated with the cyclic electron 

 transfer process. This accounts for the slower rates observed in gen- 

 eral for the photooxidation and photo reduction reactions with/?, ruhruni 

 chromatophores. 



The same general pattern applies to C/zrowa^mw, with the exception 

 that in this case the slow enzymatic reactions are not observed at all. 

 Some essential cofactor must be lost or destroyed during preparation 

 of the chromatophore. 



There is an alternative explanation for the data presented above. 

 There is ample evidence for "reverse electron flow" in mitochondrial 

 systems, in which case the electrons are transferred from either suc- 

 cinate or cytochrome to NAD (45). This transfer being against the 

 thermodynamic gradient, energy is required and the energy is supplied 

 by ATP, Thus the requirements for electron transfer in the reverse 

 direction are the necessary enzymatic components plus energy sup- 

 plied in the form of ATP. In the present case, these conditions are 

 present. Thus one could explain the photooxidation of DPIPH2 as being 

 due to reversed electron transport via the reagents contained in the 

 chromatophore with the energy for this reverse electron transport 

 being supplied by the ATP formed in the process of photophosphoryla- 

 tion. Although on the surface this explanation appears to be tenable, 

 there are a number of reasons which lead me to believe that this can- 

 not be the case. These reasons are as follows: 



1. The photophosphorylation process is very sensitive to the re- 

 spiratory inhibitors, antimycin A and HQNO, At 10~^ M inhibitor con- 

 centration, the photophosphorylation process is almost completely in- 

 hibited, while at this concentration of inhibitor the photooxidation 

 reactions proceed with very little depression in rate. A more striking 

 example is given by the compound quinacrine which inhibits photophos- 

 phorylation by 80 per cent at a concentration of 5 x 10-4 m (40), yet 

 has no appreciable effect on the DPIPH2 fast reaction at this concen- 

 tration and actually stimulates the coupled slow reaction with fumarate. 



2. When R. riibniDi chromatophores are heated to 60°C, their abil- 

 ity to carry out photophosphorylation is completely inhibited. Never- 

 theless, such chromatophores still have the ability to catalyze the 

 fast oxidation of DPIPH2 as shown in Fig. 14. 



3. The photo reduction of NADP by DPIPH2 in the plant system 

 (utilizing the long wavelength system in chloroplasts) does not appear 

 to proceed by reverse electron flow. Thus, the photo reduction of NADP 

 by DPIPH2 results in the formation of ATP, and removal of plasto- 

 quinone from the chloroplasts prevents the ATP formation but does 

 not prevent NADPH formation (46). 



4. Addition of ATP in the dark to the system containing chromato- 

 phores, DPIPH2 and NAD does not result in the formation of NADH and 

 DPIP. The results of the experiments are shown in Fig. 15, Whereas 



