262 ELECTRON TRANSPORT 



when inhibitors such as antimycin A are present to block electron 

 transfer between cytochrome b and cytochrome c^ (23,24), 



Fig. 17 presents the basic electron transfer system proposed in 

 Fig. 16, and has included the possible sites of action of the various 

 compounds which enter into the photochemical reactions demonstrated 

 by these particles. For discussion purposes, let us begin at the lower 

 end of the potential scale. The experiments of Tagawa and Arnon im- 

 plicate ferredoxin in the hydrogen metabolism of selected bacteria, 

 both photo synthetic and nonpho to synthetic (47), To date, no evidence 

 has been presented to show that hydrogen evolution can be produced 

 with isolated R. nibnim chromatophores, although it is made in 

 copious amounts by the intact cell under various conditions (51). Be- 

 cause of the implication of ferredoxin in other hydrogen-producing 

 systems, and because of the suitability of the potential of the two sys- 

 tems, it is indicated as the precursor of hydrogen in Fig. 17. 



A number of compounds are shown as reacting with the chromato- 

 phore system at the level of ferredoxin and/or flavin. These compounds 

 include oxygen, methyl red, tetrazolium blue, sulfate ion, and the di- 

 sulfide bond contained in DTNB. The primary reason for including 

 these compounds at this position is that the photo reductions involving 

 these compounds are all heat stable and also relatively insensitive to 

 the inhibitors which affect the other photo reactions. Lindstrom has 

 shown that the photooxidation of DPIPH2 in the presence of air is very 

 stable to heat (6). Information presented above shows that aerobic 

 photooxidase is also insensitive to the inhibitors tried, such as anti- 

 mycin A, HQNO, and quinacrine. The methyl red and tetrazolium blue 

 photoreductions have also been shown in our laboratory to be stable 

 to heat. Heating chromatophores to 60 °C for five minutes destroys the 

 ability of these particles to carry out photophosphorylation and the 

 secondary slow photooxidation of DPIPH2 coupled with NAD and fumar- 

 ate, but does not appreciably affect the methyl red photoreduction. 

 Likewise, the photoreduction of methyl red and tetrazolium blue is 

 also relatively insensitive to inhibitors known to affect the other photo- 

 reactions (17). Photoreduction of sulfate and DTNB are also fairly 

 stable to heating, although heating does have more effect upon these 

 reactions than upon the others in this group (5,18). 



RHP is shown in Fig. 17 as reacting with molecular oxygen (49), 

 Succinate and fumarate are shown as reacting with the electron trans- 

 fer system through ubiquinone. This assignment is logical in view of 

 the known reaction of succinate with ubiquinone in mitochondrial sys- 

 tems (52) and in view of the recent report that ubiquinone can be re- 

 duced enzymatically in the dark by succinate with chromatophores 

 from R. rubnim (53), 



The portion of the electron transfer chain up to RHP could very well 

 be functioning as an NADH oxidase system in the dark respiratory 

 activities of this organism. The phosphorylation coupled to this span 



