PHOTOOXIDATION AND PHOTOREDUCTION REACTIONS 263 



would allow for the phosphorylation accompanying aerobic oxidation of 

 substrate molecules via NAD. Since this organism is lacking a cyto- 

 chrome oxidase in the traditional sense, one looks for another com- 

 pound to complete the electron transfer span to molecular oxygen, and 

 the logical candidate is RHP (36). For reasons discussed above, the 

 flavin listed in this scheme may well be common to both the photo- 

 chemical reduction of NAD and the various enzymatic reductase ac- 

 tivities observed in extracts of R. nihnun in the dark (36). It has been 

 reported by Cohen- Bazire and Kunisawa that the particles called 

 chromatophores, which can be isolated after the cell is broken, are 

 actually segments of the cell membrane which has been ruptured dur- 

 ing the process, and that in the intact cell the photosynthetic apparatus 

 is laid down upon this limiting membrane (54). The implication is 

 obvious, therefore, that this same membrane is involved in both the 

 oxidative reactions and the photosynthetic reactions. One would expect, 

 therefore, to find both of these functions combined in the one structural 

 component, and theutilizationof a portion of the photosynthetic electron 

 transfer chain in the oxidative metabolism would meet this goal. 



The compounds which are photooxidized in the presence of NAD and 

 fumarate are shown to interact with the electron transfer scheme at 

 the cytochrome c^ level. One of the prime reasons for proposing this 

 site of entry into the chain is that the photooxidation of these com- 

 pounds is not affected appreciably by the respiratory inhibitors anti- 

 mycin A and HQNO at concentrations which almost completely inhibit 

 the photo phosphorylation process. The data of Nishimura point toward 

 the site of action of these inhibitors somewhere between cytochrome 

 b and cytochrome C2 (23,24). The entry of DPIPH2 and the other photo- 

 oxidizable compounds at the cytochrome c^ locus is also consistent 

 with the fact that this cytochrome is not firmly bound in the chromato- 

 phore system and can be removed by relatively easy treatments such 

 as extraction with citrate buffer, etc. This indicates that the cyto- 

 chrome is exposed to the aqueous medium in the chromatophore and 

 would logically be a site of action for these compounds. Furthermore, 

 Jacobs has shown that in a rat-liver mitochondrial system, TMPD 

 reacts by reducing the cytochrome c on the particle (27). 



Fig. 17 shows two sites for ATP formation along the electron 

 transfer chain. This is in accord with the data of Baltscheffsky and 

 Arwidsson (56), who have studied the effect of the inhibitor valinomycin 

 upon R. nibnim chromatophores in the photophosphorylation reaction. 

 Also, the data of Nishimura (34) on the amount of ATP formed per 

 flash of light with R. nibnim chromatophores indicate that two sites 

 for ATP formation are to be found. If PMS overcomes antimycin A 

 inhibition by means of serving as a bypass for the inhibited site be- 

 tween cytochrome b and cytochrome c^, then there must be a phos- 

 phorylation site before the site where PMS is reduced. The scheme in 

 Fig. 17 accommodates this. 



