236 ELECTRON TRANSPORT 



(5). He has also studied the stability of the photooxidase system in 

 R. rubrum chromatophores (6). 



In a series of investigations, which were designed primarily for 

 studying the photo reduction system of chromatophores, Frenkel dem- 

 onstrated that a photoreduction of NAD could be coupled to a photo- 

 oxidation of reduced FMN (7). Succinate also supported the photore- 

 duction of NAD, and a photooxidation of this compound was implied, 

 Vernon and Ash also studied the photoreduction of NAD, which was 

 coupled to a photooxidation of succinate (8) . 



Ample evidence has accumulated during the past several years 

 showing that whole cells of the photosynthetic bacteria are capable of 

 catalyzing a photooxidation of intracellular cytochrome components. 

 Duysens (9) and Chance (10) observed an oxidation of cytochrome upon 

 illuminating cells of R. rubrum under anaerobic conditions. This ob- 

 servation was later confirmed by the experiments of Chance and Smith 

 (11). The light-induced oxidation of intracellular cytochrome has been 

 extensively investigated for the bacterium Chromatium (12,13,14). Since 

 this photooxidation proceeds at temperatures as low as 80 °K, this 

 reaction is probably one of the primary photoreactions taking place 

 after absorption of a light quantum by the chromatophore (12). 



Photoreduction Reactions: 



In the intact cell, the reducing phase of photosynthesis is evidenced 

 in terms of carbon dioxide reduction. At the chromatophore level, the 

 earlier experiments of French (2) and Vernon and Kamen (3) showed 

 a photoreduction of oxygen. A photoreduction of NADP by chromato- 

 phores of R. rubrum was reported by Vernon (15). The experiments 

 of Frenkel (7) showed that NAD is photo reduced by R. rubrum chroma- 

 tophores coupled with a photooxidation of added FMNH2. Subsequent 

 experiments (8) showed that NAD photoreduction could be coupled with 

 succinate, and that NAD was the nicotinamide nucleotide of choice in 

 these reactions. Nozaki et al. (16) demonstrated that NAD photore- 

 duction could also be coupled to the oxidation of DPIPH2 in the 

 presence of ascorbate. Other photo reductions observed with /^. rub- 

 rum chromatophores have involved methyl red and tetrazolium blue 

 (17), the disulfide DTNB (18) and sulfate ion (5). A summary of these 

 photoreactions and the rates which have been observed to date is 

 shown below in Table 6. 



The photoreduction of intracellular NAD by R. rubrum cells was 

 shown in the investigation of Duysens and Sweep (19). A similar photo- 

 reduction was observed with Chromatium cells by Olson (20), and 

 Amesz (21) has recently completed an extensive investigation on NAD 

 photoreduction with R. rubrum cells. Evidence has been presented 

 for a photochemical reduction of ubiquinone contained within the chro- 

 matophores of Chromatium and Rhodopseudomonas spheroides (22), 



