PHOTOOXIDATION AND PHOTOREDUCTION REACTIONS 261 



chloroplasts is the corresponding compound photosynthetic pyridine 

 nucleotide reductase (48). For these reasons ferredoxin is presented 

 as the compound being reduced by light- activated chlorophyll. It should 

 be mentioned, however, that in the experiments designed to demon- 

 strate a ferredoxin in R. nibnim fractions, the assay employed was 

 the photoreduction of NADP by spinach chloroplasts. No stimulation 

 has been observed in the R. rubrum photo reactions by addition of R. 

 nihnim ferredoxin fractions. 



The position of the other compounds in this scheme is less certain. 

 The inclusion of a flavoprotein between ferredoxin and NAD is made 

 on the basis that the NAD- coupled photooxidation of DPIPH2 is stimu- 

 lated by FMN at levels which are consistent with its functioning as a 

 coenzyme to a flavoprotein. In a similar system involving the ferre- 

 doxin from Clostridium , a flavoprotein was shown to be required for 

 the reduction of NADP (47), 



Fig, 16 represents two pathways leading to ubiquinone, one coming 

 from ferredoxin and the other from the flavoprotein. The evidence 

 presented above on the slow photooxidation of DPIPH2 coupled to fu- 

 marate and to NAD indicates separate pathways for these two reactions. 

 Thus, FMN stimulates the NAD- supported reaction and has no effect 

 on the fumarate- supported reaction, Quinacrine inhibits the NAD re- 

 action and stimulates the fumarate- supported reaction. 



There is considerable evidence that ubiquinone occupies a central 

 position in accepting electrons on the reducing side of the scale. Thus 

 Clayton has shown that in the case of Chromatiiim one of the primary 

 reactions which occurs following illumination is most likely the photo- 

 reduction of ubiquinone (22). The high level of ubiquinone contained in 

 R. nibntni chromatophores (33), and especially the large increase 

 observed in light-grown cells over dark-grown cells indicates a prom- 

 inent role for this compound (35). The scheme presents ubiquinone as 

 transferring electrons to RHP. RHP is unique to photosynthetic bac- 

 teria, is oxidized by molecular oxygen (49), and has been shown to be 

 essential for the photophosphorylation process (36). The assignment 

 of sequence of electron transfer from ubiquinone to RHP is not in 

 agreement with the reported oxidation potentials of -0.008 volts for 

 RHP (49) and 0.098 volts for ubiquinone (50). The potentials observed 

 in the isolated compound may not accurately represent the potential of 

 the same compound in situ in the chromatophore structure, however. 

 The main reason for choosing this sequence is that it allows ubiquinone 

 to act very near the chlorophyll system, which is in agreement with 

 present information. 



Cytochrome b is included in the pathway between RHP and cyto- 

 chrome cp. This is the usual sequence encountered in mammalian 

 tissues, and the spectroscopic evidence obtained by Nishimura, al- 

 though not conclusive, indicates that cytochrome b contained in/?, 

 nibnim chromatophores can be reduced under the influence of light 



