144 METABOUSM AND PHYSIOLOGY 



flow in bacterial photosynthesis. Chance and Olson (60) have proposed 

 that a process of this kind can account for the kinetics, obtained by 

 dynamic spectrophotometry, of light- stimulated NAD reduction in 

 intact cells of purple bacteria. Relevant evidence has also been obtained 

 with a model cell-free "photohydrogenase" system in which light 

 stimulates the oxidation of H2 with fumarate (61). Pigmented particles 

 isolated from R. ruhrum do not reduce fumarate with H2 unless a 

 suitable electron carrier is added. When a dye of relatively low redox 

 potential such as brilliant cresyl blue (BCB; Eq = +0.047v.) is sup- 

 plied, a rapid reaction occurs in darkness as shown in Fig, 3. 



Illumination has no effect on the rate of the reaction with BCB 

 serving as the mediator. On the other hand, with DPIP( Eq = +0.22v,) 

 negligible fumarate- reducing activity is observed in the dark and the 

 reaction is now dependent on, or stimulated by, light. Although ferri- 

 cyanide is rapidly reduced by the particulate hydrogenase, this com- 

 pound does not catalyze fumarate reduction in dark or light. 



According to our interpretation, when DPIP is the mediator, light 

 provides energy for driving hydrogen transfer against an unfavorable 

 thermodynamic gradient, i,e,, from DPIPH2 to fumarate. Further evi- 

 dence for this view has been obtained by Bose (62) using N,N,N',N'- 

 tetramethyl-p-phenylenediamine (^TMPD; Eq = ~ +0,26v,) as the cata- 

 lyst for light- dependent reduction of fumarate by H2. Table 4 sum- 

 marizes some of his results showing the effects of various uncouplers 

 of phosphorylation on the light- stimulated oxidation- reduction reac- 

 tion. 



It can be seen that with the first three compounds both the oxidation 

 of H2 and light-induced phosphorylation were inhibited. The results 

 suggest that of the two processes, photophosphorylation is perhaps 

 somewhat more sensitive to inhibition. With appropriate concentra- 

 tions of oligomycin, on the other hand, photophosphorylation was in- 

 hibited while the oxidation- reduction reaction was consistently stimu- 

 lated; separate experiments disclosed similar effects with atebrin and 

 gramicidin D. These data show a striking parallelism with the re- 

 ported (58,63,64) effects of these inhibitors on oxidative phosphoryla- 

 tion and energy-linked "reverse" electron transfer in mitochondria 

 and, consequently, lend further support to the suggested interpretation. 



There is good reason to expect that the availability of active cell- 

 free systems and new experimental techniques will lead to an intensi- 

 fication in study of the basic features of light- activated electron trans- 

 fer pathways in bacterial photosynthesis. Many significant questions 

 remain unanswered, which is not surprising in view of the fact that 

 our understanding of electron transfer in nonphotosynthetic systems 

 is still far from complete in spite of decades of active investigation. 

 One of the prominent unresolved problems in bacterial photosynthesis 

 concerns the nature and extent of cross-connections between the elec- 

 tron carrier systems associated with net electron transfer (i.e., from 



