PHOTOOXIDATION AND PHO TOR EDUCTION REACTIONS 237 



Also, Nishimura has reported a photo reduction of cytochrome b in 

 cells of R. nibnim which have been poisoned with antimycin A or 

 HQNO (23,24). In this case, the cyclic electron transport system is 

 apparently blocked, allowing a direct demonstration of cytochrome b 

 photoreduction. 



PHOTOOXIDATION REACTIONS CATALYZED BY 

 R. RUBRUM CHROMATOPHORES 



It is apparent that a number of photochemical oxidation and re- 

 duction reactions are now available for use in investigation of the 

 electron transport system contained in chromatophores of the photo- 

 synthetic bacteria. I would like to present some detailed information 

 on one of these reactions, namely the photooxidation of DPIPH2. A 

 preliminary report of some of these data has already appeared, and 

 the experimental methods used to obtain the data reported here are 

 essentially those which were described in this previous communication 

 (25). The experiments were performed under anaerobic conditions with 

 chromatophores prepared by sonic oscillation followed with two wash- 

 ings by centrifugation of the particles sedimenting in the centrifugal 

 range of 20,000 to 100,000 x g. 



The use of a modified Spectronic 505 recording spectrophotometer 

 (25) permitted the photooxidation of DPIPH2 to be followed in detail. 

 Fig. 1 presents the results obtained with and without an added oxidant 

 present. With only chromatophores present in the reaction system, a 

 fast initial reaction was observed which saturated after two to three 

 seconds of reaction time. The presence of either NAD or fumarate in 

 the reaction system allowed a secondary slower reaction to take place 

 following the initial fast reaction. In all cases a dark back- reaction 

 was observed when the light was turned off. In the reaction system 

 containing NAD, the NADH formed in the reaction was immediately 

 converted back to NAD by means of an enzyme system consisting of 

 lactic dehydrogenase and pyruvate. In the absence of this trapping 

 system the secondary slow reaction was not observed, since there is 

 an active NADH-DPIP diaphorase present in the chromatophores. The 

 secondary slow reactions which are coupled to NAD and fumarate are 

 no different from the coupled photoreactions previously observed (4, 

 8,16), both in mechanism and rate, as shown below. 



The most interesting aspect of Fig. 1 is the initial fast photooxida- 

 tion of DPIPH2. The rapid and definite saturation of this reaction in- 

 dicates that the oxidation of the DPIPH2 is coupled to the reduction of 

 components contained within the chromatophore. Fig. 2 shows the re- 

 lationship of this initial fast reaction to the concentration of chromato- 

 phores contained within the reaction system. Not only is the rate of 

 the initial fast reaction proportional to the chromatophore concentra- 



