PHOTOOXIDATION AND PHOTOREDUCTION REACTIONS 245 



The reason for the failure to show NAD reduction with Chromatium 

 chromatophores is not immediately apparent. Arnon reported that he 

 was able to obtain a photoreductionof NADin the presence of ascorbate 

 and DPIP (30). In our laboratory, however, we have not been able to 

 obtain a photo reduction of ISi AD with Chromatium chromatophores under 

 any circumstance. This is surprising, since this bacterium would be 

 expected to have an active system for NAD photo reduction, because it 

 must reduce carbon dioxide via the photosynthetic route for all of its 

 carbon compounds. 



Another photosynthetic bacterium, Rliodopseudomonas spheroides, 

 was examined in the usual photooxidation systems with the results 

 given in Fig. 9, Again, this organism showed the capacity to photo- 

 oxidize DPIPH2 in a fast reaction which soon saturated in the usual 

 manner. Like Chromatium, this bacterium did not have the ability to 

 sustain the secondary coupled reactions with either NAD or fumarate, 

 although there was a hint of a slow coupled reaction when fumarate 

 was added to the system. In this case also, it was not possible to 

 demonstrate directly a photoreductionof NAD when DPIP and an excess 

 of ascorbate were present as the electron- donating system (31). The 

 reason for inactivity in NAD reduction in the case of both Chromatium 

 and Rliodopseudomonas spheroides is not known. One should probably 

 look at the method of chromatophore preparation to see if inactivation 

 of enzymes or other factors is involved in this situation. 



D I . NO OXIDANT ^ 



LIGHT ON 



-«• 



^>i 



LIGHT OFF 

 + FUMARATE y 



r 



NAD 



4''6 8 10 



MINUTES 



12 14 



Fig. 9. Photooxidation of DPIPH2by Rps. spheroides chromato- 

 phores. The experimental conditions given for Fig. 1 were em- 

 ployed, except that Rps. spheroides chromatophores equal tc 

 0.110 jumoles of BChl per 4 ml were employed. 



