ELECTRON TRANSPORT IN R. RUBRUM 287 



activity of endogenous oxygen uptake in darkness as well as in light, 

 doubtless because of the lack of endogenous, oxidizable substances. 

 Under dark, aerobic conditions, dialyzedchromatophores slowly oxidize 

 reduced cytochrome c? externally added; in most cases the dark cyto- 

 chrome C2 oxidation proceeds at approximately one-twentieth the rate 

 of the aerobic, dark oxidation of succinate, which agrees with Smith 

 (34). 



Under aerobic conditions at room temperature, cytochrome C2 

 oxidation is stimulated approximately tenfold upon illumination, but 

 not under anaerobic conditions at all. Both the cytochrome c? oxida- 

 tions in darkness and in light are inhibited by cyanide, with a Km for 

 cyanide of approximately 5 x 10"" M and 8 x 10"3 M, respectively. 

 Cytochrome c from bovine heart muscle can be oxidized in the same 

 manner as Cytochrome c^.. The aerobic, dark oxidation of cytochrome 

 c is about twice as fast as that of cytochrome c^. 



Using R. nibnim, Frenkel (49,50) has shown that chromatophores 

 can catalyze photo reduction of NAD in the presence of either FMNH2 

 or succinate with an accumulation of NADH. His findings have been 

 extended by Vernon (51) and Vernon and Ash (52). Many investigations 

 have also shown that a heme protein (s) and NAD present in the cells 

 are oxidized and reduced, respectively, upon illumination with actinic 

 light (53,37,54,55). 



In the presence of succinate or ascorbate, chromatophores from 

 R. nibnim reduce NAD under anaerobic conditions, as expected (Fig, 

 8). The NAD photo reduction was inhibited by ADP in the presence and 

 absence of Pi, with a Km for ADP of approximately 5 x 10-3 m (Fig. 

 9) (56). ATP and pyrophosphate (PP) inhibited to the same extent as 

 ADP, while AMP and adenosine were less active. On the other hand 

 photosynthetic ATP formation from ADP and Pi in chromatophores was 

 enhanced in rate with an increasing concentration of ADP (the highest 

 concentration tested was 10-2 M), the Km for ADP ("bound" plus 

 "free") being about 10-5 M (57)), providing a strong argument against 

 the postulation that NAD might be part of the photosynthetic, cyclic 

 electron transport chain. 



In response to variations in concentration of succinate or NAD, the 

 photo reduction of NAD reached a steady state in which about half of 

 the NAD present was reduced, indicating the Eq value of the photo- 

 chemically produced reductant to be around-0.32 V. In the presence of 

 NADH/NAD (1:1) (Eq, -0.32 V), the NADH was oxidized by succinate/ 

 fumarate (Eq, + 0.02 V) anaerobically in darkness, providing that the 

 Eh value of the succinate/fumerate system was more positive than 

 -0.04 V. This suggests there is an enzyme system which catalyzes 

 oxidation of NADH by fumarate anaerobically in darkness. It seems, 

 therefore, most likely that there is a regulatory mechanism in the 

 enzyme system which catalyzes anaerobic oxidation of NADH/NAD by 



