252 ELECTRON TRANSPORT 



of inhibitors used (lO-5 M) ATP formation is inhibited over 90 per 

 cent and the reported photooxidation reactions are only slightly de- 

 creased. 



The effect of ADP and Pi on the various reactions is shown in Table 

 3. Again a distinction is apparent between the NAD-coupled and the 

 fumarate- coupled slow reactions. Whereas the NAD reaction was in- 

 hibited by the addition of these components, the fumarate reaction was 

 stimulated. If it is true that stimulation of a reaction by ADP and Pi 

 indicates a phosphorylation in that reaction sequence, this would indi- 

 cate that the photo reduction of NAD accompanying DPIPH2 oxidation 

 does not involve a phosphorylation, whereas the photoreduction of 

 fumarate is accompanied by ATP formation. These conclusions should 

 be considered as only tentative, however, since ADP can have effects 

 other than that of stimulating a reaction coupled to ATP formation, 



TABLE 3. 



Effect of ADP + Pi on the photo reactions of R. rubrum chromatophores. The 

 reaction conditions were those given for Fig. 1 with a BChl concentration of 



0.21 mg. 



Concentration Per Cent Stimulation 



NAD Fumarate 



Fast RX Slow RX Fast RX Slow RX 



10-3 M^ p^Qp 

 10-3 M. Pi 



22 -69 12 32 



25 -40 22 38 



However, it should be noted that the concentrations used in these 

 experiments are below those reported by Horio to give inhibition of 

 NAD reduction associated with succinate oxidation (37). 



The activity of heated chromatophores is shown in Fig. 14. For 

 these experiments aliquots of chromatophores were heated at the in- 

 dicated temperatures for five minutes, following which the reaction was 

 run in the usual fashion. The rates of the slow reactions were multi- 

 plied by ten in order to place them on the same scale as the fast re- 

 actions. From this information it is apparent that heating to 40°C was 

 sufficient to destroy the reaction coupled to NAD, and heating to 50°C 

 inactivated the system involving fumarate. Heating to 60 °C had very 

 little effect upon the fast reaction observed. Indeed, to inactivate the 

 fast reaction with R. ruhnini chromatophores, a temperature of 80°C 

 for five minutes must be employed. This emphasizes the stability of 

 the fast reaction, and again implicates a very stable and perhaps frac- 



