298 



ELECTRON TRANSPORT 



It was found that R. rubrum chromatophores contained a small but 

 definite amount of "bound" ADP which could not be washed out by re- 

 peating the centrifugal washing procedures (57). Without the addition 

 of external ADP, well-washed chromatophores could photosynthesize 

 ATP32 from the "bound" ADP and added Pi32 (Fig, 16). Most of the 

 "bound" ADP present in the chromatophores appeared to be esterified. 

 Chromatographic surveys have failed to detect photosynthetic forma- 

 tion of IDP-p32 in extracts from well- washed chromatophores which 

 had been illuminated with Pi32, These negative data favor the postula- 

 tion that ADP is the direct phosphate acceptor. It should be noted that 

 photophosphorylation of the "bound" ADP is optimal in the presence 

 of a much lower concentration of ascorbate (about 10~3 M) than photo- 

 phosphorylation of externally added ADP. 



The ATP formed from the "bound" ADP ("bound" ATP) in the 

 light appeared to be rapidly replaced with externally added ATP, but 

 was also rapidly decomposed into "bound" ADP and free Pi when 



Fig. 18. Effect of "light on" and "light off on ATP formation from 

 bound ADP in "intact" chromatophores of R. nibrnDi. Experimental 

 conditions were the same as for Fig. 17, except that the concentration 

 of ascorbate was fixed at 1 niM. Control runs were carried out in 

 darkness, and differences between tests and controls were plotted, 

 although the values for control were hardly altered (74). 



