I08 SUBCELLULAR PARTICLES 



The action spectrum for photophosphorylation coincides with the absorption 

 spectrum of the bacteriochlorophyll (12, 16). 



Requirements for Photophosphorylation. Photophosphorylation is an anaerobic 

 process and is inhibited by oxygen ( 12, 19, 34). In addition to ADP and 

 inorganic phosphate, magnesium ion is required (32, 11, 2). To activate the 

 washed chromatophores of R. rubrum, small or catalytic amounts of succinate or 

 DPNH (but not DPN) are needed. Arnon, Whatley and Allen (5) have shown 

 that TPN rather than DPN is a specific catalyst in chloroplast photophosphoryla- 

 tion. A soluble TPNH-linked dehydrogenase is present in chloroplast supernatant 

 extracts which is required to demonstrate the TPN requirement (26). San Pietro 

 and Lang have described recently a light-catalyzed pyridine nucleotide reductase 

 in spinach chloroplast extracts which is more active with TPN than DPN (26). 

 Extension of these studies to the bacterial systems is indicated. 



Tn Chionnitium chromatophore fragments, succinate is ineffectual, while other 

 reducing agents, such as thiol compounds and DPNH, work well. The most 

 dramatic effects are noted with compounds of intermediate electrochemical po- 

 tential, such as phenazine methosulfate and ascorbate (12, 20). Detailed studies 

 with both R. rubrum and Chromatium preparations are in accord, generally, in 

 ascribing the effect of either of these agents to an interaction with some com- 

 ponent of the electron transport chain, whereby the component is brought to an 

 optimal value of electrochemical potential (20). 



Interactions With Inhibitors. The effect of various inhibitors, other than oxy- 

 gen, are generally similar whether using preparations from bacteria or green 

 plants. Thus, all observers are agreed on the failure of the well-known respiratory 

 inhibitors (cyanide, azide and carbon monoxide) to affect photophosphorylation 

 when present in concentrations usually almost 100 per cent effective in stopping 

 oxygen uptake. Responses to other inhibitors, especially those blocking electron 

 transport in oxidative phosphorylation, depend on the nature of the preparation. 

 In R. rubrum, for instance, the succinate- or DPNH-catalyzed photophosphoryla- 

 tion is inhibited by antimycin at levels which are effective in blocking electron 

 transport, but the phenazine methosulfate-catalyzed reaction is not (12). 



Inhibitors of oxidative phosphorylation, like dinitrophenol, the butyl ester of 

 3,5-hydroxybenzoic acid, dicumarol and various dyes (2,6-dichlorophenolindo- 

 phenol, methylene blue, brilliant cresyl blue, pyocyanine, etc.) are all effective as 

 inhibitors of photophosphorylation (12), although in some cases the concentration 

 required for inhibition may be an order of magnitude greater than that observed 

 with oxidative phosphorylation (20). One notable difference between Chromatium 

 and R. rubrum preparations is that 2,6-dichlorophenolindophenol in trace amounts 

 is an activator rather than an inhibitor for the former. 



Arsenite fails to affect photophosphorylation, indicating strongly that participa- 



