264 ELECTRON TRANSPORT 



Fig. 18 shows the probable sites of action of various inhibitors 

 which are effective with R. nibrum chromatophores. PMAis shown as 

 inhibiting NAD reduction, since it was shown to be an effective in- 

 hibitor in the photoreduction of NAD by both succinate and ascorbate- 

 DPIP by Nozaki et al. (16). Also, as shown above, this was an effective 

 inhibitor of the NAD-supportedphotooxidationofDPIPH2. Amytal would 

 also be expected to inhibit at this position. Geller showed amytal to be 

 an inhibitor of the dark aerobic oxidation of ^ ADU hy R. nibnim 

 chromatophores, while it has no effect upon the photophosphorylation 

 reactions (35). 



Since quinacrine has an inhibitory effect upon the photophosphory- 

 lation process, it probably acts at another site in addition to the flavo- 

 protein designated in Fig. 18, Quinacrine is shown as an inhibitor at 

 the flavoprotein level, since it inhibits the photooxidation of DPIPH2 

 coupled to NAD, yet stimulates the reaction coupled to fumarate. The 

 stimulating effect of FMN on the NAD-supported photooxidation of 

 DPIPH2 also indicates a flavoprotein acting at this site. 



The spectroscopic evidence of Nishimura and Chance (23,24) points 

 to the site of action of antimycin A and HQNO as being between cyto- 

 chrome b and cytochrome c^. In addition, PMA has been shown to re- 

 sult in a photooxidation of cytochrome c^anda photoreduction of cyto- 

 chrome b in R. nibrum cells, and probably acts at this locus also. 

 Since PMS has been shown to overcome the inhibition of photophos- 

 phorylation by antimycin A, it bypasses the site which is inhibited by 

 antimycin A. The most logical mechanism for this requires that PMS 

 be reduced by ubiquinone and reoxidized by cytochrome c^. 



Recent evidence obtained by Bose andGest (57) indicates that DPIP, 

 when present in the oxidized form, can also serve as a bypass and 

 overcome the inhibition of antimycin A upon photophosphorylation. This 

 again is consistent with its photooxidation at the cytochrome C9 level 

 and its photoreduction at a prior point, most likely at the ubiquinone 

 point in the electron transfer chain. 



It is anticipated that someof the reaction sequences and components 

 listed in Figs. 16-18 will be changed as additional information becomes 

 available. Not all of the evidence available could be reconciled with the 

 scheme as presented. Thus, Nishimura says that RHP is probably not 

 located between ferredoxin and cytochrome b, since carbon monoxide 

 (which does combine with RHP) does not affect the reduction of cyto- 

 chrome b (23,24). Nishimura also states that quinacrine does not have 

 any effect upon the absorption changes caused by illumination of /?. 

 nibrum chromatophores, as would be expected if quinacrine does in- 

 hibit at some site other than the flavoprotein designated in Fig. 16, 



One main question to be resolved is whether there is one electron 

 transport system serving both the photochemical and oxidative path- 

 ways in R. nibrum, or whether separate pathways are involved. As 

 stated above, the schemes outlined in Figs. 16-18 accommodate the 



