220 METABOLISM AND PHYSIOLOGY 



addition of DPIP it can be observed that the inhibitory effect of anti- 

 mycin with ascorbate alone is partially relieved, and furthermore the 

 inhibitory effect of UQ2 is bypassed, just as in the case without anti- 

 mycin (compare vessels 2 and 4, Table 2). On the other hand, a com- 

 parison of vessels 3 and 4 also shows that UQ2 acts to bypass an 

 antimycin inhibition of LIP with the ascorbate-DPIP couple. 



The results outlined in Tables 1 and 2 indicate that, even in the 

 presence of antimycin, agents affecting redox balance can produce 

 stimulation or inhibition of LIP, depending on the particular redox 

 balance established in the system. These findings extend the observa- 

 tions of Bose and Gest (7) on the effect of DPIP in bypassing an in- 

 hibition of electron transport by antimycin. The stimulatory effect of 

 UQ2 on LIP in the presence of succinate appears to be due to an effect 

 on the redox balance of an electron carrier in the particles. In this 

 connection, it is of interest that succinate readily reduces UQ;i^o ^^ 

 the dark in chromatophore particles (10). In the case of the stimulation 

 by UQ2 in the presence of PMS the effect may also possibly be directly 

 related to the new redox balance established as a result of interaction 

 of UQ2 with the light- reduced dye. It seems reasonable to speculate 

 that the effects observed with UQ2 and PMS in the presence of anti- 

 mycin are due to this dye interaction and may not necessarily involve 

 direct reaction with a second electron carrier site different from that 

 where UQ2 and succinate interact. 



The quinone normally present in R. rubnim is UQiq- Since the Eq 

 of UQ2 would not be expected to vary greatly from the +0.100+0.01 

 volts calculated for UQ^q (9), it could be assumed that the redox bal- 

 ance at the UQjo site in the cyclic electron transport scheme would 

 be directly affected by the addition of UQ2.^ On the basis of E^ values 

 UQj^O would fit in somewhere between Rhodospirillum heme protein 

 (+0,01 volts) and cytochrome c^ (+0.310 volts). Since the DPA particles 

 have greatly reduced amounts of UQj^g ^he addition of UQ2 would cer- 

 tainly influence the redox balance to a much greater extent than in the 

 case of the normal particles, where little effect of UQ2 can be ob- 

 served. UQio is insoluble in aqueous media, which may explain why the 

 addition of this substance has a negligible effect on LIP. As in the case 

 of PMS, it is difficult to distinguish whether the effects of UQ2 on the 

 interaction of the ascorbate-DPIP couple in LIP are due to direct 

 reaction with these reagents or to some action on an electron transport 

 carrier in the particles. Nonetheless, a most important point resulting 

 from the foregoing experiments, and one which requires emphasis, is 

 that evidence for the existence of noncyclic photophosphorylation in 

 bacterial systems (8) based on the inhibitory effect of antimycin must 



^ Moret et al. (11) have measured the redox potential of a series of UQ analogues 

 with varying isoprenoid side chains. They found that the length of the isoprenoid 

 chain did not influence the final value of +0.098 volts at pH 7.4. 



