PHOTOOXIDATION AND PHO TOR EDUCTION REACTIONS 251 



The effect of respiratory inhibitors upon both the fast and coupled 

 slow reactions is shown in Table 2, Phenylmercuric acetate (PMA), a 

 compound which inhibits by combination with sulfhydryl groups, in- 

 hibited both the NAD and fumarate slow reactions. It was less effective 

 on the fast reaction, and had little effect on the aerobic photooxidation. 

 As shown below, the fast reaction and the aerobic photooxidation re- 

 actions are stable activities which are resistant to heating. This indi- 

 cates that the fast reactions and the aerobic reaction involves only a 

 portion of the electron transport chain, and this portion is relatively 

 inert to various treatments and outside agents. 



The inhibitions caused by antimycin A and HQNO also reveal that 

 the coupled slow reactions are most sensitive. However, in general the 



TABLE 2. 



Effect of inhibitors upon the photoreactions ofR. rubrum chromatophores. The 

 reaction conditions were as given for Fig. 1. The concentration of BChl was 



0.20 mg. 



Per Cent Inhibition 

 NAD Fumarate Aerobic 



Fast RX Slow RX Fast RX Slow RX 



reactions reported here are less sensitive to both antimycin A and 

 HQNO than either the photophosphorylation process or the photore- 

 duction of NAD coupled to succinate. 10"^ M antimycin A inhibits over 

 90 per cent of the activity in theNAD-succinate system (16) and in the 

 photophosphorylation process (16,40), HQNO is also over 90 per cent 

 effective in these reactions at a concentration of 10"^ M (16,40), It 

 would appear, therefore, that the antimycin A and HQNO inhibitions ob- 

 served in the present case are of a different nature from the inhibitions 

 observed in the photophosphorylation process. This has significance 

 when considering the mechanism of the reduction of fumarate and NAD 

 by DPIPH2. As discussed below, the inhibition pattern is not consistent 

 with the idea that the reduction of these compounds is due to a re- 

 versed electron transfer coupled to ATP utilization, since at the level 



