511 



Leo P. Vernon, Waldo S. Zaugg and Elwood Shaw 



was reduced photochemically by white light in the absence of air showed that 

 the quinone must be present for the photooxidation of PMSH2 to proceed. 

 Turning off the light allowed a back reaction to be observed, in which PMS 

 was rereduced by the reduced quinone to give the original equilibrium con- 

 centration. The addition of DCMU, a specific inhibitor for photos ynthetic 

 oxygen evolution y^^', had no effect upon the photoreaction. This was ex- 

 pected since this reaction should not involve the entire photos ynthetic 

 machinery, but only the long wavelength system. Quantasomes frop poke 

 weed were prepared according to the directions of Park and Pon ^ ' and were 

 found to be active. This indicates that the entire chloroplast fragment is not 

 required for this reaction, but only the primary photos ynthetic system in- 

 volved in photos ynthetic electron transfer reactions. 



The effect of detergents upon the photoreaction is shown in Figure 2. 



>- 



o 



z 

 < 

 aa 



IT 



o 

 </) 

 m 

 < 



PLUS 

 DEOXYCHOLATE 



ON 



ON 



2 3 4 



MINUTES 



± 



012 1 



MINUTES 



Fig. 2. (Left) PMS photoreactions in the presence of spinach 

 chloroplasts, UQ2 and detergents. Conditions as for Figure 1 

 were used, and where present Triton X-100 was 0. 02 per cent 

 and deoxycholate was 0. 4 per cent. 



Fig. 3. (Right) Photoreactions of PMS with poke weed chloro- 

 plasts and UQ^ or TMQ (trimethylbenzoquinone). Reaction con- 

 ditions as for Figure 1. 



Triton X-100 at a concentration of 0. 02 per cent markedly stimulated the 

 rate of PMSH2 photooxidation. The addition of deoxycholate at a final con- 

 centration of 0. 4 per cent had just the reverse effect, influencing the reac- 

 tion system to cause a photoreduction of PMS. It should be noted that the 

 PMS was initially reduced by adding UQ2 in the reduced form, allowing the 

 UQ2H2 to react with PMS to an equilibrium position. Sufficient oxidized PMS 

 remains to allow for a net photoreduction in the presence of deoxycholate. 



Figure 3 shows that UQg in the absence of added detergents is unable to 



