515 



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



ability to catalyze the same photoreactions as does the intact chloroplast or 

 the chloroplast in the presence of detergent. Thus, both UQ2 and UQ^ 

 support a photooxidation of PMSH2 while TMQ supports a photoreduction of 

 the dye. In the presence of deoxycholate all three quinones support the 

 photoreduction of PMS. For these experiments chlorophyll a was added to 

 the detergent solution from a methanol solution. The resultant preparations 

 were optically clear, and were carried through the manipulations in the 

 ordinary fashion. 



Although other porphyrins have not been investigated in as much detail, 

 it should be mentioned that this reaction is not restricted to chlorophyll a. 

 In the presence of Triton X- 100 the following porphyrins support a photo" 

 oxidation of PMSH2 in the presence of UQ2: chlorophyll a, chlorophyll b, 

 bacteriochlorophyll, pheophytin a, chlorophyllin a, protoporphyrin IX, and 

 tetraphenylporphyrin. It is interesting that the water soluble porphyrin, 

 hematoporphyrin, is inactive. This is unusual since this compound has been 

 shown to catalyze the photoreduction of methyl red, tetrazolium blue, and 

 NADP in the presence of ascorbate ^^^' '^'^'. 



Under the experimental conditions employed the reaction was not satu- 

 rated with respect to light intensity. This agrees with the fact that PMS 

 stimulated photophosphorylation does not show the usual light saturation 

 response ^"^^i. A preliminary study of the effect of pH upon the reaction 

 revealed that the nature of the reaction was sensitive to the pH of the medium. 

 Thus, with UQ^ at pH values between 6. 6 and 7. 6 the usual photooxidation of 

 PMSH2 was observed in Triton X-100 solutions. When the pH was lowered to 

 6. 0, however, the reaction changed to one of photoreduction of PMS. This 

 again indicates that the reaction is very sensitive to the ionic species in the 

 reaction mixture. 



DISCUSSION 



PMS has played a singular role in the biochemical investigations of 

 photophosphorylation and associated electron transfer reactions in both 

 plants and bacteria. In the plant system PMS effectively catalyzes the pro- 

 cess of photophosphorylation l"*. ^. 2 8) j^y virtue of its being alternately oxi- 

 dized and reduced, thus completing the cycle of electron flow driven by the 

 photochemical system. Evidence for direct interaction of PMS with the 

 components of the photos ynthetic apparatus is presented from the extensive 

 experiments of Witt and collaborators as summarized in reference 3. These 

 data show that PMS can be oxidized either through the endogenous cytochrome 

 (cytochrome J) or by chlorophyll a directly. The compound also operates in 

 the bacterial system to catalyze cyclic electron flow in the presence of the 

 inhibitor antimycin A, which inhibits one step of the cyclic electron transfer 

 system. PMS serves as a bypass for this sensitive site, thus completing 

 the cycle and allowing cyclic electron flow and photophosphorylation to 

 proceed '2^' ^^'. 



The behavior of PMS in the photophosphorylation system is unique in its 



