PHOTOOXIDATION OF REDUCED PMS BY CHLOROPLASTS AND 

 CHLOROPHYLL A_ UNDER ANAEROBIC CONDITIONS IN THE PRESENCE 



OF QUINONES 



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



INTRODUCTION 



It is currently held that photosynthesis in green plants comprises two 

 separate light reactions which couple through enzymatic components to 

 complete the electron transfer span from water to reduced pyridine nucleo- 

 tide. The primary photochemical act for the long wavelength system results 

 in transfer of electrons from a cytochrome situated at the reaction center to 

 an electron acceptor such as ferredoxin, which can then couple chemically 

 with other electron acceptors such as NADP '^"'*). The long wavelength 

 system in plant photosynthesis resembles in many respects the photosyn- 

 thetic apparatus found in bacteria (4-7)_ 



The photosynthetic process in bacteria involves the oxidation of cyto- 

 chrome coupled to the reduction of some endogenous component within the 

 chromatophore fragment, such as ubiquinone '^8-10) j^i studying this reac- 

 tion in detail, Zaugg demonstrated that bacterial chromatophore fragments 

 catalyze the photooxidation of added cytochrome c coupled to the photo- 

 reduction of added UQ2 or UQ^ under anaerobic conditions '^^\ PMSH2 can 

 substitute for cytochrome c in this reaction, becoming oxidized in a coupled 

 reaction with UQ2. ~ 



Similar experiments have been attempted with chloroplasts. With deter- 

 gents present and under aerobic conditions it was possible to demonstrate the 

 photooxidation of ferrocytochrome c previously reported ^^^' ^^'. It was not 

 possible to demonstrate a cytochrome oxidation coupled with quinone reduc- 

 tion under anaerobic conditions. When PMS was substituted for cytochrome 

 c, however, a coupled oxidation of PMSH2 and reduction of UQ2 was demon- 

 strated with spinach or poke weed chloroplasts. UQ^ was less active than 

 UQ2 in these reactions, but a marked stimulation was obtained upon the addi- 

 tion of the detergent Triton X-100. With this detergent present either UQ2 

 or UQ^ supported the photooxidation of PMSH2. A rapid back reaction was 

 observed in a subsequent dark period with both quinones. Chlorophyll a in 

 the presence of Triton X-100 also catalyzes the reaction. Since this reaction 

 is representative of the primary photochemical reaction thought to occur in 

 the long wavelength system, it has been investigated in some detail. 



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