ANDRE T. JAGENDOlib AND CAOIUUO lORTI 583 



TABLE 2 

 Stimulation ok O2 Uptake and Piioroi'iiosiMioRvi.ATioN by 

 Photosynthi-.tic Pyridine Nucleotide Reductase (PPNR) 



Oxygen uptake ATP formed 



/xatoms Minolcs 



2.0 0.30 



3.0 1.01 



4.4 1.58 



The chloroplasts, prepared in sucrose-Tris-NaCl-Versene (11), were incubated in 

 manometric vessels at 15°C, with a light intensity of 4500 foot-candles at vessel height. 

 The reaction mixture was made to a final volume of 3.0 ml, containing an excess of 

 hexokinasc, and NaCl .03 .\/, Tris-HCl />H 8.0 .01 M, MgCh .005 M, ADP .002 M, 

 phosphate 'abeled with P'^ .005 M and glucose .025 M. 0.5 mg of chlorophyll was 

 present in each flask. Reaction time, one hour. 



the appropriate concentration of ascorbate, oxygen was required and 

 CMU inhibited phosphorylation very greatly. Thus it seems to us 

 to be likely that ascorbate, especially in the presence of PPNR, de- 

 presses the Mehler reaction sequence (eq. 1, 2, 3, and 5fl) and en- 

 courages or sets up an oxygen exchange reaction (eq. 1, 2, 3, and 5) 

 as an alternative. 



A clear-cut distinction between the two types of oxygen involve- 

 ment may be possible in the future. Some preliminary indication of 

 this is found in the fact that KCN has little effect on the phosphoryla- 

 tion without ascorbate where Oo is consumed, but inhibits ATP 

 formation very markedly in the presence of ascorbate (Table 3) . 



Of the numerous compounds which have been used as catalytic 

 redox cofactors in photosynthetic phosphorylation, so far we can 

 say that only PMS and its derivative pyocyanine participate in reac- 

 tion •/; whereas FMN, menadione, indigo carmine, vitamin K-„ and 

 methyl and ben/yl viologcns run through the oxygen exchange .se- 



TABLE 3 

 Effect of CN~ on Ascorbate-stimulated Photophosphorylation 



Ascorbate 

 Additions consumed formed disappeared 



KCN, M/\00 



Ascorbate, .002 M 1.0 4.50 .75 



Ascorbate, .002 yVf plus KCN, M/1 00 5.8 1.44 3.00 



ixmoles 



Conditions as in Table 1 . A dark control with cyanide and ascorbate present showed 

 no oxygen uptake, ascorbate disappearance, or ATP formation. 



