LIGHT-INDUCED PHOSPHORYLATION 437 



together with the above-mentioned, more stable site. If the curve in Fig. 5 

 for "the physiological pathway" represents the decrease of phosphoryla- 

 tion at two sites, one of them being the same as that in the curve indicating 

 a high stability of the phosphorylation in the system containing PMS and 

 HOQXO, the curve for the stability of the other phosphorylation is given 

 by the broken line in Fig. 5. 



TABLE III 



Requirement for a Reducing Agent in Aerobic LIP in 

 Chromatophores of R. riibrum 



The experimental details were as described in ref. [3]. Twice w-ashed "chro- 

 matophores" were used. The final concentrations of the various agents were, 

 where added: 33 x io~* m ATP, 3-3 x io~* M succinate, 33 x io~^ m DPNH, 

 10"^ M ascorbate, 3 3 x 10^ m PMS, and 2 x lo^" m HOQNO. The time for the 

 experiment was 20 min. in the absence and 6 min. in the presence of PMS. The 

 6-min. values have been recalculated to 20-min. values, assuming earlier shown 

 linearity. 



..... Per cent orthophosphate 



Additions .r- , 



estenned 



— 0-2 



Succinate 9 



DPNH 9 



Ascorbate 10 



PMS + HOQNO 20 



Succinate + PMS + HOQNO 22 



It was found by Frenkel and by Geller and Lipmann in the early days 

 of research on light-induced phosphorylation [17] that washed chromato- 

 phores of R. ruhrum under aerobic conditions needed catalytic amounts of 

 hydrogen donor, for example DPXH or succinate, in order to produce 

 ATP in the light. Ascorbate may be used instead of these agents [18] as 

 is shown in Table III. High concentrations of ascorbate are needed to give 

 maximum effect (Fig. 6). "The PMS-pathway ", however, functions 

 without any addition of hydrogen donor under our standard conditions 

 (Table III). The reason for this can be ascribed to light-induced reduction 

 of PMS, which then provides the necessary reducing equivalents to the 

 phosphorylation system. This explanation has recently been given by 

 Geller and Lipmann [19], who have demonstrated that an added hydrogen 

 donor is indeed needed for phosphorylation when the light-induced 

 reduction of PMS is inhibited by avoiding light of wavelengths where 

 this compound absorbs. Their conclusion was tested with our system and 

 confirmed. 



