247 



F. R. Whatley 



imal, but smaller amounts of oxygen produced a large stimulatiou 

 10 pinoles oxygen added to a vessel of 20 ml volume corresponds 

 to about 1^ oxygen in the gas phase. 



Table 2 Experimental condi- 



Effect of small amounts o f oxvaen on tions were as given in 

 cyclic photophosphorylation catalyzed legend to Table 1. 5 

 b y a limiting amount of PMS under argon ^g,-anis PMS were added 



in each vessel. The 

 reaction vessels were 

 capped with a serum 

 bottle cap and flushed 

 with purified argon. 

 Small amounts of oxy- 

 gen gas were then in- 

 jected into the gas 

 phase through the cap. 



These results appear similar to those obtained with the FMN- 

 catalyzed pseudo-cyclic photophosphorylation, which has been 

 shown to require oxygen as a terminal electron acceptor. As 

 already stated, however, the oxygen exchange data do not support 

 a similar interpretation for the PMS system. Consequently, an 

 alternative explanation was sought. If the effect of oxygen in 

 the PMS system were due to the oxidation of some component of 

 the system, it seemed probable that the addition of another oxi- 

 dant would produce a similar stimulation of the ATP formation. 

 Since ferricyanide is not toxic to photophosphorylation by 

 chloropiasts (e.g., it supports ATP formation in noncyclic 

 photophosphorylation) it was chosen as a suitable substitute 

 for oxygen. When tested in a system provided with a limiting 

 amount (;> digrams) of PMS, ferricyanide was indeed capable of 

 stimulating cyclic photophosphorylation with PMS under argon, 

 as is shown in Table 3. The table indicates very clearly that 

 the ferricyanide acts in a "catalytic" fashion, and not as a 

 substrate; the addition of 1 pinole ferricyanide (which in non- 

 cyclic photophosphorylation might have given 0.5 lamoles ATP) 

 caused an increment of 5 lomoles ATP. The effect of ferricyanide 

 is not due to the oxygen which it will produce via a Hill re- 

 action when the light is turned on. This will be seen by com- 

 paring the results in Tables 2 and 3 (both experiments carried 

 out at the same time) ; 1 (.unole ferricyanide, which gave a large 

 stimulation, would yield 0.25 iimoles oxygen, an amount much less 

 than is needed to give a measurable effect when injected into 

 the gas phase. 



