592 LIGHT AND LIFE 



trons required for the reduction of the substrate (presumably aceto- 

 acetate) would be derived from the photochemical reductant and 

 acetate would be oxidized by the j^hotochemical oxidant, most likely 

 by way of the tricarboxylic acid cycle (6) (Fig. 3B) . The other ex- 

 treme possibility would be that none of the electrons required for 

 this reduction come from the photoreductant, but that the electron 

 pair of the succinate-fumarate couple could be upgraded by ATP, 

 similar to a reaction studied by Chance and HoUunger in mitochon- 

 dria (5) , and thus all required electron pairs would be produced 

 in the dark (Fig. 3A) . At the present time there is no clear-cut evi- 

 dence which would permit one to eliminate any of these schemes (Fig. 

 3 A-C) , or to permit the postulate that only one of them is operative 

 during the photometabolism of acetate. Since cell-free preparations 

 of Rhodospirillum rubriim can both produce ATP and bring about 

 the reduction of DPN in the light, it appears likely that both of these 

 light-induced processes could occur in the intact cell; but how much 

 reduced pyridine nucleotide is produced by the photochemical re- 

 ductant and how much is formed through the anaerobic dismutation 

 of acetate may be difficult to decide. Tracer experiments in due time 

 may yield information which may give us a somewhat better insight 

 into this problem. 



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