ALBERT \V. I'RENKEL 



591 



pairs obtained liom the dismutation of one molecule of acetate (Fig. 

 3A) , namely, the flavin couple associated with the succinate-fumarate 

 couple, is not of a sufficiently high chemical potential to bring about 

 the reduction ol acetoacetate to beta-hydroxybutyrate. Consequently 

 they postulate that at this j^oint photochemically produced reductants 

 come into play, possibly as indicated by Fig. 3C, a reaction which has 

 been observed with isolated chromatophores (8) . Thus, Stanier and 

 coworkers still require a photoreductant for bacterial photosynthesis, 

 but invoke it only for one-fourth of the electron pairs required in 

 this particular reaction system. In this connection, it may be worth- 

 ^\•hile to consider other means by which all or part of these hydrogens 

 coidd be generated. First, according to van Niel's theory all the elec- 



(A) 



(Dark 



(B) 8x hv 



8 (H) J8 (OH) 



4DPN,V \ ,AC 



3 PN" 



3 PNH +3H''" 



4DPNH + 4H + 



2 COg + e H2O 



(C) 



2x hY 

 2(H)^ 2 (OH) 



DPN 



PNH-I-H + 



FMNH. 



DPNH+H+ FMN 



Fig. 3. Possible means of forming reduced pyridine nucleotides in the anaerobic 

 photometabolism of acetate (.\c) by Rhodospinllum nibntm (17). 



A. Dark production of three molecules of reduced pyridine nucleotide (PNH) 

 and of one molecule of reduced flavin (FIMNH,.) from one molecule of acetate via 

 the tricarboxylic acid cycle. The reducing equivalents of FMNH2 then may be 

 "upgraded" to the PXH level, the required energy being supplied by a mole- 

 cule of ATP (5). (For the formation of enzyme-bound DPXH in catalytic amounts, 

 no ATP may be required; however, for the formation of reduced DPN in sub- 

 strate quantities even one equivalent of energy-rich phosphate per mole of DPNH 

 formed would appear barely adequate; two equivalents of energy-rich phosphate 

 per mole of DPNH formed would appear to be more than sufficient). 



B. Reduction of pyridine nucleotides in the light, according to scheme of van 

 Niel. Diphosphopyridine nucleotide (DPN) reduced by the photochemical re- 

 ductant (H), acetate oxidized to COo and water by the photochemical oxidant 

 (OH), most likely by way of the tricarboxylic acid cycle, x = number of quanta 

 required by the photochemical system for the production of one reducing equiva- 

 lent. 



C. Possible manner of "upgrading" the reducing equivalents of FMNH2 pro- 

 duced in (A) through a photochemical reaction. FMNH, is oxidized by the photo- 

 chemical oxidant with the simultaneous reduction of DPN to DPNH (8). 



