METABOLIC ASPECTS 143 



explain the unsolved problem (56) of how certain chemosynthetic auto- 

 trophs generate reduced pyridine nucleotide from the oxidation of in- 

 organic electron donors of relatively high redox potential. 



Reduced pyridine nucleotide would be utilized for a variety of re- 

 ductive biosyntheses, e.g., conversion of CO2 to organic compounds, 

 transformation of C2 and C3 intermediates to reserve materials (5, 

 13,27,57), or reductive aminations to provide amino acids for protein 

 synthesis. If, however, the illuminated cell is producing "excess" ATP 

 and NADH2 relative to the demands of the biosynthetic machinery, the 

 reduced electron carrier would be reoxidized, through liberation of 

 molecular hydrogen, by an energy-dependent process. Conditions of 

 this kind apparently obtain during photosynthetic growth with certain 

 amino acids serving as the nitrogen source, and in illuminated resting 

 cell suspensions which are rapidly metabolizing oxidizable compounds 

 in the absence of utilizable nitrogen sources. According to the fore- 

 going conception, H2 evolution is interpreted as the reflection of a 

 kind of regulatory device which maintains ATP and reduced pyridine 

 nucleotide at levels consistent with the overall rate of biosynthetic 

 activity (13,24,33), Chance and Hollunger (58) have recently suggested 

 that a similar type of control mechanism may operate in mitochondria, 

 i.e., the succinate-dependent reduction of NAD shows great sensitivity 

 to "uncouplers" of oxidative phosphorylation and this could provide a 

 mechanism for delicate regulation of the concentration of NADH2 with- 

 in the mitochondrion. 



The relatively simple scheme shown in Fig, 2 provides a working 

 hypothesis which appears to be compatible with the salient oxidation- 

 reduction features of bacterial photosynthesis. With particular refer- 

 ence to light-dependent H2 formation, the scheme predicts that this 

 process should be inhibited by compounds which "discharge" or "de- 

 energize" the postulated ATP-precursors, Certain inhibitors of light- 

 induced phosphorylation (such as antimycin A and redox dyes) are, in 

 fact, potent inhibitors of H2 evolution (24). Addition of such compounds 

 to illuminated intact cells of R. rubrum not only abolishes H2 forma- 

 tion, but usually also causes the cells to resort to an anaerobic fer- 

 mentation of endogenous reserves (to fatty acids). Such fermentation, 

 which is characteristic of dark anaerobic metabolism, normally does 

 not occur to an appreciable extent during illumination (59), In other 

 words, it appears as if inhibition of the phosphorylating system can 

 have the effect of suppressing overall photometabolism, which in turn 

 results in the appearance of a fermentative pattern frequently seen in 

 heterotrophic anaerobes. This striking phenomenon suggests that 

 photophosphorylation activity inhibits fermentation and, accordingly, 

 the induced transition could be characterized as a "photosynthetic 

 Pasteur effect" (47), 



It is encouraging that other types of experiments have given results 

 consistent with the occurrence of energy-linked "reverse" electron 



