METABOLIC ASPECTS 135 



quantity. With glutamate serving as the nitrogen source, the growth 

 rate is somewhat reduced and an additional metabolic product is now 

 observed, viz., molecular hydrogen (34), The source of this hydrogen 

 and the mechanism of its formation are problems of great interest 

 because of their relationships with the energy metabolism of photo- 

 synthetic bacteria. 



Hydrogen production by R. rubrum is dependent on light and, except 

 for cells which contain readily expendable endogenous reserves, on the 

 addition of oxidizable organic substrates, e,g., citric acidcycle inter- 

 mediates (16,34), Maximal yields of gas are observed with truly 

 resting cells and the quantities produced closely approximate those 

 predicted on the basis of complete conversion of the organic com- 

 pound to H2 and CO2 (24), i,e.: 



Acetate: 



Succinate: 



Fumarate: 



L-Malate: 



'=2«4°2 * 2H2O 2CO2 + 4H2 



'=4«604 " ""2° "- *'=°2 * ™2 



C^H^O^ + 4H2O 4CO2 + 6H2 



C4H6°5 * '"2° ^ ^^°2 * ««2 



Studies with intact cells have led to the conclusion that these re- 

 markable conversions occur through the reactions of the well-known 

 citric acid cycle coupled with an additional light-dependent process 

 which, either directly or indirectly, effects the oxidation of reduced 

 pyridine nucleotide by liberation of H2 (13,24), If certain utilizable 

 nitrogen sources such as N2 or NH4+ are added to cells metabolizing 

 in this fashion, the evolution of H2 is completely inhibited (34) and the 

 quantity of CO2 produced is greatly diminished (35), These effects 

 are especially clear-cut with N2, which establishes inhibition very 

 rapidly. In our earliest publications (16,17,36; see also ref. 37) on 

 this phenomenon, we made the obvious suggestion that the inhibition 

 was due to diversion of a reductant, created by light, from the 

 hydrogen- evolving system to reactions of reductive amination of keto 

 acids derived from the organic substrate. 



