The Transitionary Stage from Anaerobiosis to Aerobiosis 597 



transfer it to various hydrogen acceptors, as the partial pressure of hydrogen in 

 the environment gradually increased. Thus, the hydrogenase system came to 

 play a new important role as reductant in biological reduction systems. 



The hydrogenase system in some Clostridium spp. cannot utilize methylene 

 blue as a hydrogen acceptor, but does utilize methylviologen having a low redox 

 potential. But some of the strict anaerobes (Micrococcus lactilyticus [ii]) and the 

 facultative anaerobes {coli-aerogenes group) can utilize methylene blue as a 

 hydrogen acceptor. Thus, the redox potential of the biological reduction system 

 gradually shifted towards higher levels during the course of biological evolu- 

 tion. 



A series of metabohc processes involving the hydrogenase system do not only 

 liberate molecular hydrogen from carbohydrates and organic nitrogen com- 

 pounds, but also can reduce various types of intermediates by activating mole- 

 cular hydrogen that has been evolved during the fermentation process. Though 

 it appears that there are various organic compovmds from which molecular hy- 

 drogen is evolved in the fermentation process ; the principle of the hydrogen- 

 evolving processes is essentially the same mechanism that evolved molecular 

 hydrogen from the C2 compound, and it is fundamentally the H2-evolving process 

 from pyruvate in Clostridium butyricum, mentioned above. 



Curiously, various types of organisms employ the same general mechanisms, 

 the same pathways, and hence must have similar metabolic processes. In short, 

 all life is alike in spite of differences which are 'in reahty' minor, and at the same 

 time each organism, or group of organisms, differs very markedly because the 

 line of biological evolution has exploited diverse principles. 



On the other hand CO2, which was evolved with H2 into the environment, 

 enables the living beings to acquire the new abiUty to assimilate CO2. Thus, the 

 stage of heterotrophic CO2 assimilation originated on the Earth. The process 

 that was investigated in detail am.ong the CO2 assimilation processes of hetero- 

 trophs is the following reductive ^-carboxylation, 



TPNH TPN 



pyruvate + CO., + or ^ malate + or 

 DPNH DPN 



The equiUbrium point of this reaction is far over to the left, and consequently 

 it is necessary to reproduce DPNH (TPNH) by other processes in order to 

 assimilate the CO2 progressively in this reaction. It is apparent that this reaction 

 may proceed in vitro by coupling the following reaction, 



DPN(TPN) + glucose 6-phosphate ^ 

 DPNH(TPNH) + gluconate 6-phosphate 



It is likely that reduction of the pyridine nucleotide coenzyme systems has an 

 important significance for the mechanism of electron transport in all types of 

 CO2 reduction processes. From the existence of the hydrogen- evolving process 

 as reductant it may be inferred that the generation of electrons (reducing power) 



