604 YOSHIHARU ODA 



N2-fixation mechanism. In anaerobic N2-fixing bacteria such as CI. pasieurianum 

 O2 cannot be utilized and so the maintenance of the reduced state for N2 reduc- 

 tion, that is, the reduction of Xi, X2, occurs by reactions (i), (4) of the diagram, 

 resulting in the evolution of molecular hydrogen. The equiUbrium of the hydro- 

 genase system is far over to the side of H2 evolution, and so there is no effect of 

 H2 on anaerobic N2 fixation. On the contrary, in aerobic N2-fixing bacteria such 

 as Azotobacter, the reduced state for N2 reduction is kept reversely by oxidative 

 reactions (i), (2), (5) which onesidedly transfer electrons from hydrogen donors 

 to O2 through the powerful respiratory process characteristic of this microbe. 

 It is very surprising that it has the most powerful respiration capacity in the 

 kingdom of living beings, respiring 10 times as vigorously as ordinary microbes. 

 Accordingly, it may be inferred that the hydrogenase system is not directly 

 concerned with nitrogen fixation, but may have the function of keeping other 

 enzymes or intracellular hydrogen donors in the active reduced state. And its 

 presence may indicate that Xi, Xo are constantly kept in the reduced state. 

 Therefore, in the presence of molecxilar hydrogen in the atmosphere, reaction 

 (i) is depressed by the rapid proceeding of reactions (4), (2), (5), and N2 fixation 

 is inhibited by blocking of the acceptor system of the fixed nitrogen compound. 

 In this coimection, it was actually demonstrated that even in the cells of 

 Azotobacter grown in conditions excluding No fixation, a complete synthesis of 

 the hydrogenase system was observed upon anaerobic incubation in the proper 

 conditions. 



Thus, in aerobic Azotobacter, a striking contradiction is involved between the 

 development of energy efficiencies by the utihzation of O2 and the difficulties in 

 maintenance of reducing power for N2 reduction. It is very interesting that the 

 succinic oxidase system in Azotobacter has no cytochrome c-cjaochrome c 

 oxidase system and not the saturation point of 0> pressure for its activity, 

 differing from the system of the strict aerobes. 



These facts show that Azotobacter may be closely related to the group of 

 facultative anaerobes. 



On the basis of the observations mentioned above, it is conceivable that the 

 hydrogenase system has played an important role in the various biological 

 reductions as the reductant of lowest potential in the stage of anacrobiosis. 



But because of the remarked contradictions between the acquisition of poten- 

 cies for O2 utilization and the role of reducing agent in biological reductions the 

 hydrogenase system could not universally be distributed among the strict aerobes, 

 probably losing the specificities of H2 evolution and activation inherent to this 

 system in the line of biological evolution. But the flavoprotein that is inherent 

 to the system has played a role for the appearance of the highly organized 

 respiratory process as the electron-transporting system linking pyridine nucleo- 

 tide coenzyme systems with the cytochrome system, and now docs so as an 

 important electron carrier in the present respiratory process. 



In conclusion, in this report an attempt has been made to understand the 

 significance of molecular hydrogen metabolism in the transitionary stage from 

 anacrobiosis to aerobiosis in the fine of biological evolution. This proposition 

 should certainly serve as a sufficient challenge to provide further tests of the 



