596 YOSHIHARU ODA 



posed to the formation of H2, CO2 and volatile acids by uninhibited cells. The 

 same result can also be achieved by controlling the metal nutrition of this organ- 

 ism; thus, cells from a medium rich in iron ferment glucose to C2, CO2 and 

 volatile acids, while those from a medium deficient in iron carry out lactic acid 

 fermentation. All these observations show that in some organisms of the clos- 

 tridial group, butyric acid fermentation can potentially be switched back to the 

 lactic acid fermentation by environmental conditions [9]. 



Thus, in opposition to Prof Oparin's opinion, it may be inferred that homo- 

 lactic acid fermentation is a more primitive one than the clostridial type of 

 fermentation and that the latter has developed secondarily from the former, 

 acquiring new metabolic systems additively. The new enzyme system that 

 played the important role in this transition from the former to the latter was the 

 hydrogenase system itself. Thus it follows that the fact that fermentation types 

 containing the hydrogenase system were unstable and heterogeneous might be 

 a factor in development of their versatiHty and their complexity inherent to the 

 fermentation types in present hving beings. 



Next, the hydrogenase system has played an important role in raising the 

 level of energy-yielding efficiency of fermentation types, by Unking with pyridine 

 nucleotide coenzyme systems (DPN, TPN) which were hydrogen carriers par- 

 ticipating in the process of EMP system. The diverse types of fermentations 

 that occur in present hving beings have developed due to the fact that the 

 electrons released in a series of dehydrogenation reactions could be transferred 

 to a variety of intermediates and to the hydrogenase system as the ultimate 

 acceptors, and that frequently several alternative routes were possible. In a 

 number of fermentation processes, the end products are quahtatively or quan- 

 titatively related with the alternative paths of hydrogen transfer and their balance 

 sheet. These facts are not only inherent in the strict anaerobes such as 

 Clostridium^ but also in facultative anaerobes such as the coli-aerogeties group, 

 when they are adapted to anaerobic conditions, which are to be considered as 

 a pre-stage in their evolutionary Hne. 



From several points of view, it is convenient to consider the level of hydro- 

 genase activity as the factor which controls the chaimelling of electrons into 

 possible alternative routes in the stage of anaerobiosis. In this sense, the hydro- 

 genase system can be considered as a substitute for cytochrome oxidases which 

 have thus far not been detected in the strict anaerobes, and it is not only the 

 actual controUing factor but a leaking system which enables the organism to 

 discard excess electrons. 



BIOLOGICAL REDUCTION 



As the results of anaerobic fermentation by the primitive living beings, CO2 

 and H2 gases were evolved and gradually accumulated on the surface of the 

 Earth. Thereupon, the living beings became able to acquire the new capacity of 

 activating molecular hydrogen directly. In other words, the hydrogenase system 

 which had been concerned solely with accommodating electrons during the 

 fermentation process as molecular hydrogen, came to activate H2 reversely and 



