Meaning of Nitrate and Sulphate 



Reduction in the Process of 



Metabohc Evolution 



M. ISHIMOTO AND F. EGAMI 



Faculty of Science, Nagoya University Chikusa, Nagoya, Japan 



There are a great variety of metabolic pathways in the present living organisms. 

 This complexity is not due to the chemical nature of the substrates, but it was 

 formed gradually in the long history of living organisms, in the course of evo- 

 lution of the metabolic enzyme systems. 



It has been known that the evolution of Hfe on the Earth proceeded from 

 anaerobes to aerobes and from heterotrophs to autotrophs [i], and some specu- 

 lations were made on the meanings of nitrate and sulphate reduction in that 

 process [2]. The authors have been studying the enzymic reduction of nitrate 

 and sulphate at Nagoya and Tokyo Universities. Now we try to summarize the 

 development in these fields and compare it with other metabohc processes and 

 speculate about the evolutionary processes leading to the formation of respiratory 

 systems. 



Biological reductions of nitrate and sulphate which take place in organisms 

 have several meanings for their hfe. Nitrate and sulphate are abundant on the 

 Earth and the latter is the sole mineral source of sulphur available to organisms. 

 Organisms must reduce and convert them into organic nitrogen and sulphur 

 compounds, indispensable for the building up of their cells. On the other hand, 

 by the metabohc evolution, ultimate hydrogen acceptor systems were simul- 

 taneously acquired, which may function as an energy-yielding mechanism instead 

 of oxygen respiration. 



Studies on the mechanisms of nitrate and sulphate reduction indicate that 

 enzyme systems responsible for assimilation are quite different from those of 

 energy supply. So we shall discuss them separately; firstly the energy-yielding 

 systems and secondly the assimilative systems. 



The free-energy change in reduction of nitrate is less than in that of oxygen, 

 but fairly higher than in the reduction of organic compoimds. The potential 

 of sulphate reduction is much lower and comparable to that of the organic 

 metabohtes (Table i). 



These values decide the characteristic pattern of nitrate and sulphate reduc- 

 tion. The metabohc systems concerning both have double characters. They 

 resemble respiratory systems since inorganic substances are involved as vdtimate 

 hydrogen acceptors hke oxygen. On the other hand, from the view of the redox 



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