Process of Metabolic Evolution 559 



potential {Eq = 200 mV). In living cells, it is reduced by hydrogen and formate 

 and oxidized by sulphate, sulphite and tliiosulphate, etc. Its rôle as the inter- 

 mediary carrier can be suggested. Reductases for sulphite and thiosulphate, 

 hydrogenase and formic and lactic dehydrogenase were extracted from the cells 

 [16]. Ishimoto et al. showed that an intermediary electron carrier was necessary 

 for the thiosulphate reduction with hydrogen [17] and that the purified cyto- 

 chrome was utilizable as the carrier [18]. These experiments offer direct proof 

 for the participation of the cytochrome in electron transfer in the anaerobic 

 reaction of strict anaerobes. 



The cytochrome acts also in hydrogen production. When the cytochrome is 

 reduced enzymically with formate and formic dehydrogenase system or chemi- 

 cally with sodiimi dithionite, production of molecular hydrogen is observed in 

 the presence of hydrogenase [19]. This cytochrome is autoxidizable and can be 

 oxidized with colloidal sulphur, hydroxylamine and nitrite. 



Experimental results indicating phosphate incorporation in the cells, during 

 the reduction of sulphate with hydrogen, suggest the presence of a phosphory- 

 lation process, not on the level of substrate dehydrogenation, but on the level 

 of electron transfer in the sulphate reduction [20]. 



There are many autotrophic strains among Desulfovibrio [21] and even some 

 heterotrophic strains can grow under supply of hydrogen, sulphate, carbon 

 dioxide, inorganic nitrogen source and small amounts of grovrth factors [22]. 

 Phosphate incorporation was observed even in the heterotrophic strain employed 

 in the above experiments. 



Now, we try to propose a scheme for the evolution of respiratory systems by 

 the comparison of different types of cytochrome. There are several groups of 

 cytochromes which are different in their chemical nature and physiological 

 fimction. The first group is that of Desulfovibrio, which has very low potential 

 and strong reducing abihty for many inorganic substances through corresponding 

 reductases. The second is that of photoreductive and photosynthetic organisms 

 including higher plants, algae and bacteria. A kind of them was found also in 

 anaerobic photoreductive bacteria Chlorobium [23]; they have high redox 

 potential and are generally conceived to be oxidized photochemically and 

 reduced by suitable substrates to yield free energy. Some species of anaerobic 

 photosynthetic bacteria with such cytochrome have the abihty to oxidize various 

 substances including fatty acids, which are not usually oxidized by nonphoto- 

 synthetic anaerobes [24]. The former may occupy the position between aerobes 

 and anaerobes. The third group includes the cytochrome systems of aerobic 

 respiration. They are miscellaneous in their nature. Frequently, several sorts 

 of cytochromes co-operate in series, e.g. those of a, b, c type in mammalian 

 muscle and yeast. The common function of the c}n:ochrome systems is transfer 

 of electrons in biological oxidoreduction and formation of energy-rich phos- 

 phate bonds accompanying it. 



According to the above grouping, we can speculate on the course of evolution 

 of cytochromes in the evolution of metabohsm. Cytochrome had been playing 

 a rôle as an electron carrier already in anaerobic primaeval organisms. It may 

 be necessary for the phosphorylation in electron transfer, a new type of energy- 



