SUCCINIC ACID FORMATION 135 



alternatively, rigid removal of carbon dioxide during fermen- 

 tation results in suppression of succinic acid production. This 

 suggests that carbon dioxide is assimilated during the fermenta- 

 tion and that the ultimate fate of the assimilated carbon 

 dioxide is to form succinic acid. This suggestion has been 

 investigated in detail by Prof. Werkman and his colleagues 

 at Iowa State College. They showed that if pyruvic acid 

 is fermented in the presence of carbon dioxide containing 

 isotopic C^^, then some of the heavy-carbon is assimilated 

 and appears in one of the — COOH groups of succinic acid. 

 They suggest the following scheme for the reaction: — 



C*02 C*OOH C*OOH C*OOH C*OOH 



+ 11 II 



CH3 CH2 CH2 CH Succinic CH2 



I I > I > II > 1 (7) 



CO Oxalacetic CO Malic CHOH Fumarase CH dehydro- CHg 



I decar- 1 dehydro- 1 1 genase i 



I boxylase I genase I I I 



COOH COOH COOH COOH COOH 



Oxalacetic Malic Fumaric Succinic 



acid acid acid acid 



(C* = C13 isotope) 



CO2 is first fixed by combination with pyruvic acid to 

 form oxalacetic acid. The enzyme involved in this fixation 

 process is oxalacetic acid decarboxylase working in reverse. 

 Oxalacetic acid is then reduced to malic acid by malic dehydro- 

 genase in reverse. Malic acid is converted to fumaric acid 

 by the action of fumarase and, finally, fumaric acid is reduced 

 to succinic acid by succinic dehydrogenase acting in reverse. 

 All these enzymes have been demonstrated in the bacteria 

 concerned, and it is probable that the " Wood- Werkman 

 scheme," as this is usually called, is responsible for part, at 

 least, of the succinic acid formation in these organisms. 



This scheme accounts for succinic acid formation by the 

 fixation of carbon dioxide, but studies involving isotopic CO2 

 show that, although some of the succinic acid arises in this 

 fashion, the total formation of succinic acid cannot be 

 accounted for in this way. Werkman and his colleagues have 

 now demonstrated that there is a second method whereby 



