206 



A SYMPOSIUM ON RESPIRATORY ENZYMES 



carbonate appears in the synthesized alpha-ketoglutaric acid. Un- 

 fortunately these data are difficult to interpret in terms of any 

 precise mechanism of alpha-ketoglutarate formation. Since all the 



Table 3.— Synthesis of alpha-ketoglutarate in radioactive 

 bicarbonate medium* 



* 7.6 gm. minced pigeon liver in 50 ml. calcium-free bicarbonate saline, pH. 7.4, 

 containing radioactive C"; 1.7 ml. 0.1 M malonate; 10 ml. 0.02 M pyruvate; malonate 

 added directly to tissue; pyruvate added after 10 minutes; temperature, 40° C; gas 

 phase, 5 per cent carbon dioxide; experimental period, 40 minutes. 



t Activities are expressed in divisions per second (Lauritsen electroscope) ; and are 

 corrected for decay so that the values are comparable. 



X Activities after successive recrystallizations of alpha-ketoglutarate dinitrophenyl- 

 hydrazone. 



radioactivity is lost on oxidation of alpha-ketoglutaric acid to suc- 

 cinic acid, we can definitely conclude that not more than one carbon 

 atom of the five of alpha-ketoglutarate is derived from the medium. 

 From the quantity of radioactivity present in the alpha-ketoglutarate 

 we can calculate (the details of these very approximate calculations 

 are given elsewhere [11]) that approximately one carbon atom in 

 ten of the alpha-ketoglutarate is derived from the inorganic car- 

 bonate of the medium. Since this figure involves very approximate 

 calculations, and is of value only in suggesting the order of magni- 

 tude of the reaction, we are inclined to believe, as a working 

 hypothesis, that one mole of carbon dioxide from the medium is 

 assimilated per mole of alpha-ketoglutarate synthesized. 



The synthesis of alpha-ketoglutarate is by no means the limit of 

 carbon dioxide assimilation. Under our experimental conditions we 

 find that alpha-ketoglutaric acid represents only about 25 per cent 

 of the radioactivity which has been assimilated, that is, activity 

 which can no longer be released as carbon dioxide on the addition 



