38 



CARBON METABOLISM III 



Amino acids 



Formate 



Glucose 



*" Gluconate 



Pyruvate 



Fumarate 



Lactate 



Itaconate 



Isocitrate 



^C0 2 



Succinate 



T 



a-Ketoglutarate 



Amino acids 



C0 2 

 Figure 1. The interrelations of the organic acids in metabolism. 



1. Citric acid is shown as arising by the condensation of oxalacetate 

 and acetyl-coenzyme A; an origin from succinate and glyoxalate is a 

 second possibility (405). 



2. In order for citrate to accumulate, oxalacetate must therefore be 

 provided; in Figure 1 its only indicated source is the carboxylation of 

 pyruvic acid. Whether this mechanism is quantitatively adequate for 

 the high yields of citrate which have been reported (553) is not finally 

 certain, although there is no doubt that carbon dioxide does enter 

 into the citrate molecule in substantial amount (369). 



3. Citrate may be oxidized further, i.e., it is not an end product. 

 This is borne out both by studies of the time course of citrate ac- 

 cumulation (47, 58a, 213) and by isotope studies (110, 337). 



4. Especially during growth, the carbon of citric acid cycle inter- 

 mediates is drawn upon for amino acid and lipid synthesis. The 



