56 



David Shemin and Jonathan Wittenberg 



per se in porphyrin formation. It is merely used to indicate 

 the participation of the tricarboxyhc acid cycle as a whole in 

 this synthesis. For example, if one starts with methyl labelled 

 acetate with a relative activity of 10 in the methyl group after 

 endogenous dilution, the a-ketoglutaric acid formed on the 

 first turn of the cycle would contain ^*C activity only in the 

 y-carbon atom, and the relative activity would be 10 (see 

 Fig. 5 and Table V). When the a-ketoglutarate has been 



CH2-C00H 



HOOC-CHp-C-COOH £i 



I 

 OH 



CITRATE 



CH3-COOH 

 ACETATE 



\| 



N 



CH2-COOH 



I 



HOOC-CO- C-COOH 



I 

 H 



OXALOSUCCINATE 



[\ 



CH2-COOH 



HOOC-CO-CH2 



Oi KETOGLUTARATE 



-7 - V 



HOOC-CH2-CO-COOH ^ 



OXALACETATE 



N 



OH 



I 

 HOOC-CH2-C-COOH 



I 

 H 



MALATE 



\[ 



HOOC-CH=CH-COOH 

 FUMARATE 



-> HOOC-CH2-CH2-COOH 



CH3-CO-COOH 

 PYRUVATE 



SUCCINATE 



Fig. 5. Abridged scheme of the tricarboxylic acid cycle. 



converted to the symmetrical succinic acid, the activities of 

 the methylene carbon atoms would (ignoring dilution by 

 endogenous succinic acid) be 5 and 5, those of the oxaloacetate 

 eventually formed would contain half of the activity of the 

 y-carbon atom of the a-ketoglutarate. The recycling of this 

 oxaloacetate with the labelled acetate into the tricarboxylic 

 acid cycle would result in a-ketoglutarate having the relative 

 activities shown in Table V for the second cycle. On the 

 third and after an infinite number of cycles, therefore, the 

 activities of the carbon atoms of the a-ketoglutarate would 



