254 Charles Heidelberger and Van R. Potter 



surprising result shown in Fig. 12 is that although the citric 

 acid is indeed asymmetric, it is in the opposite direction from 

 that which might have been predicted. 



One common finding in the citric acids derived from acetate 

 both in vitro and in vivo, is that there is a much higher isotopic 

 content in the "a" carboxyl than in the tertiary carboxyl 



CH^COCOOE 



' i 



HOOCCH^CO + (-CH^50X) <— CH^SoOH 



2i 

 COOH 



EOOCCH^COHCEoCOOH 

 2| 2 



COOH 



0-5% 



HOOCCOCH^CH^COOH 



2^^"^' ^^ ^^2 



Rat Kidney 

 Eomogenate I, 



1. Rat Kidney Homogenate II. 



2. Pigeon Liver Extract 



I 



KMnOij. 



CO^ + EOOCCH^CH^COOH 



15-23^ 85-775^ 



Fig. 11. Distribution of label in citrate and a-ketoglutarate 

 from isotopic acetate and pyruvate (Potter and Heidelberger, 



1951, unpublished). 



carbon and suggests that there may be a common mechanism 

 for the process. 



Several explanations may be advanced for the finding of 

 an isotopic content 20 per cent in the "a" carboxyl of citrate 

 in the acetate and pyruvate experiments. An artifact in the 

 permanganate oxidation of the a-ketoglutarate dinitro- 

 phenylhydrazone has been ruled out by the demonstration 

 that under very drastic conditions labelled succinic acid is 

 converted to CO 2 to the extent of only 3 per cent. Recycling 



