ASYMMETRIC CITRIC ACID 



CHARLES HEIDELBERGER and VAN R. POTTER 



The classical investigations of Krebs on the oxidation of 

 carbohydrates and their metabolites led him to formulate the 

 now widely accepted citric acid cycle that bears his name. 

 The subsequent use of isotopic tracers has in many cases 



COg + CH^CCXIOOH 



HOOCCHoCO + (-CHoCOX) 



COOH ^ 



HOOC-CHgCOHCHgCOOH 



COOH 

 (Not isolated) 



HOOCCOCHoCHoCOOH 

 I 



Pigeon liver 

 mince 



COg + HOOCCH2CH2COOH 

 100^ 0% 



Fig. 1. Distribution of COg fixed into a-ketoglutarate (Evans 

 and Slotin, 1941; Wood et ah, 1942). 



The percentage values showTi on this and the following figures 



denote the proportion of the total isotope content of a substance 



found in individual chemical groupings. 



strengthened the evidence for this cycle. The important 

 demonstration by Evans and Slotin (1941) and by Wood and 

 his group that animal tissues are able to incorporate carbon 

 dioxide into intermediary metabolites marked the beginning 

 of the use of carbon isotopes in animal metabolism. When 

 ^^COg or ^^C02 was incubated with pigeon liver minces the 

 isotope was present in the a-ketoglutarate, but when this 



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