250 Charles Heidelberger and Van R. Potter 



b, and with the hydroxyl group at c above the plane of the 

 paper. When dehydration occurs specifically between the 

 hydroxyl group and the hydrogens of the acetate group at 

 point b, the m-aconitic acid w^ill have its double bond 



CH3COCOOH 

 Pyruvic acid 



Step l||c*02 



HOOC\ 



>CO 



hooc*ch/ 



Oxalacetic acid 



10 



CH3CO— X 



/-Malic acid 

 Furaaric acid 



ati 



Succinic acid 

 7 



aHOOC 

 bHOOC 



OOCv ,.CH 

 Citric acid 



.CHjCOOH 



c 



HOOC*— CO 



/ 



t-co, 



/CHjCOOH 

 CH,/ 



a-Ketoglutaric 

 ejj— CO, 



HOOCv yy 



W 



-UiO 



HOOC^/ 



HOOC*C^ 

 H 



c»5-Aconitic acid 



CH2COOH -^ 



CH2COOH 



HOOC*— CO/ 



Oxalosuccinic acid 



5J|-2H 



HOOC*— CH ^ 



\ 



OH 



Isocitric acid 



Tj r\ yCH 



HOOCv yCHjCOOH 



Fig. 5. The Krebs citric acid cycle. 



situated towards the labelled primary carboxyl group. Once 

 the double bond has been formed, the other reactions leading 

 to a-ketoglutarate are straightforward. In Fig. 6 is formulated 

 the enzymatic dehydration of the antipodally labelled citric 

 acid, so that the double bond is situated away from the 



