Fig. 1.

Movement of labelled carbons during one turn of the Krebs cycle. Black represents a ¹³C labelled carbon, and red represents an unlabelled one. Labelled citrate with + 2 amu, formed from oxaloacetate and [1,2-¹³C]acetyl-CoA, is equally likely to produce one of two different isotopologues of oxaloacetate with + 2 amu