... the label does not leave the cycle via α-ketoglutarate, but passes into fumarate. Here, things get a little complicated, since fumarate is a symmetrical molecule, so that the 13C label may subsequently end up at position 2 or 3 of malate. In a simplifying manner this will be disregarded for the further treatment and it will be assumed that via malate the label appears at C2 of oxaloacetate. The 13C-labelled oxaloacetate may now ...