... meet a acetyl coenzyme A "freshly" provided by β-oxidation of labelled palmitate. This will result in the formation of [3,4,5-13C3]glutamate – if the "fresh" acetyl coenzyme A is supplied by glycolysis, [4,5-13C2]glutamate will be generated. The respective coupling pattern of the individual isotopomers unambiguously reveals, from which substrate the label at C4 originates.