... meet a "freshly" labelled acetyl coenzyme A. If that "fresh" label originates from glucose (as in the scheme above), this will result in a double labelling at positions C3 and C4 – if the "fresh" label arises from palmitate, a triple labelling at C3, C4, and C5 will be formed. By means of the coupling pattern of the different glutamate isotopomers, it therefore can be unequivocally determined which substrate served as source of the carbon incorporated at C4.