Estimation of fluxes into the PEP pool. The fractional contribution of each alternative pathway (X, Y, Z) directly follows from the mass pattern of the [f302] fragments of the precursors 3-phosphoglycerate (serine), oxaloacetate (aspartate), pyruvate (alanine) and the product PEP (phenylalanine). The corresponding isotopomer of each molecule is illustrated next to the experimental data (mass spectra). White circles represent 12C whereas black circles indicate labelled 13C. The numbers given reflect the position of the carbon atom within the molecule. PEPCk: phosphoenolpyruvate carboxykinase; PPDK: pyruvate-orthophosphate dikinase.