Figure 1: Principle of 13C-based metabolic flux analysis. (a) Principle of 13C-based metabolic flux analysis (13C-MFA). Isotopic enrichment of alanine depends on metabolic flux via the Embden-Meyerhof-Parnas (EMP) pathway or the pentose phosphate (PP) pathway. (b) The configuration of the model is described in “Metabolic_network.xlsx.” The metabolic reactions and the carbon transfer are described in the “Rxns” and “Carbon_transitions” columns, respectively. Detailed rules are provided in the tutorial on the project home page. ((c) and (d)) Metabolic steady state and isotopically stationary. The isotopic labeling experiment is performed under metabolic steady state. After feeding 13C-labeled glucose, isotopic labeling enrichment changes in a time-dependent manner and then reaches a stationary condition. Whereas cells are sampled under isotopically stationary conditions in conventional 13C-MFA, time courses of isotopic labeling enrichment during an isotopically transient state are used for INST-13C-MFA.