NONSTEADY STATE MODEL APPLICABLE TO NMR-STUDIES FOR CALCULATING FLUX RATES IN GLYCOLYSIS, GLUCONEOGENESIS, AND CITRIC-ACID CYCLE

We present a mathematical model for calculating most reaction rates of glycolysis, gluconeogenesis and citric acid cycle in mammalian cells. The model also includes cycles such as the ''phosphoenolpyruvate (PEP ) --> pyruvate --> oxaloacetate --> PEP'' cycle and the ''pyruvate --> acetyl-CoA --> citrate --> citric acid cycle --> oxaloacetate --> PEP --> pyruvate'' cycle. The model, which does not require steady state conditions, is based on a set of equations, each one describing the fa tes of a given carbon of a selected intermediate. These fates are expr essed as ratios of integrated transfer of this carbon to corresponding carbons in subsequent metabolites. At each bifurcation, the sum of al l proportions adds up to 1. Among several calculation routes to determ ine a proportion value, we chose the one that was based on the most re liable parameter determined experimentally. The data introduced in the model are the micrograms of atom of traced carbon measured on each ca rbon of a number of products (corrected for natural tracer abundance). These incorporations can be measured by C-13 NMR, gas chromatography- mass spectroscopy, or C-14 counting. Thanks to its flexibility, this m odel can be applied to data obtained with substrates other than glucos e under many experimental conditions.