The extension of metabolite balancing with car-need for precise and extensive quantitation methods in bon labeling experiments, as described by Marx et al.
biotechnology. However, only recently have experimen- , results in a much more tal methods become available, enabling a sufficient detailed stationary metabolic flux analysis. As opposed amount of high precision measurement data to be gathto basic metabolite flux balancing alone, this method enered for this task (Rizzi et al., 1996; Weuster-Botz and ables both flux directions of bidirectional reaction steps to be quantitated. However, the mathematical treatment de Graaf, 1996;Wiechert and de Graaf, 1996). of carbon labeling systems is much more complicated, This contribution concentrates on the carbon isotope because it requires the solution of numerous balance labeling approach for flux quantitation under metabolic equations that are bilinear with respect to fluxes and fracsteady-state conditions (Marx, 1996; Wiechert and de tional labeling. In this study, a universal modeling frame-Graaf, 1996). In this situation, neither mechanistic aswork is presented for describing the metabolite and carbon atom flux in a metabolic network. Bidirectional sumptions about enzyme and transport kinetics, nor the reaction steps are extensively treated and their impact knowledge of energy yields are required for flux deteron the system's labeling state is investigated. Various mination. kinds of modeling assumptions, as usually made for met-Consequently, stationary flux estimates computed abolic fluxes, are expressed by linear constraint equafrom labeling experiments are expected to exhibit a high tions. A numerical algorithm for the solution of the resulting linear constrained set of nonlinear equations is degree of reliability. The practical aspects of stationary developed. The numerical stability problems caused by flux analysis and its relation to dynamic modeling are large bidirectional fluxes are solved by a specially develdiscussed in Wiechert and de Graaf (1996).
oped transformation method. Finally, the simulation of carbon labeling experiments is facilitated by a flexible software tool for network synthesis. An illustrative simu-Metabolite Flux Balancing lation study on flux identifiability from available flux and labeling measurements in the cyclic pentose phosphate
The quantitation of intracellular fluxes in vivo requires pathway of a recombinant strain of Zymomonas mobilis measurement techniques to be available that do not concludes this contribution.