Quantitation of metabolic activity from isotope tracing data using automated methodology

Abstract

Metabolic flux analysis (MFA) is a computational approach to deciphering labelling patterns based on machine learning principles. Differing from typical machine learning algorithms that train a model from known datasets to make predictions, the commonly used MFA algorithm trains a metabolic network with data from isotope tracing experiments and directly outputs the learned information — that is, all fluxes in the network that best fit data^3,5 (Fig. 1b). However, as a machine learning algorithm, current MFA methods often lack systematic evaluation and benchmarking, a standard practice in broader machine learning and artificial intelligence applications⁶. Issues such as algorithmic convergence, flux estimation accuracy and result robustness in MFA studies have been raised but remain largely unaddressed³, limiting the effectiveness and broader adoption of these automated tools in metabolic research.

To advance the capabilities of MFA for complex metabolic networks and extensive isotope tracing datasets, we developed an automated analysis methodology alongside a large-scale metabolic network model. This model comprises over 100 fluxes across key pathways, including glycolysis, the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway (PPP), one-carbon metabolism, and several amino acid (AA) biosynthetic pathways (Fig. 1c, Supplementary Methods). Compared to contemporary MFA tools^7,8,9, a notable feature of our methodology is the incorporation of organelle compartmentalization, facilitating accurate quantification of exchange fluxes between mitochondria and cytosol in eukaryotic cells (Fig. 1c). While other tools typically require tens of minutes to obtain a solution^7,9, our methodology can generate an optimized solution, with fluxes that accurately explain the labelling pattern from a ¹³C tracing experiment on cultured cell lines, within 2 s on a desktop computer¹⁰ (Supplementary Fig. 1a–d). Nonetheless, a challenge arose from the observation that these optimized solutions could diverge significantly, showing considerable variability in certain net fluxes even with similar loss values (Fig. 1d, Supplementary Fig. 1e,f).To address this problem, we developed an optimization-averaging algorithm that refines the computation process by selecting a subset of solutions with minimal loss (selected solutions) from the pool of optimized solutions and averaging them to produce a new, more stable solution set (averaged solutions) (Fig. 1e, Supplementary Methods). These solutions, along with those generated using the typical strategy used in contemporary software (Supplementary Methods, Supplementary Fig. 1c), were benchmarked using simulated ¹³C tracing datasets generated from a known flux vector (Supplementary Fig. 2a). The results demonstrated that, relative to the benchmark, the optimization-averaging algorithm effectively reduced flux variability and improved the accuracy of the results in approximating the known flux, even with varying levels of data availability (Fig. 1f, Supplementary Figs. 2b–e and 3a–e).