Stable isotope tracers for metabolic pathway analysis

Stable isotope tracing allows a metabolic substrate to be followed through downstream biochemical reactions, thereby providing unparalleled insights into the metabolic wiring of cells. This approach stops short of modeling absolute fluxes but is relatively straightforward and has become increasingly accessible due to the widespread adoption of high-resolution mass spectrometers. Analysis of both dynamic and steady-state labeling patterns in downstream metabolites provides valuable qualitative information as to their origin and relative rates of production. Stable isotope tracing is, therefore, a powerful way to understand the impact of genetic alterations and defined perturbations on metabolism. In this chapter, we describe a liquid chromatography-mass spectrometry (LC-MS) protocol for stable isotope tracing using ¹³C-l-arginine in a macrophage cell line. A similar approach can be used to follow other stable isotope tracers, and notes are provided with advice on how this protocol can be generalized for use in other settings.