Carbon-13 nuclear magnetic resonance for analysis of metabolic pathways

Carbon tracers have long been used to probe biochemical pathways. The central concept is that the distribution of the carbon tracer in product molecules encodes the underlying pathways. The traditional approach that defined much of our textbook knowledge of metabolism was to provide a ¹⁴C-labeled molecule to a system and to measure the presence or distribution of ¹⁴C in a product molecule (1–4), an often painstaking procedure. Based on the results of this measurement, mathematical models of varying complexity were used to interpret the labeling patterns of the tracer in downstream metabolites and thereby detect metabolism in various pathways. This ¹⁴C approach serves well for studies of a single substrate in isolated enzyme preparations or isolated organelles where alternative substrates and interactions among metabolic networks can be minimized. However, as biological systems increase in complexity, multiple metabolic pathways and competing substrates come into play. In vivo, metabolism becomes even more complex as the effects of different organ systems or the effects of nutritional state and disease must be considered. The information provided by radiotracer studies generally is insufficient for interrogating these systems. The challenge is to identify methods to help assess the myriad metabolic states that are generated in vivo by disease, drugs, various physiological states, and gene manipulation. ¹H nuclear magnetic resonance (NMR) has been widely used to globally profile small-molecule metabolites in biofluids and tissues (5); these are described in sections 3 and 4 of this book. The complementary power of ¹³C NMR is to target specific metabolic pathways; therefore, this method plays an important role in the exploration of the metabolome. The combination of a ¹³C tracer with detection by NMR offers numerous advantages, including the simplicity and safety of dealing with a stable isotope. Because detection of nuclei by NMR is frequency-dependent, experiments can be performed simultaneously with other stable isotopes, such as ²H, and each isotope can be detected and quantified independently, which is an advantage compared with mass spectrometry (MS). The NMR method itself offers the advantage that purification and chemical degradation of metabolic products is generally not required. Furthermore, in vivo kinetic studies in preparations ranging in complexity from cells in bioreactors to large animals and humans are feasible (6,7). Because of these advantages compared with alternative tracers and detection methods, ¹³C NMR spectroscopy is now widely used to probe metabolic pathways.