Integration of 13C isotopomer methods and hyperpolarization provides a comprehensive picture of metabolism

Intermediary metabolism can be described by a complex network of intertwined pathways in tissues, so understanding and measuring flux in these pathways is challenging. Infusion of 13C-enriched tracers combined with high-resolution 13C NMR spectra of tissue extracts provides a quantitative way to measure fluxes through metabolic pathways in simple tissues such as the heart, yet the heterogeneity of cell types in tissue such as the brain and recirculation of labeled metabolites from one tissue to another make quantitative measurements a challenge. Details encoded in a 13C NMR spectrum often bring new questions about the impact of cell organization on metabolic fluxes, mixing of intermediates between common pathways, flux control by individual enzymes, the importance of genetic variants of enzymes involved in metabolic pathways, and the effects of drug and other therapies on metabolism. DNP of 13C tracers now offers the exciting possibility of measuring fluxes in real time in intact tissues, yet these same tissue complexities make proper interpretation of dynamic data a real challenge. Nevertheless, by combining kinetic measurements using hyperpolarized 13C tracers with 13C isotopomer data collected from the same tissues under steady-state metabolic conditions, one can evaluate fluxes through all pathways contributing to a specific metabolic end-product such as bicarbonate to gain new insights into spin relaxation of hyperpolarized materials as they pass through alternative metabolic pathways.