C-13 Nmr Of Glutamate For Monitoring The Pentose Phosphate Pathway In Myocardium

After administration of C-13-labeled glucose, the activity of the pentose phosphate pathway (PPP) is often assessed by the distribution of C-13 in lactate. However, in some tissues, such as the well-oxygenated heart, the concentration of lactate may be too low for convenient analysis by NMR. Here, we examined C-13-labeled glutamate as an alternative biomarker of the PPP in the heart. Isolated rat hearts were perfused with media containing [2,3-C-13(2)]glucose and the tissue extracts were analyzed. Metabolism of [2,3-C-13(2)]glucose yields [1,2-C-13(2)]pyruvate via glycolysis and [2,3-C-13(2)]pyruvate via the PPP. Pyruvate is in exchange with lactate or is further metabolized to glutamate through pyruvate dehydrogenase and the TCA cycle. A doublet from [4,5-C-13(2)]glutamate, indicating flux through the PPP, was readily detected in C-13 NMR of heart extracts even when the corresponding doublet from [2,3-C-13(2)]lactate was minimal. Benfotiamine, known to induce the PPP, caused an increase in production of [4,5-C-13(2)]glutamate. In rats receiving [2,3-C-13(2)]glucose, brain extracts showed well-resolved signals from both [2,3-C-13(2)]lactate and [4,5-C-13(2)]glutamate in C-13 NMR spectra. Assessment of the PPP in the brain based on glutamate had a strong linear correlation with lactate-based assessment. In summary, C-13 NMR analysis of glutamate enabled detection of the low PPP activity in isolated hearts. This analyte is an alternative to lactate for monitoring the PPP with the use of [2,3-C-13(2)]glucose.