A novel 13C MRS-based marker of pyruvate cycling in perfused mouse liver using (2-13C) pyruvate and 13C MRS

Introduction The pathways by which pyruvate is recycled from the TCA cycle (PEPCK and malic enzyme) have received recent attention. Despite being in principle a futile cycle (i.e. resulting in only a net waste of ATP), flux through pyruvate recycling pathways have been shown to correlate with essential physiological processes such as glucose-stimulated insulin release from pancreatic cell lines. Current methods to measure pyruvate cycling rely on the introduction of uniformly labeled 13C-tracers and isotopomer analysis of the C-C couplings in glutamate and/or glucose. Here we propose a simpler marker of pyruvate cycling based on the introduction of [2C] pyruvate and the subsequent appearance of C-label in the C3 position of pyruvate which can only occur via label scrambling in the TCA cycle followed by recycling back to pyruvate. As pyruvate is consumed quite rapidly in the liver, the C-labeling in lactate (a stable direct conversion product of pyruvate) is used as surrogate.