title: Production of hyperpolarized13CO2from [1-13C]pyruvate in perfused liver does reflect total anaplerosis but is not a reliable biomarker of glucose production.
abstract: In liver,13CO2can be generated from [1-13C] pyruvate via pyruvate dehydrogenase or anaplerotic entry of pyruvate into the TCA cycle followed by decarboxylation at phosphoenolpyruvate carboxykinase (PEPCK), the malic enzyme, isocitrate dehydrogenase, or α-ketoglutarate dehydrogenase. The purpose of this study was to determine the relative importance of these pathways in production of hyperpolarized (HP)13CO2after administration of hyper-polarized pyruvate in livers supplied with a fatty acid plus substrates for gluconeogenesis. Isolated mouse livers were perfused with a mixture of thermally-polarized13C-enriched pyruvate, lactate and octanoate in various combinations prior to exposure to HP pyruvate. Under all perfusion conditions, HP malate, aspartate and fumarate were detected within ~ 3 s showing that HP [1-13C]pyruvate is rapidly converted to [1-13C]oxaloacetate which can subsequently produce HP13CO2via decarboxylation at PEPCK. Measurements using HP [2-13C]pyruvate allowed the exclusion of reactions related to TCA cycle turnover as sources of HP13CO2. Direct measures of O2consumption, ketone production, and glucose production by the intact liver combined with13C isotopomer analyses of tissue extracts yielded a comprehensive profile of metabolic flux in perfused liver. Together, these data show that, even though the majority of HP13CO2derived from HP [1-13C]pyruvate in livers exposed to fatty acids reflects decarboxylation of [4-13C]oxaloacetate (PEPCK) or [4-13C]malate (malic enzyme), the intensity of the HP13CO2signal is not proportional to glucose production because the amount of pyruvate returned to the TCA cycle via PEPCK and pyruvate kinase is variable, depending upon available substrates.
authors.affiliation: Advanced Imaging Research Center, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8568, USA.
authors.forename: Karlos X
authors.initials: KX
authors.lastname: Moreno
authors.affiliation: Department of Chemistry, University of Texas at Dallas, Richardson, TX 75080, USA.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Moore
authors.affiliation: Advanced Imaging Research Center, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8568, USA.
authors.forename: Shawn C
authors.initials: SC
authors.lastname: Burgess
authors.affiliation: Advanced Imaging Research Center, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8568, USA.
authors.forename: A Dean
authors.initials: AD
authors.lastname: Sherry
authors.affiliation: Advanced Imaging Research Center, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8568, USA.
authors.forename: Craig R
authors.initials: CR
authors.lastname: Malloy
authors.affiliation: Advanced Imaging Research Center, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8568, USA.
authors.forename: Matthew E
authors.initials: ME
authors.lastname: Merritt
keywords: Gluconeogenesis; Hyperpolarization; Hyperpolarized 13CO2 production; Liver metabolism; Pyruvate cycling