title: Flux through hepatic pyruvate carboxylase and phosphoenolpyruvate carboxykinase detected by hyperpolarized 13C magnetic resonance.
abstract: In the heart, detection of hyperpolarized [(13)C]bicarbonate and (13)CO(2) by magnetic resonance (MR) after administration of hyperpolarized [1-(13)C]pyruvate is caused exclusively by oxidative decarboxylation of pyruvate via the pyruvate dehydrogenase complex (PDH). However, liver mitochondria possess alternative anabolic pathways accessible by [1-(13)C]pyruvate, which may allow a wider diagnostic range for hyperpolarized MR compared with other tissue. Metabolism of hyperpolarized [1-(13)C]pyruvate in the tricarboxylic acid (TCA) cycle was monitored in the isolated perfused liver from fed and fasted mice. Hyperpolarized [1-(13)C]pyruvate was rapidly converted to [1-(13)C]lactate, [1-(13)C]alanine, [1-(13)C]malate, [4-(13)C]malate, [1-(13)C]aspartate, [4-(13)C]aspartate, and [(13)C]bicarbonate. Livers from fasted animals had increased lactate:alanine, consistent with elevated NADH:NAD(+). The appearance of asymmetrically enriched malate and aspartate indicated high rates of anaplerotic pyruvate carboxylase activity and incomplete equilibration with fumarate. Hyperpolarized [(13)C]bicarbonate was also detected, consistent with multiple mechanisms, including cataplerotic decarboxylation of [4-(13)C]oxaloacetate via phosphoenolpyruvate carboxykinase (PEPCK), forward TCA cycle flux of [4-(13)C]oxaloacetate to generate (13)CO(2) at isocitrate dehydrogenase, or decarboxylation of [1-(13)C]pyruvate by PDH. Isotopomer analysis of liver glutamate confirmed that anaplerosis was sevenfold greater than flux through PDH. In addition, signal from [4-(13)C]malate and [4-(13)C]aspartate was markedly blunted and signal from [(13)C]bicarbonate was completely abolished in livers from PEPCK KO mice, indicating that the major pathway for entry of hyperpolarized [1-(13)C]pyruvate into the hepatic TCA cycle is via pyruvate carboxylase, and that cataplerotic flux through PEPCK is the primary source of [(13)C]bicarbonate. We conclude that MR detection of hyperpolarized TCA intermediates and bicarbonate is diagnostic of pyruvate carboxylase and PEPCK flux in the liver.
authors: Advanced Imaging Research Center, Department of Molecular Biophysics, University of Texas Southwestern Medical Center, Dallas, TX 75390-8568, USA. Matthew.Merritt@UTSouthwestern.edu
authors: Matthew E
authors: ME
authors: Merritt
authors: Crystal
authors: C
authors: Harrison
authors: A Dean
authors: AD
authors: Sherry
authors: Craig R
authors: CR
authors: Malloy
authors: Shawn C
authors: SC
authors: Burgess
mesh_terms: D000818:Animals; D002247:Carbon Isotopes / Q000378:metabolism; D002952:Citric Acid Cycle / Q000502:physiology*; D005215:Fasting / Q000378:metabolism; D005943:Gluconeogenesis / Q000502:physiology*; D008099:Liver / Q000201:enzymology* / Q000502:physiology; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D018345:Mice, Knockout; D010729:Phosphoenolpyruvate Carboxykinase (GTP) / Q000235:genetics / Q000378:metabolism*; D011766:Pyruvate Carboxylase / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D010729:Phosphoenolpyruvate Carboxykinase (GTP); D011766:Pyruvate Carboxylase