Flux through hepatic pyruvate carboxylase and phosphoenolpyruvate carboxykinase detected by hyperpolarized 13C magnetic resonance

Proc Natl Acad Sci U S A. 2011 Nov 22;108(47):19084-9. doi: 10.1073/pnas.1111247108. Epub 2011 Nov 7.


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.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Carbon Isotopes / metabolism
  • Citric Acid Cycle / physiology*
  • Fasting / metabolism
  • Gluconeogenesis / physiology*
  • Liver / enzymology*
  • Liver / physiology
  • Magnetic Resonance Spectroscopy
  • Mice
  • Mice, Knockout
  • Phosphoenolpyruvate Carboxykinase (GTP) / genetics
  • Phosphoenolpyruvate Carboxykinase (GTP) / metabolism*
  • Pyruvate Carboxylase / metabolism*


  • Carbon Isotopes
  • Phosphoenolpyruvate Carboxykinase (GTP)
  • Pyruvate Carboxylase