TY - JOUR
T1 - Flux through hepatic pyruvate carboxylase and phosphoenolpyruvate carboxykinase detected by hyperpolarized 13C magnetic resonance
AU - Merritt, Matthew E.
AU - Harrison, Crystal
AU - Sherry, A. Dean
AU - Malloy, Craig R.
AU - Burgess, Shawn C.
PY - 2011/11/22
Y1 - 2011/11/22
N2 - In the heart, detection of hyperpolarized [ 13C]bicarbonate and 13CO 2 by magnetic resonance (MR) after administration of hyperpolarized [1- 13C]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- 13C]pyruvate, which may allow a wider diagnostic range for hyperpolarized MR compared with other tissue. Metabolism of hyperpolarized [1- 13C]pyruvate in the tricarboxylic acid (TCA) cycle was monitored in the isolated perfused liver from fed and fasted mice. Hyperpolarized [1- 13C]pyruvate was rapidly converted to [1- 13C]lactate, [1- 13C]alanine, [1- 13C] malate, [4- 13C]malate, [1- 13C]aspartate, [4- 13C]aspartate, and [ 13C] 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 [ 13C]bicarbonate was also detected, consistent with multiple mechanisms, including cataplerotic decarboxylation of [4- 13C] oxaloacetate via phosphoenolpyruvate carboxykinase (PEPCK), forward TCA cycle flux of [4- 13C] oxaloacetate to generate 13CO 2 at isocitrate dehydrogenase, or decarboxylation of [1- 13C]pyruvate by PDH. Isotopomer analysis of liver glutamate confirmed that anaplerosis was sevenfold greater than flux through PDH. In addition, signal from [4- 13C] malate and [4- 13C]aspartate was markedly blunted and signal from [ 13C]bicarbonate was completely abolished in livers from PEPCK KO mice, indicating that the major pathway for entry of hyperpolarized [1- 13C]pyruvate into the hepatic TCA cycle is via pyruvate carboxylase, and that cataplerotic flux through PEPCK is the primary source of [ 13C]bicarbonate. We conclude that MR detection of hyperpolarized TCA intermediates and bicarbonate is diagnostic of pyruvate carboxylase and PEPCK flux in the liver.
AB - In the heart, detection of hyperpolarized [ 13C]bicarbonate and 13CO 2 by magnetic resonance (MR) after administration of hyperpolarized [1- 13C]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- 13C]pyruvate, which may allow a wider diagnostic range for hyperpolarized MR compared with other tissue. Metabolism of hyperpolarized [1- 13C]pyruvate in the tricarboxylic acid (TCA) cycle was monitored in the isolated perfused liver from fed and fasted mice. Hyperpolarized [1- 13C]pyruvate was rapidly converted to [1- 13C]lactate, [1- 13C]alanine, [1- 13C] malate, [4- 13C]malate, [1- 13C]aspartate, [4- 13C]aspartate, and [ 13C] 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 [ 13C]bicarbonate was also detected, consistent with multiple mechanisms, including cataplerotic decarboxylation of [4- 13C] oxaloacetate via phosphoenolpyruvate carboxykinase (PEPCK), forward TCA cycle flux of [4- 13C] oxaloacetate to generate 13CO 2 at isocitrate dehydrogenase, or decarboxylation of [1- 13C]pyruvate by PDH. Isotopomer analysis of liver glutamate confirmed that anaplerosis was sevenfold greater than flux through PDH. In addition, signal from [4- 13C] malate and [4- 13C]aspartate was markedly blunted and signal from [ 13C]bicarbonate was completely abolished in livers from PEPCK KO mice, indicating that the major pathway for entry of hyperpolarized [1- 13C]pyruvate into the hepatic TCA cycle is via pyruvate carboxylase, and that cataplerotic flux through PEPCK is the primary source of [ 13C]bicarbonate. We conclude that MR detection of hyperpolarized TCA intermediates and bicarbonate is diagnostic of pyruvate carboxylase and PEPCK flux in the liver.
KW - Dynamic nuclear polarization
KW - Gluconeogenesis
KW - Metabolic flux
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U2 - 10.1073/pnas.1111247108
DO - 10.1073/pnas.1111247108
M3 - Article
C2 - 22065779
AN - SCOPUS:82755168823
SN - 0027-8424
VL - 108
SP - 19084
EP - 19089
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 47
ER -