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.
|Original language||English (US)|
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Nov 22 2011|
- Dynamic nuclear polarization
- Metabolic flux
ASJC Scopus subject areas