In liver, <sup>13</sup>CO<inf>2</inf> can be generated from [1-<sup>13</sup>C]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) <sup>13</sup>CO<inf>2</inf> after administration of hyperpolarized pyruvate in livers supplied with a fatty acid plus substrates for gluconeogenesis. Isolated mouse livers were perfused with a mixture of thermally-polarized <sup>13</sup>C-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-<sup>13</sup>C]pyruvate is rapidly converted to [1-<sup>13</sup>C]oxaloacetate which can subsequently produce HP <sup>13</sup>CO<inf>2</inf> via decarboxylation at PEPCK. Measurements using HP [2-<sup>13</sup>C]pyruvate allowed the exclusion of reactions related to TCA cycle turnover as sources of HP <sup>13</sup>CO<inf>2</inf>. Direct measures of O<inf>2</inf> consumption, ketone production, and glucose production by the intact liver combined with <sup>13</sup>C 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 HP <sup>13</sup>CO<inf>2</inf> derived from HP [1-<sup>13</sup>C]pyruvate in livers exposed to fatty acids reflects decarboxylation of [4-<sup>13</sup>C]oxaloacetate (PEPCK) or [4-<sup>13</sup>C]malate (malic enzyme), the intensity of the HP <sup>13</sup>CO<inf>2</inf> signal 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.
- Hyperpolarized <sup>13</sup>CO<inf>2</inf> production
- Liver metabolism
- Pyruvate cycling
ASJC Scopus subject areas
- Clinical Biochemistry
- Endocrinology, Diabetes and Metabolism