Abstract
The activity of specific enzyme-catalyzed reactions may be detected in vivo by (13) C NMR of hyperpolarized (HP) substrates. The signals from HP substrates and products, acquired over time, have been fitted to a number of different mathematical models to determine fluxes, but these models have not been critically compared. In this study, two-pool and three-pool first-order models were constructed to measure flux through lactate dehydrogenase in isolated glioblastoma cells by NMR detection of lactate and pyruvate following the addition of HP [1-(13) C]pyruvate. Mass spectrometry (MS) was used to independently monitor (13) C enrichment in intra- and extracellular lactate. Six models were evaluated using time-dependent pyruvate C2 and lactate C1 HP NMR data acquired by the use of selective excitation pulses, plus (13) C enrichment data from intracellular and extracellular lactate measured by MS. A three-pool bidirectional model provided the most accurate description of pyruvate metabolism in these cells. With computed values for T(1) of pyruvate and lactate, as well as the effect of pulsing, the initial flux through lactate dehydrogenase was well determined by both the two-pool bidirectional and unidirectional models when only HP data were available. The three-pool model was necessary to fit the combined data from both MS and HP, but the simpler two-pool exchange model was sufficient to determine the (13) C lactate concentration when the lactate appearance was measured only by HP.