Simultaneous detection of cerebral metabolism of different substrates by in vivo 13C isotopomer MRS

Abstract

In this report a new method is introduced for simultaneous detection of the metabolism of two 13C-labeled subtracts in brain in vivo. We recognized and experimentally demonstrated that when a 13C-labeled substrate generates [1,2-13C2]acetylCoA ([1-13C]acetylCoA) only, glutamate C5, glutamine C5 and apsartate C4 doublets (singlets) are formed exclusively, regardless of the number of turns of the tricarboxylic acid cycle. We utilized the large one-bond 13C-13C homonuclear J coupling between a carboxylic/amide carbon and an aliphatic carbon (~50 Hz) and demonstrated that it is feasible to simultaneously detect the labeling of brain metabolites by two different substrates using different isotopomer signals of the same carbon atom. Uniformly labeled glucose was used to generate the doublets and a second substrate ([2-13C] lactate or [1,3-13C2]β-hydroxybutyrate or [1-13C] acetate) was used to generate the singlets. It was shown that contribution to cerebral metabolism from different substrates can be simultaneously measured in vivo.

Glutamate Metabolism in Major Depressive Disorder

“…Rising evidence suggest astroglial changes in major depression (16, 36) including 13 C MRS studies in a rodent chronic unpredictable stress model (33). To overcome this limitation, future studies may employ recently developed 13 C methods of combined labeling by 13 C-glucose and 13 C-acetate (37). Acetate oxidation is limited to astroglial cells, so therefore the simultaneous administration of 13 C-glucose and 13 C-acetate allows separate measurements of neuronal and astroglial energy production, respectively (14).…”

Spectral editing for in vivo 13C magnetic resonance spectroscopy

“…Total number of scans was 32. Unlike uniformly 13 C-labeled glucose which produces only a doublet at glutamate C5 [15], infusion of [1,2- 13 C 2 ]acetate leads to both a doublet and a singlet at glutamate C5 due to pyruvate recycling [18]. Because of the close proximity of glutamate C5 to acetate C1 as shown in Fig.…”

“…The spectral overlap between glutamate C5 at 182.0 ppm and acetate C1 at 182.15 ppm, however, cannot be resolved by existing techniques. When [1- 13 C] acetate is used care has to be taken to ensure that tissue concentration of acetate is sufficiently low so as not to contaminate the glutamtate C5 signal [15]. …”

In vivo detection of 13C isotopomer turnover in the human brain by sequential infusion of 13C labeled substrates

“…When 13 C-labeled glucose is administered the glutamate and glutamine C4-C5 moiety and the aspartate C3-C4moiety are labeled by glucose C1-C2 and/or C5-C6, regardless of the number of the turns of the tricarboxylic acid (TCA) cycle. As a result, when the input is [1,2- 13 C 2 ]acetylCoA glutamate C5, glutamine C5 and aspartate C4 can only form doublet signals (18). Vice versa, when the input is [2- 13 C]acetylCoA glutamate C5, glutamine C5 and aspartate C4 can only form singlet signals (18).…”

“…As a result, when the input is [1,2- 13 C 2 ]acetylCoA glutamate C5, glutamine C5 and aspartate C4 can only form doublet signals (18). Vice versa, when the input is [2- 13 C]acetylCoA glutamate C5, glutamine C5 and aspartate C4 can only form singlet signals (18). This unique feature of the carboxylic/amide carbons makes it possible to simultaneously and unambiguously detect the labeling of glutamate, glutamine and aspartate by two substrates with the second substrate (e.g., acetate, ethanol, β-hydroxybutyrate or lactate) producing singlets only as demonstrated recently using isoflurane anesthetized rats and an 11.7 Tesla spectrometer (18).…”