Identification of new resonances in downfield 1H MRS of human calf muscle in vivo: Potentially metabolite precursors for skeletal muscle NAD+

Abstract

PurposeThe purpose of this study was to identify and characterize newly discovered resonances appearing in the downfield proton MR spectrum (DF 1H MRS) of the human calf muscle in vivo at 7T.MethodsDownfield 1H MRS was performed on the calf muscle of five healthy volunteers at 7T. A spectrally selective 90° E‐BURP RF pulse with an excitation center frequency at 10.3 ppm and an excitation bandwidth of 2 ppm was used for DF 1H MRS acquisition.ResultsIn all participants, we observed new resonances at 9.7, 10.1, 10.3, and 10.9 ppm in the DF 1H MRS. Phantom experiments at 37°C strongly suggest the new resonance at 9.7 ppm could be from H2‐proton of the nicotinamide rings in nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) while the resonance at 10.1 ppm could be attributed to the indole –NH proton of L‐tryptophan. We observed that the resonances at 10.1 and 10.9 ppm are significantly suppressed when the water resonance is saturated, indicating that these peaks have either 1H chemical exchange or cross‐relaxation with water. Conversely, the resonances at 9.7 and 10.3 ppm exhibit moderate signal reduction in the presence of water saturation.ConclusionWe have identified new proton resonances in vivo in human calf muscle occurring at chemical shifts of 9.7, 10.1, 10.3, and 10.9 ppm. These preliminary results are promising for investigating the role of NR/NMN and L‐tryptophan metabolism in understanding the de novo and salvage pathways of NAD+ synthesis in skeletal muscle.