Identification of l-Tryptophan by down-field 1H MRS: A precursor for brain NAD+ and serotonin syntheses
Ravi Prakash Reddy Nanga, Mark A. Elliott, Anshuman Swain, Neil Wilson, Sophia Swago, Narayan Datt Soni, Walter R. Witschey, Ravinder Reddy
Abstract
Purpose
To explore the presence of new resonances beyond 9.4 ppm from the human brain, down-field proton MRS was performed in vivo in the human brain on 6 healthy volunteers at 7 T.
Methods
To maximize the SNR, a large voxel was placed within the brain to cover the maximal area in such a way that sinus cavities were avoided. A spectrally selective 90° E-BURP pulse with an excitation bandwidth of 2 ppm was used to probe the spectral chemical shift range between 9.1 and 10.5 ppm. The E-BURP pulse was integrated with PRESS spatial localization to obtain non-water-suppressed proton MR spectra from the desired spectral region.
Results
In the down-field proton MRS obtained from all of the volunteers scanned, we identified a new peak consistently resonating at 10.1 ppm. Protons associated with this resonance are in cross-relaxation with the bulk water, as demonstrated by the water saturation and deuterium exchange experiments.
Conclusion
Based on the chemical shift, this new peak was identified as the indole (–NH) proton of l-tryptophan (l-TRP) and was further confirmed from phantom experiments on l-TRP. These promising preliminary results potentially pave the way to investigate the role of cerebral metabolism of l-TRP in healthy and disease conditions.