In vivo 2 H ‐ MR spectroscopy and imaging of hepatic metabolic formation of trimethylamine‐N‐oxide

Abstract Purpose Despite growing evidence of the link between elevated levels of trimethylamine‐N‐oxide (TMAO) and multiple diseases, there is no method with which to spatially monitor its hepatic formation from the interstitially produced trimethylamine (TMA). This study aimed to develop a deuterium metabolic spectroscopy (DMS) and imaging (DMI) approach to detect the TMA‐to‐TMAO metabolism in vivo. Methods The metabolism of 2 H 9 ‐TMA (TMA‐ d 9 ) to 2 H 9 ‐TMAO (TMAO‐ d 9 ) in cells overexpressing the hepatic enzyme flavin‐dependent monooxygenase 3 (FMO3) was monitored in vitro with 2 H‐NMR. Using an ultrahigh‐field (15.2T) MRI scanner, the hepatic metabolism of the orally administered TMA‐ d 9 to TMAO‐ d 9 was studied in mice with DMS and DMI. Results The spectrally resolved 2 H‐NMR peaks of intracellularly produced TMAO‐ d 9 (3.1 ppm) from that of supplemental TMA‐ d 9 (2.7 ppm) could be detected only in cells that overexpressed FMO3. In vivo, DMS and DMI experiments performed after oral administration of TMA‐ d 9 revealed the conversion to high TMAO‐ d 9 levels in the liver of females, which express high levels of FMO3. In contrast, there was no indication of TMAO‐ d 9 production in the liver of males, in agreement with reports of the role of testosterone in downregulating the expression of FMO3. Conclusion This work shows the ability to use 2 H‐MR‐based methodologies to spatially monitor the TMA‐to‐TMAO metabolic pathway in vivo, and thus should be explored further to investigate the role of TMAO in diverse pathologies.