Milacemide metabolism in rat liver and brain slices by solids NMR

The metabolism of ¹³C- and ¹⁵N-labeled milacemide, 2-(pentylamino)- acetamide, has been studied in rat liver and brain slices using solid-state NMR. This analysis is fast and efficient and can be used to monitor both major and minor metabolic pathways in mammalian tissue culture. The NMR work reported herein involves both conventional cross-polarization magic-angle spinning ¹³C and ¹⁵N NMR spectra and rotational-echo double resonance ¹³C-¹⁵N experiments. The latter measure quantitatively the breaking of isotopically labeled carbon-nitrogen chemical bonds. Our results, which are consistent with suggestions from previous metabolic studies, show that the first step in the breakdown of milacemide is the breaking of the pentylamine nitrogen bond to yield pentanoic acid and glycinamide. Total incorporation of ¹⁵N label from the resulting glycinamide fragment is comparable in rat liver and brain. In both tissues, considerably more of the ¹⁵N label from glycinamide is incorporated than the corresponding ¹³C label. Differences between the liver and brain tissue are also observed, with more synthesis incorporating the ¹³C labels taking place in the liver.