Mass Spectrometric Enzyme Histochemistry Method Developed for Visualizing In Situ Cholinesterase Activity in Mus musculus and Drosophila melanogaster.

Enzyme histochemistry facilitates enzyme activity visualization in situ; however, as it is a color-based method, molecular quantification is prohibitive. This study aimed to develop a semiquantitative, mass spectrometry imaging (MSI)-based enzyme histochemistry method to determine endogenous cholinesterase (ChE) activity. Using deuterium-labeled acetylcholine (ACh-d9) as a substrate to distinguish ACh-d9 and choline-d9 from endogenous acetylcholine and choline, respectively, the heterogeneous localization of de novo ChE activity was visualized using MSI, devoid of interferences from in situ factors. Furthermore, a tissue inhibitor assay involving two ChE inhibitors in the mouse brain revealed specific ChE inhibition in the corpus callosum. To the best of our knowledge, this study is the first to report a visualization method for total ChE activity in the ganglia and abdomen in Drosophila melanogaster, indicating its applicability among different animals. The present results provide novel insights into the applicability of enzyme histochemistry via MSI to the metabolism of low-molecular-weight organic compounds (i.e., "small molecules") and semiquantitative capability, suggesting that MSI enzyme histochemistry may become a powerful tool for heterogeneous tissue studies.