Matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI) is presently used in physiological evaluations for visualisation of targets in organs. In the present study, MALDI-MSI was used as a visualisation technique to investigate the intestinal absorption of polyphenols. Nifedipine/phytic acid-aided MALDI-MSI was performed to visualise theaflavin-3′- O -gallate (TF3′G) and epicatechin-3- O -gallate (ECG) in the rat jejunum for 50-µM, 60-min transport experiments. Non-absorbable TF3′G was successfully visualised at the apical region, whereas absorbable ECG was detected throughout the rat jejunum. MALDI-MSI was also performed to determine the transport routes of the target metabolites. Signals corresponding to TF3′G and ECG in the membranes were diminished following treatment with inhibitors targeting the monocarboxylic acid transporter and organic anion transporting polypeptides. Enhanced visualisation of TF3′G was achieved by inhibiting efflux routes. Our findings demonstrated that the present MALDI-MSI can provide critical spatial informations on intestinal absorption of targets, by which TF3′G and ECG were incorporated into intestinal tissues, followed by efflux back to the apical compartment. In addition, MALDI-MSI analyses suggested that TF3′G was resistant to phase II metabolism during the influx/efflux processes, whereas ECG was susceptible to methylation and sulphation reactions. In conclusion, inhibitor-aided MALDI-MSI could serve as a powerful in situ visualisation technique for verifying intestinal transport routes and investigating the metabolism of penetrants.

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Matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) is widely used for the detection and analysis of ionizable compounds. However, the method has less potential for the analysis of neutral compounds, such as polyphenols, owing to their lack of favorable proton-attachment or -removal groups. In this study, we reported for the first time that nifedipine (2,6-dimethyl-3,5-dicarbomethoxy-4-(2-nitrophenyl)-1,4-dihydropyridine), which is a strong photobase generator commonly used in polymerization, can abstract protons from neutral compounds in negative mode-MALDI experiments. When nifedipine (5 mg/ml) was used as a matrix reagent, the limit of detection (LOD) for epigallocatechin-3-O-gallate (EGCG) was determined to be 100 fmol/spot, which constitutes >50-fold improvement compared to the LOD obtained when trans-3-indoleacrylic acid, a matrix reagent previously reported for polyphenol detection, was used. Of the dihydropyridines investigated, only nifedipine facilitated the detection of EGCG, suggesting that the nitrosophenyl pyridine derivative of nifedipine formed by photoreduction under laser irradiation at 355 nm plays a crucial role in detecting polyphenols in negative mode. Reduced MS detection of 5-O-methylnaringenin indicated that nifedipine may preferably remove a proton from the 5-position OH group in the A ring of the flavonoid skeleton. The significant MS detection by nifedipine was extensively observed for polyphenols including flavones, flavonones, chalcones, stilbenoids and phenolic acids. In conclusion, nifedipine can act as a novel matrix for improving polyphenol detection by MALDI-MS in negative mode. Copyright © 2016 John Wiley & Sons, Ltd.

Food-derived polyphenols have health benefits owing to their ability to prevent some diseases. Knowledge of their bioavailabilities and metabolism is important for understanding the mechanisms underlying their benefits. Thus, mass spectrometry (MS)-based analytical techniques, such as liquid chromatography-MS (LC-MS), that are capable of highly sensitive and simultaneous detection of absorbed polyphenols and their metabolites are essential tools in this field. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging is currently used in biological sciences for visualizing the spatial information of various compounds, such as peptides, proteins, lipids, drugs, and food compounds, in various biological tissues. However, the application of MALDI-MS imaging to polyphenols is challenging owing to the lack of appropriate matrix reagents for visualizing (or detecting) polyphenols absorbed in target biological tissues. This review highlights the development of the MALDI-MS imaging technique for the detection and visualization of food-derived polyphenols. First, the development of new matrices and matrix additives for the detection and visualization of polyphenols is discussed, followed by their introduction to inhibitor-aided MALDI-MS imaging for the elucidation of intestinal absorption routes and the metabolic behaviors of polyphenols. The MALDI-MS imaging technique shows great potential as a novel analytical tool for elucidating the bioavailabilities of food-derived polyphenols.

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