Selective enrichment of microbiota-derived tryptophan metabolites in mouse faeces based on molecularly imprinted solid-phase extraction for HPLC analysis

Gut microbiota-derived tryptophan (TRP) metabolites, especially the indole derivatives, function as critical modulators of intestinal immune function and integrity via modulating the aryl hydrocarbon receptor pathway. Selective enrichment of these indole metabolites in biological samples is important for quantitative determination by routine methods such as HPLC. Here, we report a molecularly imprinted polymer (MIP) for solid-phase extraction (SPE) of TRP-derived indole metabolites from faeces of mice. The MIP was synthesized by surface polymerization by using indole-3-acetic acid (IAA) and acrylamide after cross-linking by ethylene glycol dimethacrylate (EGDMA) and initialed by 2',2-azobisisobutyronitrile (AIBN). The MIPs were then characterized by transmission electron microscope (TEM) and fourier transform infrared spectroscopy (FTIR), and the adsorption capacity, selectivity and reusability of MIPs were systematically evaluated. Results showed that IAA-MIPs showed a uniformly distributed nanoshell layer with a thin shell thickness of 26 nm. The IAA-MIPs could selectively adsorb IAA, indole-3-propionic acid (IPA), and indole-3-lactic acid (ILA) in a mixed solution that also contains TRP and tyrosine. The adsorption capacity of IAA-MIPs only slightly decreased with the increase of recycling use. As purification material for SPE, the IAA-imprinted polymers (IAA-MIPs) were successfully applied to extract IAA, IPA, and ILA from normal and colitis mice for HPLC determination. Collectively, these surface molecularly imprinted polymers could find extensive use to selective enrichment of microbiota-derived indole metabolites in biological samples.