Affinity Selection-Mass Spectrometry for the Discovery of Pharmacologically Active Natural Products
Although originally invented for high-throughput screening of combinatorial libraries, affinity selection-mass spectrometry (AS-MS) is ideal for the discovery of lead compounds from chemically diverse sources such as botanical, fungal and microbial extracts. Based on binding interactions between macromolecular receptors and ligands of low molecular mass, AS-MS facilitates the affinity isolation of pharmacologically active small molecules from complex mixtures for mass
spectrometric characterization and identification. AS-MS requires no radiolabels, no UV or fluorescent chromophores, and is compatible with all classes of receptors, enzymes, incubation buffers, cofactors, and ligands. The most successful types of AS-MS include pulsed ultrafiltration (PUF) AS-MS, size exclusion chromatography (SEC) AS-MS, and magnetic microbead affinity selection screening (MagMASS), which differ mainly in their approaches for separating the ligand-receptor complexes from the non-binding compounds in mixtures. After affinity isolation, the ligand(s) from the mixture are characterized using high resolution UHPLC-MS and tandem mass spectrometry on instruments such as the Shimadzu LCMS-9030 QToF MS.
Based on HRMS elemental composition and tandem mass spectrometric structural data, the identities of the lead compounds are determined through dereplication by searching on-line databases for known natural products and by comparison with standards. For novel natural products, AS-MS provides UHPLC retention times of active compounds for chromatographic isolation and then complete structure determination using a combination of spectroscopic techniques. Advantages of ASMS over conventional high-throughput screening are speed, reduced consumption of reagents, compatibility with any receptor and ligand, and the ability to discovery allosteric as well as orthosteric ligands to any receptor, whether or not any ligands are already known.
