Comprehensive screening and identification of natural inducible nitric oxide synthase inhibitors from Radix Ophiopogonis by off-line multi-hyphenated analyses

Off-line multi-hyphenated methods including affinity-ultrafiltration mass spectrometry, molecular docking, and reverse-phase high-performance liquid chromatography-fluorescence detection (RP-HPLC-FD) were used to rapidly screen and identify natural inducible nitric oxide synthase (iNOS) inhibitors isolated from Ophiopogon japonicus (Thunb.) Ker-Gawl. Ultrafiltration high-performance liquid chromatography quadrupole time-of-flight mass spectrometry was used to screen and identify the potential ligands with high binding affinity to iNOS. Seven compounds (three saponins and four flavonoids) were found in Radix Ophiopogonis. Molecular docking identified the binding sites and found that the binding affinity of the three ophiopogonis ligands was higher than that of the four homoisoflavonoids. Pre-column derivatization of l -citrulline and RP-HPLC-FD verified that the in vitro inhibition activity of the three ophiopogonis compounds was stronger than that of the four homoisoflavonoids, which was similar to the screened results. In conclusion, combining an in vitro iNOS inhibition assay and affinity-ultrafiltration mass spectrometry with molecular docking was a powerful tool for rapid screening and identification of iNOS inhibitors isolated from this traditional Chinese medicine.