Rapid discovery of natural antioxidants in Hypericum japonicum: Dual roles of the liquid phase mobile phase as extraction and separation solvent

Hypericum japonicum is a traditional folk medicine with various bioactivities such as hepatoprotective, antioxidant, and anti-tumorous. The antioxidant effect of H. japonicum is one of the most prominent effects due to its responsibility for many of its activities. To clarify active natural substance, the antioxidant properties of H. japonicum were preliminarily assessed by ferric reducing–antioxidant power (FRAP), 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS) and Oxygen radical absorbance capacity (ORAC), as well as superoxide dismutase (SOD). Then, a straightforward and effective method named online liquid extraction-high performance liquid chromatography combined with ABTS antioxidant assay and mass spectrometry (OLE-HPLC-ABTS/Q-TOF-MS) was developed to swiftly and directly discover the antioxidants in H. japonicum. Using mobile phase as extraction and separation reagent, coupled with online activity analysis and compounds identification by high-resolution MS, the online system enables rapid screening of natural antioxidant bioactives from complex mixture. By using it, a total of 9 compounds including flavonoids and phenolic acids characterized by retention time, precise mass, and fragmentation ions in MS/MS spectra showed antioxidant action. Finally, the antioxidant and SOD activity of main found active compounds were validated by in vitro experiment assay and molecular docking. In summary, these results suggested that H. japonicum could be considered as a potential source of natural antioxidants, and the online integrated system might become a promising candidate for the natural antioxidants discovery in the future.