Triterpenoid saponins and flavonoids from licorice residues with anti-inflammatory activity

Industrial processing of licorice (Glycyrrhiza glabra L.) leads to a considerable quantity of residues which are normally discarded randomly or treated by landfill. Therefore, the aim of this study was to develop licorice residues as a source of bioactive compounds with potentially applications. Chemical investigation of licorice residues led to the isolation of four new triterpenoid saponins (1–3, 12), along with ten known saponins (4–11, 13–14) and five flavonoids (15–19). The structures were established by comprehensive spectroscopic analyses. All the isolated compounds were tested for their inhibitory effects on nitric oxide (NO) production induced by lipopolysaccharide (LPS) in RAW 264.7 cells. Among the results, compound 18 displayed the most potent NO inhibitory effect (IC50 9.89 μM) compared with the positive control drug minocycline (IC50 33.20 μM). Further studies showed that the levels of pro-inflammatory cytokines interleukin (IL)-1β and interleukin (IL)-6 and inducible enzymes inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) were markedly down-regulated by compound 18. In addition, the Western blotting assay showed that compound 18 might reduce the inflammatory effect by down-regulating autophagy level. Overall, this study suggest that licorice residues are a promising waste and a valuable source of bioactive compounds for the pharmaceutical industry.