C-ring Cleavage Of Liquiritigenin Extracted From Licorice Roots By An Oxygen-tolerant Bovine Rumen Bacterium Strain Aeroto-Niu-O16

Licorice root, the main active components of which are flavonoids, is one of the most popular used Chinese traditional medicine. Liquiritigenin (LG), which belongs to the group of dihydroflavones, is one of the main ingredients of licorice flavonoids, mainly occurring as LG glucoside (i.e. liquiritin) in nature. Liquiritin is searched in high concentration in licorice root, the content of which reaches 3%. Previous studies have shown that licorice root was able to be transformed to different metabolites, including glycyrrhetinic acid and davidigenin (DG), by human and rat intestinal microflora. The metabolite DG was reported to be of different bioactivities which are different from those of LG, such as apoptotic induction of in human lung fibroblasts, therapeutic effect in the treatment of diabetes, relatively high antispasmodic activity, etc. However, to date, DG has not been found in licorice root.Aeroto-Niu-O16 is an oxygen-tolerant bovine rumen bacterium capable of aerobically reducing isoflavones daidzein and genistein to dihydrodaidzein and dihydrogenistein through catalytic hydrogenation. In this study, we found that bacterium strain Aeroto-Niu-O16 was able to cleavage the C-ring of LG, one of the main biologically active components of licorice roots in the presence of atmospheric oxygen. The substrate LG was prepared by acid hydrolysis of the crude extract of licorice roots. The metabolite of LG by strain Aeroto-Niu-O16 was identified as DG based on the UV spectrum, MS spectrometry, 1H and 13C nuclear magnetic resonance (NMR) analyses. The maximal concentration of LG that strain Aeroto-Niu-O16 was able to transform effectively was 0.8 mmol/L and the average productivity of the metabolite DG was 71.7%. Furthermore, when 0.1% (m/v) of L-cysteine or sodium thiosulfate was added in the cultural medium, the average bioconversion rate of the substrate LG was increased from 71.7% to 78.3% and 77.2%, respectively. The in vitro antioxidant investigation showed that 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity of DG was extremely significantly higher than that of LG at concentrations ranging from 0.4 mmol/L to 1.6 mmol/L. We here show for the first time that LG can be converted to DG, the biological activities of which is stronger and wider than that of LG, through microbial biotransformation method.