C-ring cleavage of liquiritigenin extracted from licorice roots by an oxygen-tolerant bovine rumen bacterium strain Aeroto-Niu-O16

Aeroto-Niu-O16, an oxygen-tolerant bovine rumen bacterium, is capable of aerobically reducing isoflavones daidzein and genistein to dihydrodaidzein and dihydrogenistein through catalytic hydrogenation.  In this study, it was found that bacterium strain Aeroto-Niu-O16 was able to cleavage the C-ring of liquiritigenin (LG), which is one of the main biologically active components of licorice roots, in the presence of atmospheric oxygen.  LG was prepared by acid hydrolysis of the crude extract of licorice roots.  The metabolite of LG obtained in strain Aeroto-Niu-O16 was identified as davidigenin (DG) based on the data of UV, MS, ¹H and ¹³C NMR.  The maximal concentration of LG that the strain Aeroto-Niu-O16 was able to transform effectively was 0.8 mmol·L⁻¹ 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 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 significantly or extremely significantly higher than that of LG at the concentrations from 0.2 mmol·L⁻¹ to 1.6 mmol·L⁻¹.  We discoverd for the first time that LG can be converted to DG, which has stronger and wider biological activities, through microbial biotransformation method.