Synthesis and characterization of novel astragalin galactosides using β-galactosidase from Bacillus circulans

Astragalin (kaempferol-3-O-β-d-glucopyranoside, Ast) is a kind of flavonoid known to have anti-oxidant, anti-HIV, anti-allergic, and anti-inflammatory effects. It has low solubility in water. In this study, novel astragalin galactosides (Ast-Gals) were synthesized using β-galactosidase from Bacillus circulans and reaction conditions were optimized to increase the conversion yield of astragallin. Purified Ast-Gal1 (11.6% of Ast used, w/w) and Ast-Gal2 (6.7% of Ast used, w/w) were obtained by medium pressure chromatography (MPLC) with silica C₁₈ column and open column packed with Sephadex LH-20. The structures of Ast-Gal1 and Ast-Gal2 were identified by nuclear magnetic resonance (NMR) to be kaempferol-3-O-β-d-glucopyranosyl-(1→6)-β-d-galactopyranoside and kaempferol-3-O-β-d-glucopyranosyl-(1→6)-β-d-galactopyranosyl-(1→4)-β-d-galactopyranoside, respectively. The water solubility of Ast, Ast-Gal1, and Ast-Gal2 were 28.2±1.2mg/L, 38,300±3.5mg/L, and 38,800±2.8mg/L, respectively. The SC₅₀ value (the concentration required to scavenge 50% of the ABTS+) of Ast, Ast-Gal1, and Ast-Gal2 were 5.1±1.6μM, 6.5±0.4μM, and 4.9±1.1μM, respectively. The IC₅₀ values (the half maximal inhibitory concentration) of Ast, Ast-Gal1, and Ast-Gal2 against angiotensin converting enzyme (ACE) were 171.0±1.2μM, 186.0μM, and 139.0±0.2μM, respectively.