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 ± 1.2 mg/L, 38,300 ± 3.5 mg/L, and 38,800 ± 2.8 mg/L, respectively. The SC₅₀ value (the concentration required to scavenge 50% of the ABTS[rad]+ ) of Ast, Ast-Gal1, and Ast-Gal2 were 5.1 ± 1.6 μM, 6.5 ± 0.4 μM, and 4.9 ± 1.1 μM, respectively. The IC₅₀ values (the half maximal inhibitory concentration) of Ast, Ast-Gal1, and Ast-Gal2 against angiotensin converting enzyme (ACE) were 171.0 ± 1.2 μM, 186.0 μM, and 139.0 ± 0.2 μM, respectively.