Characterization of Novel Astragalin Galactosides Synthesized by β-Galactosidase from Bacillus circulans

Abstract

Astragalin (kaempferol-3-O-β-D-glucopyranoside) is a kind of flavonoid known to have anti-oxidant, anti-HIV, anti-allergic, and anti-inflammatory effects and low solubility in water. In this study, novel astragalin galactosides (Ast-Gals) were enzymatically synthesized with β-galactosidase from Bacillus circulans. Reaction conditions were optimized to maximize the conversion yield of astragalin. 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)-O-β-D-galactopyranoside and kaempferol-3-O-β-D-glucopyranosyl-(1→6)-O-β-D-galactopyranosyl-(1→4)-O-β-D-galactopyranoside, respectively. The water solubility of Ast, Ast-Gal1, and Ast-Gal2 were 28.2 mg/L, 38,300 mg/L, and 38,800 mg/L, respectively. The antioxidant activities (The half maximal scavenging capacity, SC50) of Ast, Ast-Gal1, and Ast-Gal2 based on ABTS∙+ radical scavenging activity were 5.05 µM, 6.54 µM, and 4.91 µM, respectively. The angiotensin converting enzyme inhibition activities (ACE, the half maximal inhibitory capacity, IC50) of Ast, Ast-Gal1, and Ast-Gal2 were 171 µM, 191 µM, and 138 µM, respectively.