Structural and quantitative analysis of antioxidant and low-density lipoprotein-antioxidant flavonoids from the grains of sugary rice

Grains of sugary rice were extracted with 80% aqueous methanol, and the concentrated extracts were successively partitioned using ethyl acetate, n-butanol, and water. From the n-butanol fractions, four flavonoid glycosides were isolated through repeated silica gel, octadecyl silica gel, and Sephadex LH-20 column chromatographies. Based on the nuclear magnetic resonance, mass spectrometry, and infrared spectroscopic data, the chemical structures of the compounds were determined to be taxifolin-7-O-β-d-glucopyranoside (1), hyperin (2), isoquercitrin (3), and quercetin gentiobioside (4). These compounds were isolated from the grains of sugary rice for the first time. All isolated compounds were tested for antioxidant activity and low-density lipoprotein (LDL)-antioxidative activity using 1,1-diphenyl-2-picrylhydrazyl (DPPH) and LDL assays. Compound 1 exhibited a strong scavenging effect on DPPH, with a 50% inhibition concentration (IC ₅₀) value of 8.1 μM, and also inhibited LDL oxidation with an IC ₅₀ value of 40.0±20 μM. A simple and efficient high-performance liquid chromatography/diode array detection method for the simultaneous determination of the four bioactive flavonoids (1-4) has been developed and applied to their content determination in the sugary rice. The grains were extracted by 80% methanol, and the contents of 1, 2, 3, and 4 were determined to be 1.12±0.045, 0.65±0.011, 0.68±0.032, and 0.89±0.021 mg/g, respectively.