Optimization of alkali extraction, structure, and antioxidant activity of protein-bound polysaccharide from seeds of Plantago ovata Forssk

Abstract

In this study, the effects of different solvents on the viscosity of the seeds of Plantago ovata Forssk (POF) were compared to select the extraction solvent; then, a three-level and four-variable Box–Behnken design (BBD) was applied to determine the optimum extraction conditions for the maximum yields of polysaccharides from POF. Two novel protein-bound heteropolysaccharides (POFG-1a and POFG-2a) with molecular weights of 118 kDa and 113 kDa were isolated and purified by ethanol fractionation precipitation, DEAE-650 M anion exchange column chromatography, and Sephadex G-100 column chromatography. POFG-1a and POFG-2a contain 49.6% and 17.2% of polysaccharide, 26.9% and 56.4% of protein, 9.1% and 5.0% of uronic acid, respectively; they are rich in monosaccharides, Ara, Gal, and Rha, and amino acids, Ser, Pro, Ile, and Phe. Based on FT-IR, β-elimination, methylation analysis, NMR spectroscopy, and some literature data, it was found that POFG-1a is a homogeneous acidic protein-bound heteropolysaccharide with O-glycosidic linkage. The main chain of POFG-1a is connected by 1→4 Araf and 1→5 Araf with a high degree of branching, connected to the branch at the O-2 and O-3 positions of 1→5 Araf. The POFGs exhibited strong scavenging activity on DPPH, ABTS, and NO free radicals, which is rarely observed in other plant polysaccharides. These results indicate that POFGs might have potential applications in the medical and food industries as novel natural antioxidants.