Ultrasonic extraction of polysaccharides from Laminaria japonica and their antioxidative and glycosidase inhibitory activities

In the present study, ultrasonic extraction technique (UET) is used to improve the yield of polysaccharides from Laminaria japonica (LJPs). And their antioxidative as well as glycosidase inhibitory activities are investigated. Box-Behnken design (BBD) combined with response surface methodology (RSM) is applied to optimize ultrasonic extraction for polysaccharides. The optimized conditions are obtained as extraction time at 54 min, ultrasonic power at 1050 W, extraction temperature at 80A degrees C and ratio of material to solvent at 1:50 (g mL(-1)). Under these optimal ultrasonic extraction conditions, an actual experimental yield (5.75% +/- 0.3%) is close to the predicted result (5.67%) with no significant difference (P > 0.05). Vitro antioxidative and glycosidase inhibitory activities tests indicate that the crude polysaccharides (LJP) and two major ethanol precipitated fractions (LJP1 and LJP2) are in a concentration-dependent manner. LJP2 (30%-60% ethanol precipitated polysaccharides) possesses the strongest alpha-glucosidase inhibitory activity and moderate scavenging activity against hydroxyl radicals (66.09% +/- 2.19%, 3.0 mg mL(-1)). Also, the inhibitory activity against alpha-glucosidase (59.08% +/- 3.79%, 5.0 mg mL(-1)) is close to that of acarbose (63.99% +/- 3.27%, 5.0 mg mL(-1)). LJP1 (30% ethanol precipitated polysaccharides) exhibits the strongest scavenging activity against hydroxyl radicals (99.80% +/- 0.00%, 3.0 mg mL(-1)) and moderate alpha-glucosidase inhibitory activity (47.76% +/- 1.92%, 5.0 mg mL(-1)). LJP shows the most remarkable DPPH scavenging activity (66.20% +/- 0.11%, 5.0 mg mL(-1)) but weakest alpha-glucosidase inhibitory activity (37.77% +/- 1.30%, 5.0 mg mL(-1)). However, all these LJPs exert weak inhibitory effects against alpha-amylase. These results show that UET is an effective method for extracting bioactive polysaccharides from seaweed materials. LJP1 and LJP2 can be developed as a potential ingredient in hypoglycemic agents or functional food for the management of diabetes. This study provides scientific evidence and advances in the preparation technology and a hypoglycemic activities evaluation method for seaweed polysaccharides, especially glycosidase inhibition in combination with an antioxidative activity evaluation method.