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Three novel acidic polysaccharide fractions(OFPP-1,OFPP-2,OFPP-3)with different molecular weights(803.7,555.1 and 414.5 kDa)were isolated from the peeled Opuntia dillenii Haw.fruits by alkali-extraction,graded alcohol precipitation and column chromatography.Structural analysis indicated that OFPPs were pectic polysaccharides consisting of rhamnose,arabinose and galactose residues.The backbone of OFPP-1 consisted of a repeating unit →6-α-D-GalpA-(1→2)-α-L-Rhap-(1→ with T-α-D-GalpA-(1 →6)-α-D)-GalpA-(1→4)-α-D-Glcp-(1→,T-β-D-Xylp-(1→6)-α-D-GalpA-(1→4)-α-D-Glcp-(1→ or T-α-D-GalpA-(1→3)-α-L-Araf-(1→as the side chains.The backbone of OFPP-2 consisted of a disaccharide repeating unit →2)-α-L-Rhap-(1→4)-β-D)-GalpA-(1 → with T-β-L-Araf-(1→ as the branches substituted at the O-4 position of →2,4)-α-L-Rhap-(1→.Whereas the backbone of OFPP-3 was →2,4)-α-L-Rhap-(1→2)-α-L-Rhap-(1--→3)-β-L-Araf-(l→or →2,4)-α-L-Rhap-(1 →2)-α-L-Rhap-(1→4)-β-D-GalpA-(1→,which was branched at the O-4 position of→2,4)-α-L-Rhap-(1 →.Moreover,these three polysaccharide fractions could protect Huh-7 cells against H2O2-induced oxidative stress to different extents by decreasing the MDA content and increasing the SOD,CAT,GSH-Px activities and the GSH level in the Huh-7 cells.These results suggest that OFPPs have the potential to be used as natural antioxidants.