Antioxidant and immunostimulating activities in vitro of sulfated polysaccharides isolated from Gracilaria rubra

•Three polysaccharides (GRPS) were obtained from Gracilaria rubra by hot water.•GRPS-1-1, GRPS-2-1 and GRPS-3-2 were all sulfated polysaccharides.•GRPS-2-1 and GRPS-3-2 had little toxicity to L-02 cells.•GRPS-3-2 exhibited excellent antioxidant and immunostimulating activities in vitro. In this work, crude polysaccharides from Gracilaria rubra (GRPS) and its three purified fractions, GRPS-1-1, GRPS-2-1 and GRPS-3-2, were obtained through hot water extraction and chromatographic purification with DEAE-52 cellulose column and Sephadex G-50 column. The three purified fractions, being homogeneous heteropolysaccharides with average molecular weights of 1310, 691, 923kD and sulfuric radical contents of 5.96, 8.46 and 12.03%, respectively, were all composed of galactose and fucose. GRPS-2-1 and GRPS-3-2 had little toxicity to L-02 cells. Crude GRPS and its purified fractions exhibited strong scavenging activities against ABTS and superoxide radicals and lipid peroxidation inhibition. At a cellular level, GRPS-3-2 showed the strongest protective effect on H2O2-induced oxidative injury in PC12 cells. Moreover, GRPS-3-2 exhibited a much stronger immunostimulating activity on RAW264.7 cells than GRPS-1-1 and GRPS-2-1. The results suggested that GRPS, especially GRPS-3-2, could be developed as a promising functional food supplement.