Abstract: In this study, the carboxymethylation of polysaccharides (LYP) extracted from longan (Dimocarpus longan) pulp was investigated using monochloroacetic acid (MCA) with polysaccharides-to-MCA ratio, reaction time and reaction temperature as independent variables. The degree of substitution (DS) was taken as response value to optimize the important reaction conditions by response surface methodology. Meanwhile, the antioxidant activity and immunoregulatory of carboxymethlated polysaccharides (CM-LYP) in vivo were measured. The results showed that the optimized conditions that provided the maximum DS of 1.053 were determined as follows: reaction time, 3.2 h; MCA concentration, 1.2 mol/L; and reaction temperature, 73 ℃. The antioxidant activity of CM-LYP was concentration dependent in a certain range of concentration. The percentage inhibition of hydroxyl and superoxide anion radicals, lipid peroxidation, red blood cell hemolysis induced by H2O2 were (42.35 ± 5.67)%, (51.91 ± 5.34)%, (67.91 ± 5.72)%, and (47.23 ± 3.5)% by LYP, and (84.39 ± 4.47)%, (87.91 ± 7.32)%, and (79.85 ± 2.92)%, and (54.66 ± 2.83) % by CM-LYP, respectively, implying that the antioxidant activity of LYP was improved as compared to the native polysaccharides. The immunoregulatory activity in vivo showed that CM-LYP significantly improved spleen index in immunosuppressed, increased serum hemolysin level and lysozyme activity in serum and spleen, and maintained Th1/Th2 balance, and the effect was better than that of LYP.

Key words: longan pulp polysaccharides, carboxymethylation, response surface methodology, immune activity, antioxidant activity