Effect of Different Protein Removal Methods on the Structure and Antioxidant Activity of the Cyclocarya paliurus Polysaccharides

Removal of proteins is essential for the purification of Cyclocarya paliurus polysaccharides (CPP), which have strong hypoglycemic and antioxidant activities. In order to explore the most suitable way of removing protein from the polysaccharides, three methods (HCl, TCA and NaCl) were used to treat the crude polysaccharides, and the structure and composition of the polysaccharides obtained by the three deproteinization methods were analyzed by high performance liquid gel chromatography, ion chromatography and Fourier infrared spectroscopy, and their antioxidant activity was determined by measuring the scavenging rate of DPPH radicals, ABTS⁺ radicals and hydroxyl radicals. The results showed that the protein removal rate of the HCl method was 73.48% and the polysaccharide residue rate was 74.86% at pH2. The protein removal rate of the TCA method was 79.88% and the polysaccharide residue rate was 81.78% at a TCA volume fraction of 2%. The protein removal rate of the NaCl method was 47.90% and the polysaccharide residue rate was 82.51% at a NaCl volume fraction of 7%. The molecular weight distribution of the polysaccharides treated by the three methods could be roughly divided into three parts, 500~750 kDa, 100~130 kDa and 30~40 kDa, but their proportions were different. The polysaccharides treated by the TCA method accounted for the largest percentage of 100~130 kDa compared with the three, and the polysaccharides treated by the NaCl method accounted for the largest percentage of 30~40 kDa. All three polysaccharides consisted of eight monosaccharides, but their molar ratios were different. Of the three deproteinization method, the HCl-treated polysaccharide had the highest content of arabinose, galactose and glucose, the TCA-treated polysaccharide had a higher content of glyoxylate, and the NaCl-treated polysaccharide had the highest content of rhamnose, xylose and mannose. The scavenging ability of the three polysaccharides against free radicals at the same concentration was compared, and the antioxidant ability of the polysaccharides treated with TCA method was the strongest, followed by HCl method and the weakest was polysaccharides treated by NaCl method. As shown by the results, different methods of protein removal have different effects on the structure and activity of CPP, among which TCA method is a better way to remove protein from CPP.