Structural characterization and immunological activity of pectin polysaccharide from kiwano (Cucumis metuliferus) peels

[Display omitted] •Two pectin polysaccharides, CMPP-1 and CMPP-2, were obtained from kiwano peels.•Chemical structure of CMPP-1and CMPP-2 were elucidated by FTIR, GPC, HPAEC and NMR.•Both of CMPP-1 and CMPP-2 possessed immune-enhancing activities on RAW264.7 cells.•The higher activity of CMPP-1 was related to its high branching structure degree. Two novel polysaccharides, namely CMPP-1 and CMPP-2, from kiwano (Cucumis metuliferus) peels were isolated through hot-water extraction, followed by ethanol precipitation and column chromatography. The results showed that CMPP-1 and CMPP-2 were hetero-galacturonans with different molecular weights of 7.35 kDa and 6.90 kDa, respectively. Both of CMPP-1 and CMPP-2 were mainly composed of glucuronic acid (45.93 % and 51.75 %, respectively), and other monosaccharides including rhamnose, arabinose, galactose, glucose, xylose, fucose, mannose, galacturonic acid, and mannuronic acid. The results of structural characterization from FT-IR and NMR confirmed that CMPP-1 and CMPP-2 were pectin with highly branched structure. Furthermore, both CMPP-1 and CMPP-2 possessed immune-enhancing activity and could enhance the secretion of nitric oxide and cytokines (TNF-α, IL-6) in a dose-dependent manner. Especially, CMPP-1 had higher immune activity than CMPP-2 as the minimum effective concentration were 0.78 μg/mL and 6.25 μg/mL, respectively. These findings provide a scientific basis for further utilization of polysaccharide from kiwano peels.