Novel Pectic Polysaccharides Isolated from Immature Honey Pomelo Fruit with High Immunomodulatory Activity

A novel pectic polysaccharide (HPP-1) with high immunomodulatory activity was extracted and isolated from the immature honey pomelo fruit (Citrus grandis). Characterization of its chemical structure indicated that HPP-1 had a molecular weight of 59,024 D. In addition, HPP-1 was primarily composed of rhamnose, arabinose, fucose, mannose, and galactose at a molar ratio of 1.00:11.12:2.26:0.56:6.40. Fourier-transform infrared spectroscopy, periodic acid oxidation, and Smith degradation results showed that HPP-1 had alpha- and beta-glycosidic linkages and 1 -> 2, 1 -> 4, 1 -> 6, and 1 -> 3 glycosidic bonds. C-13 NMR and H-1 NMR analyses revealed that the main glycogroups included 1,4-D-GalA, 1,6-beta-D-Gal, 1,6-beta-D-Man, 1,3-alpha-L-Ara, and 1,2-alpha-L-Rha. Immunomodulatory bioactivity analysis using a macrophage RAW264.7 model in vitro revealed that NO, TNF-alpha, and IL-6 secretions were all considerably increased by HPP-1. Moreover, RT-PCR results showed that HPP-1-induced iNOS, TNF-alpha, and IL-6 expression was significantly increased in macrophages. HPP-1-mediated activation in macrophages was due to the stimulation of the NF-kappa B and MAPK signaling pathways based on western blot analyses. HPP-1 extracted from immature honey pomelo fruit has potential applications as an immunomodulatory supplement.