Structural Characterization of Longan Polysaccharide and Its Activating Effect on Macrophages

To explore the structure of longan polysaccharide and its effect on macrophages, a homogenous polysaccharide (named LP) was isolated and purified from longan pulp by ethanol fractional precipitation, and the structure of LP was characterized by gel permeation chromatography with laser light scattering (GPC-LLS), infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) spectroscopy. Meanwhile, its effect on the phagocytic capacity of macrophages and the secretion of tumor necrosis factor-α (TNF-α), interleukin (IL)-1β and IL-6. The results showed that LP was a heteropolysaccharide with a weight average molecular mass of 8.42 × 10⁵ Da composed of glucose, galactose, mannose, mannuronic acid, arabinose and rhamnose at a molar ratio of 98.70% : 0.57% : 0.35% : 0.09% : 0.20% : 0.12%. NMR spectroscopic analysis showed the backbone chain structure of LP to be α-Glcp-(-1. LP at 50, 100, 200 μg/mL promoted the phagocytic capacity of macrophages and increased the secretion of TNF-α, IL-1β and IL-6. The phagocytic capacity of LP-induced macrophages increased by 95.61% compared with the untreated control group. TNF-α, IL-1β and IL-6 secretion in macrophages challenged by LP at 50, 100 and 200 μg/mL were 2.38, 10.51 and 10.67 times higher than those in the control group, respectively. These results showed that LP had an activating effect on macrophages, which can provide a scientific basis for the deep processing of longan fruit.