Structural Features and Anti-Complement Activity of an Acidic Polysaccharide from Forsythia suspensa

Abstract

Background: The complement system is an important host defence mechanism against the invasion of foreign materials. However, excessive or uncontrolled activation of the complement system might lead to severe complement-mediated disorders which are harmful to human bodies. There are still no desirable therapeutic compounds available on the market for complement inhibition. In recent years, researches have been focused on natural polysaccharides to modulate of the immune system because of their immunomodulatory activities and safety. In this study, Fs-8-ba2, a homogeneous acidic polysaccharide (MW ca. 7.2 kDa) was isolated from Forsythia suspensa and its anti-complementary effect was investigated. Method: The major primary structural features of Fs-8-ba2 were elucidated using HPGPC, IR, absolute configuration, component analysis and methylation analysis, periodate oxidation, carboxyl reduction partial acid hydrolysis, and NMR spectroscopy, etc. To identify the anti-complementary activity of Fs-8-ba2, the hemolytic assays through the classical pathway (CP) and the alternative pathway (AP) of complement system in vitro were taken. Results: The backbone of Fs-8-ba2 was composed of 7 homogalacturonan (HG) and 2 rhamnogalacturonan (RG-I) moieties with the side chains attached at O-4 of 1,2,4-linked α-L-Rha in the RG-I moieties. The anticomplement assay showed that the native acidic polysaccharide (Fs-8-ba2) possessed stronger inhibitory effect on the complement activation than the carboxyl-reduced polysaccharide (Fs-8-ba2re) through the classical (IC50: 0.311 ± 0.020 mg/mL vs. 3.292 ± 0.032 mg/mL) and alternative pathways (IC50: 0.218 ± 0.015 mg/mL vs. no activity). It indicated that the anti-complement effect of pectic polysaccharide was related to GalA content (carboxyl group). Preliminary mechanism studies by using complement-depleted sera indicated that Fs-8-ba2 selectively interacted with C1q, C1r, C1s, C2, C3 and C9, but not C4 and C5. Conclusion: These results suggested that Fs-8-ba2, an acidic polysaccharide, could be of potential benefits in the treatment of the complement-mediated diseases.