Structural features of an immunoactive acidic arabinogalactan from Centella asiatica

A water-soluble acidic arabinogalactan, named HBN with a molecular mass of 5.4×105 Da determined by HPGPC, was obtained from Centella asiatica. HBN contained Ara, Gal, Rha, GalA and Xyl in molar ratios of 1.0:1.9:0.26:0.30:0.15. The acetyl content was estimated to be 2.5%. Using methylation analysis, partial acid hydrolysis, NaIO4 oxidation–Smith degradation, RI, NMR, ESI-MS, HPGPC, pectolyase-treatment methods, the structure of HBN was elucidated. HBN had a core composed of Rha and GalA, with arabinogalactosyl and xylosyl chains were attached to this core. Seventy-six percent Ara residues were located at termini and linked to O-6 of Gal residues. HBN had remarkable immunoenhancing activities on T- and B-lymphocytes in vitro and vivo tests. It increased spleen index and inhibited the level of IgG. With the stimulation of SAC, it enhanced the secretion of IFN-γ, IL-12 and IL-6, and inhibited IL-10. Its derivatives by NaIO4 oxidation–Smith degradation and enzyme-treatment possessed immunological activities in vitro.