Structural and immunological properties of arabinogalactan polysaccharides from pollen of timothy grass (Phleum pratense L.) Original Research Article

Extracts from pollen of timothy grass (Phleum pratense L.) contain up to 20% arabinogalactan proteins (AGPs). Separation of the AGP polysaccharide moieties by tryptic digestion, size exclusion chromatography (GPC), and reverse phase HPLC yielded arabinogalactan fractions AG-1 and AG-2 with molecular weights of ∼15,000 and ∼60,000 Da, respectively. The backbones of both polysaccharides are composed of (1→6)-linked β-d-galactopyranosides with β-d-GlcUAp or 4-O-Me-β-d-GlcUAp at their terminal ends as revealed by chemical analysis, FT-IR, MALDI-MS, and NMR spectroscopy. AG-1 contains a small number of β-l-Araf side chains while AG-2 possesses a variety of (1→3)-linked units, which consist of β-l-Araf-(1→, α-l-Araf-(1→3)-β-l-Araf-(1→, and α-l-Araf-(1→5)-β-l-Araf-(1→ as well as a small number of longer arabinogalactan side chains. In contrast to crude pollen extracts, the immunological properties of the arabinogalactan mixture reveal an IgG4 reactivity instead of IgE reactivity. Structural properties of timothy pollen arabinogalactan might thus influence the immune response.