Properties of arabinogalactan-proteins in European pear (Pyrus communis L.) fruits

Arabinogalactans (AGs) and arabinogalactan-proteins (AGPs) were partially purified from an extract of fruits of the European pear (Pyrus communis L.) by DEAE-cellulose ion-exchange and Sepharose 6B gel-filtration chromatography. Among 7 AG(P)-containing fractions, a neutral AGP (SE-1) was confirmed to be highly purified (Mr 67,000) and rich in L-Ara and Gal; this fraction included a small amount (2.6%, w/w) of protein and showed the highest reactivity forming precipitate with β-Glc Yariv reagent among the 7 fractions, the intensity of which was comparable to that of gum arabic, a standard AGP. Another accompanying minor low-Mr neutral AGP (SE-2; Mr approx. 7200) still contained other polysaccharide (starch fragments) and did not show Yariv reactivity. The carbohydrate moieties of SE-1 consisted of consecutive (1 → 3)-linked β-galactosyl backbone chains substituted with side chains of (1 → 6)-linked β-galactosyl residues at O-6, to which mainly single α-l-arabinofuranosyl residues were attached through O-3. This structural feature was also observed for SE-2. Successive digestion of SE-1 with α-l-arabinofuranosidase and exo-β-(1 → 3)-galactanase with the aid of endo-β-(1 → 3)-galactanase released most (more than 98%, w/w) of the carbohydrate moieties as low-Mr fragments. These consisted of free L-Ara and Gal, and a series of β-(1 → 6)-galactooligosaccharides with degree of polymerization (dp) up to at least 17, indicative of attachment of (1 → 6)-linked β-galactosyl side chains of varying length along the (1 → 3)-linked β-galactosyl backbone chains.