Isolation of a lectin binding rhamnogalacturonan-I containing pectic polysaccharide from pumpkin

A rhamnogalacturonan-I (RG-I) containing pectic polysaccharide (PPc) was isolated from pumpkin following a low-temperature alkali treatment and a combination of gradual alcohol precipitation and ion-exchange. Monosaccharide compositional analysis of PPc revealed the presence of rhamnose, galacturonic acid, galactose, and arabinose in a molar ratio of 7.4: 25: 28: 2.6. Structural and linkage analysis by 1D NMR (¹H NMR and ¹³C NMR), and 2D NMR (COSY, TOCSY, HSQC, and elevated temperature HMBC) suggested that PPc was a RG-I-like pectic polysaccharide, branched at the C-4 of some of the (about 29% of) rhamnosyl units, with relatively long β-1,4-D-galactan side chains to which were attached, through the C-3 of β-D-Gal, terminal non reducing α-Araf units. The results of surface plasmon resonance (SPR) show that PPc binds to two types of lectin, Ricinus communis agglutinin 120 (RCA₁₂₀) and Galectin-3 (Gal-3). These binding studies show quick association and slow dissociation with a moderate binding affinity between PPc and Gal-3 of 1.26 μM. The interaction between PPc and Gal-3 suggest the potential use of pumpkin pectic polysaccharide as a Gal-3 inhibitor in functional food or drug development applications.