Structural characterization of water-soluble polysaccharides from Opuntia monacantha cladodes in relation to their anti-glycated activities

An aqueous extract of polysaccharides from Opuntia monacantha cladodes (POMC) was preliminarily purified by 5 kDa molecular weight cut-off ultrafiltration membrane to remove impurities with low molecular weight. Then the retentate was fractionated by ethanol solution and chromatographed on a DEAE Sepharose Fast Flow anion-exchange column to yield a major fraction (POMC IV) which was eluted by 0.5 M NaCl. POMC IV was subjected to further purification on a Sephadex G-50 gel filtration column. Two major fractions, POMC V and VI, were collected. By analyses using gel permeation chromatography (GPC), high-performance liquid chromatography (HPLC) and gas chromatography (GC), POMC V, which had a molecular weight of 28.7 kDa, was comprised mainly of rhamnose, arabinose and glucose in the molar ratio of 9.15:1.00:6.84, with 3.07% (w/w) of glucuronic acid, while POMC VI, which had a molecular weight of 10.8 kDa, was comprised mainly of rhamnose, mannose and glucose in the molar ratio of 8.72:1.00:6.19, with 4.68% (w/w) of glucuronic acid. Six distinct-absorbance peaks, at 1742, 1633 and 1417 cm−1 in the infrared (IR) spectra of POMC V, and at 1729, 1596 and 1407 cm−1 in the IR spectra of POMC VI, resulted from the presence of uronic acids. The peaks at 1043 and 890 cm−1 were characteristic of rhamonse and β-d-glucose, respectively. From the profiles of 13C and 1H nuclear magnetic-resonance (NMR) spectra, the main (1 → 2)-α-l-rhamnopyranose units were obviously characterized by six strong signals at 99.24 (C-1), 77.52 (C-2), 70.19 (C-3), 71.33 (C-4), 69.81 (C-5) and 17.45 ppm (C-6). The signal at 175.92 ppm was due to C-6 of β-d-glucuronic acid units. The 1H spectrum signal at 1.20 ppm was assigned to the CH3 of α-l-rhamnopyranose units. The evaluation of anti-glycation activity suggested that POMC had good potential for inhibiting the formation of advanced glycation endproducts. Time- and dose-dependent effects were also observed for all POMC samples.