Ultrafiltration isolation, structures and anti-tumor potentials of two arabinose- and galactose-rich pectins from leaves of Aralia elata

Abstract

Two novel arabinose- and galactose-rich pectic polysaccharides, AELP-B5 (M_w, 4.25 × 10⁴ g/mol) and B6 (M_w, 1.56 × 10⁴ g/mol), were rapidly obtained from the leaves of Aralia elata (Miq.) Seem. with anion resin and sequenced ultrafiltration membrane columns. The structural backbone and branched chains of AELP-B5 and B6 were preliminarily elucidated by mild acid hydrolysis with HILIC-ESI^--MS/MS. The planar structures and spatial configurations were further identified using UPLC-QDa and GC-MS for compositions, Smith degradation and methylation analysis, FT-IR, NMR (¹H/¹³C, DEPT, HSQC, HMBC, COSY, NOESY and TOCSY) and SEC-MALLS-RID. (1) AELP-B5 possessed →⁴GalA¹→ as smooth regions (HG) and a repeating disaccharide moiety of →⁴GalA¹→²Rha¹→ as hairy regions (RG-I) with a 1:5 molar ratio, whereas AELP-B6 had a distinguishing 1:1 molar ratio between the HG and RG-I; (2) complex side chains were constituted of T-α-Araf, 1,3-α-Araf, 1,5-α-Araf, T-β-Galp, 1,3-β-Galp, 1,4-β-Galp, 1,6-β-Galp, 1,3,4-β-Galp and 1,3,4,6-β-Galp connected at C-4 of the rhamnosyl units in RG-I of AELP-B5 and B6; and (3) both possessed highly branched and compact coil conformations. The CCK-8 assay illustrated that AELP-B6 possessed higher cytotoxicity against HepG2 and HT-29 than that of AELP-B5. Surface plasmon resonance showed the binding affinity of AELP-B6 to galectin-3 (6.488 × 10^-5 M) was about 10 times stronger than that of AELP-B5 (4.588 × 10^-4 M). The above findings provide a molecular structure and bioactivity basis for future potential applications of AELP in the food and medical industries.

Keywords: Aralia elata; Galectin-3 binding affinity; Pectic polysaccharides; Structural elucidation; Ultrafiltration isolations.