Abstract: A novel water-soluble polysaccharide (LKY-I) was isolated from Lobaria koorkauae Yoshim sequentially through ultrasonic-assisted water extraction, ethanol precipitation, DEAE-52 ion-exchange chromatography and gel permeation chromatography. Its molecular weight distribution and purity were determined by gel permeation chromatographic (GPC) separation with dextran-based Sephadex G-100. The chemical structure of LKY-I was studied by Fourier transform-infrared (FT-IR) spectroscopy, gas chromatography (GC), periodate oxidation, Smith degradation, and nuclear magnetic resonance (NMR). The antioxidant activity was evaluated by measuring reducing power and radical scavenging capacities against 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) free radicals (ABTS+·) in vitro. The growth inhibitory effect of LKY-I on tumor cell HepG-2, HCC827 and MCF-7 was evaluated using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay. The results revealed that the weight-average molecular mass (Mw) of LKY-I was 61 598 D, and its polysaccharide content was 88.91%. GC results showed that LKY-I contained five monosaccharides, including rhamnose (Rha), xylose (Xyl), mannose (Man), glucose (Glu), and galactose (Gal), with a molar ratio of 1.1:2.0:46.5:28.5:20.5. FT-IR analysis showed that LKY-I had characteristic absorption peaks and its monosaccharide residues existed in the form of pyran ring. Periodate oxidation, Smith degradation and NMR revealed that there were 24.68% (1→6) and (1→) glycosides bonds in LKY-I; meanwhile, LKY-I had a D-glucopyranose ring containing mannose residue, including α and β configurations. The scavenging activity of LKY-I against DPPH free radicals and ABTS+· increased with increasing its concentration from 250 to 10 000 μg/mL, with the highest inhibition percentages of 44.12% and 24.74%, respectively at a concentration of 10 000 μg/mL. At 10 000 μg/mL, the FRAP value was 0.071 17 mmol/L, which was equivalent to 100 μg/mL of VC (0.072 31 mmol/L). The inhibitory effect of LKY-I on the proliferation of cancer cells was gradually increased with increasing its concentration from 125 to 2 000 μg/mL. When the concentration was 2 000 μg/mL, the percentage inhibition of tumor cell HepG-2, HCC827 and MCF-7 proliferation was nearly 50%, indicating that LKY-I has potent antitumor activity.

Key words: Lobaria koorkauae Yoshim, polysaccharides, structure, antioxidant, antitumor