Structure, enzymatic transformation and anticancer activity of branched high molecular weight laminaran from brown alga Eisenia bicyclis

•Structure of HMW laminaran EbL from Eisenia bicyclis was established.•EbL was investigated by chemical methods, NMR spectroscopy and mass spectrometry.•Laminaran was depolymerised by endo- and exo-glucanases from marine sources.•EbL and its oligomers inhibited colony formation of melanoma and colon cancer cells. The structure of high molecular weight laminaran from brown alga Eisenia bicyclis was investigated by chemical and enzymatic methods, NMR spectroscopy and mass spectrometry. The laminaran from E. bicyclis was characterized as 1,3;1,6-β-d-glucan with the high content of 1,6-linked glucose residues (ratio of bonds 1,3:1,6=1.5:1), which are both in the branches and in the main chain of the laminaran. The degree of polymerization of fragments, building from 1,3-linked glucose residues with single glucose branches at C-6 or without it, was no more than four glucose residues. The main part of 1,3-linked glucose blocks was builded from disaccharide fragments. 1,6-Linked glucose residues were localized basically on non-reduced ends of molecules. The degree of polymerization of 1,6-linked blocks was not greater than three glucose residues. Laminaran contained laminarioligosaccharides, gentiobiose, gentiotriose and single glucose residues in the branches at the C-6. Laminaran and its products of enzymatic hydrolysis inhibited a colony formation of human melanoma SK-MEL-28 and colon cancer DLD-1 cells. It was shown that decreasing the molecular weight of native laminaran to a determined limit (degree of polymerization 9–23) and increasing the content of 1,6-linked glucose residues increased the anticancer effect. Therefore, they may be perspective antitumor agents.