Chemical structure and biological activity of a highly branched (1 → 3,1 → 6)-β-D-glucan from Isochrysis galbana

A highly branched (1 → 3,1 → 6)-β-D-glucan was isolated from the microalga Isochrysis galbana Parke (Isochrysidales, Haptophyta). The polysaccharide structure was analyzed by methylation and Smith degradation, as well as by ESI and MALDI TOF mass spectrometry and NMR spectroscopy. The glucan was shown to contain a (1 → 6)-linked backbone, where every residue is substituted at position 3 by Glc, which in turn may be substituted at C-6 by a single Glc or by rather short (up to tetrasaccharide) oligosaccharide chains. All the 3-linked Glc residues are present in these side chains. In the biological activity experiments it was demonstrated that the polysaccharide directly inhibits the proliferation of U937 human leukemic monocyte lymphoma cells and therefore has potential anti-tumor activity.