Chemical Structure of a Branched α-d-Glucan from the Eggs of Sea Urchin Tripneustes gratilla

A water-soluble high-molecular neutral polysaccharide (<b>NP</b>) was isolated from the eggs of the sea urchin <i>Tripneustes gratilla</i>. The formation of glucose only upon the treatment of <b>NP</b> by amyloglucosidase and the value of its optical rotation [α]_D +233.5 (c 0.2, water) confirmed its belonging to the family of α-d-glucans. According to the results of NMR spectroscopy and methylation analysis, the chains of <b>NP</b> are built up of non-reducing terminal, 4-linked and 4,6-disubstituted glucose residues at a ratio of 1:8:1. A branched structure with an average linear chain length of about five glucose residues was calculated from the spectrum of iodine complex. Contrary to the previously published structure of branched α-d-glucan from the sea urchin <i>Strongylocentrotus nudus</i> bearing single glucose units as branches, the polysaccharide <b>NP</b> contains oligosaccharide branches at position 6, which was confirmed by NMR data. Hence, <b>NP</b> has a glycogen-like structure with a rather high degree of branching, which markedly exceeds that of usual mammalian or fungal glycogens.