Structural elucidation and immune-enhancing activity of peculiar polysaccharides fractioned from marine clam Meretrix meretrix (Linnaeus).

The marine clam Meretrix meretrix was subjected to extraction with a combined procedure of water decoction, ethanol precipitation, and deproteinization to yield a water-soluble polysaccharide coded as MMP. The product was fractionated with DEAE-52 cellulose and resulted in two soluble fractions designated as MMP-2 and -3. The structures of the fractions were determined by chemical and physicochemical analysis, and their immunomodulation activities and cytokine production were assayed by lymphopoiesis. Results showed that MMP-2 and -3 were glucomannan and glucogalactan homologs, respectively. They possess average molecular masses of 272 and 257 kDa, respectively, and were elucidated as branched copolymers, elaborated as α(1→4)-glucan branching with mannose or galactose via β(1→6)-linkage. Bioassay indicated that MMP-2 and -3 held moderate potentials in restoring immune function because they augment splenocyte proliferation and enhance the cytokine secretion of lymphocytes dose-dependently. We speculated that their bioactivities are closely correlated with their peculiar structures.