Preparation and characterization of a novel extracellular polysaccharide with antioxidant activity, from the mangrove-associated fungus Fusarium oxysporum.

Marine fungi are recognized as an abundant source of extracellular polysaccharides with novel structures. Mangrove fungi constitute the second largest ecological group of the marine fungi, and many of them are new or inadequately described species and may produce extracellular polysaccharides with novel functions and structures that could be explored as a source of useful polymers. The mangrove-associated fungus Fusarium oxysporum produces an extracellular polysaccharide, Fw-1, when grown in potato dextrose-agar medium. The homogeneous Fw-1 was isolated from the fermented broth by a combination of ethanol precipitation, ion-exchange, and gel filtration chromatography. Chemical and spectroscopic analyses, including one- and two-dimensional nuclear magnetic resonance spectroscopies showed that Fw-1 consisted of galactose, glucose, and mannose in a molar ratio of 1.33:1.33:1.00, and its molecular weight was about 61.2 kDa. The structure of Fw-1 contains a backbone of (1 → 6)-linked β-D-galactofuranose residues with multiple side chains. The branches consist of terminal α-D-glucopyranose residues, or short chains containing (1 → 2)-linked α-D-glucopyranose, (1 → 2)-linked β-D-mannopyranose, and terminal β-D-mannopyranose residues. The side chains are connected to C-2 of galactofuranose residues of backbone. The antioxidant activity of Fw-1 was evaluated with the scavenging abilities on hydroxyl, superoxide, and 1,1-diphenyl-2-picrylhydrazyl radicals in vitro, and the results indicated that Fw-1 possessed good antioxidant activity, especially the scavenging ability on hydroxyl radicals. The investigation demonstrated that Fw-1 is a novel galactofuranose-containing polysaccharide with different structural characteristics from extracellular polysaccharides from other marine microorganisms and could be a potential source of antioxidant.