Sulfated polysaccharides of brown algae (fucoidans) attract great attention due to their high and strongly diversified biological activity. This review summarizes recent data on the structural variability of these polysaccharides and reports their anti- and proangiogenic properties. Recent publications have revealed that fucoidans isolated from different algal species may differ considerably in the structures of their backbones and branches, in both monosaccharide composition and sulfate content. It was found that the degree of sulfation significantly influences the biological properties of fucoidans. Additionally, fucoidan action in angiogenesis is highly dependent on molecular weight: antiangiogenic activity is connected with the high-molecular weight of polysaccharide molecules, whereas the low-molecular-weight fractions may act as proangiogenic agents. The influence of other fine structural details of fucoidans on angiogenesis remains to be established.

Three structurally different fucoidans from the brown seaweeds Saccharina latissima (SL), Fucus vesiculosus (FV), and Cladosiphon okamuranus (CO), two chemically modified fucoidans with a higher degree of sulfation (SL-S, CO-S), and a synthetic totally sulfated octasaccharide (OS), related to fucoidans, were assessed on anticoagulant and antithrombotic activities in different in vitro experiments. The effects were shown to depend on the structural features of the compounds tested. Native fucoidan SL with a degree of sulfation (DS) of 1.3 was found to be the most active sample, fucoidan FV (DS 0.9) demonstrated moderate activity, while the polysaccharide CO (DS 0.4) was inactive in all performed experiments, even at high concentrations. Additional introduction of sulfate groups into fucoidan SL slightly decreased the anticoagulant effect of SL-S, while sulfation of CO, giving rise to the preparation CO-S, increased the activity dramatically. The high level of anticoagulant activity of polysaccharides SL, SL-S, and CO-S was explained by their ability to form ternary complexes with ATIII-Xa and ATIII-IIa, as well as to bind directly to thrombin. Synthetic per-O-sulfated octasaccharide OS showed moderate anticoagulant effect, determined mainly by the interaction of OS with the factor Xa in the presence of ATIII. Comparable tendencies were observed in the antithrombotic properties of the compounds tested.

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The immunotropic activity of structurally different fucoidans and their derivatives towards isolated immune blood cells, effectors of innate immune system, was studied. The most potent effect was observed for high molecular weight fucoidan CF from the alga Chordaria flagelliformis, whose backbone is built of (1→3)-linked units of α-L-fucopyranose, and branches included residues of α-D-glucuronic acid and α-L-fucofuranose. This compound at the concentration of 0.05 mg/ml potentiated phagocytosis of Saccharomyces cerevisiae and Lactobacillus acidophilus by neutrophils, increasing relative quantity of phagocytes as well as their effectiveness. Along with this, 14% increase in the concentration of membrane-bound integrin CD11c molecules was observed. The systemic effect of CF at the dose of 0.01 mg/mouse i.p. led to potentiation of cytotoxic activity of spleen mononuclear leucocytes towards melanoma cells of line B16 by 1.9-fold and towards chronic myelogenous leukemia cells of line K-562 by 1.7-fold. These results indicate that fucoidan CF can stimulate anti-infective and antitumor activity of effectors of the innate immune system via CD11c integrins.