Relationships between chemical characteristics and anticomplementary activity of fucans.

We have shown previously that a low-molecular-weight fucan extracted from the brown seaweed Ascophylum nodosum strongly inhibited human complement activation in vitro and its mechanism of action was largely elucidated. We further investigated the influence of molecular weight and chemical composition of fucan on its anticomplementary activity. The capacity of 12 fragments of fucan (ranging from a molecular weight of 4100 to 214,000) to prevent complement-mediated haemolysis of sheep erythrocytes (classical pathway) and of rabbit erythrocytes (alternative pathway) increased with increasing molecular weight, and reached a plateau for 40,000 and 13,500, respectively. The most potent fucan fractions were 40-fold more active than heparin in inhibiting the classical pathway. They were, however, as active as heparin in inhibiting the alternative pathway. In addition, we have developed a haemolytic test based on the CH50 protocol, which allows discrimination between activators and inhibitors of complement proteins. Although the mannose content within the different fucan fragments did not vary, the galactose and glucuronic acid contents increased with increasing activity, suggesting that these residues should be essential for full anticomplementary activity. Meanwhile, sulphate groups appeared to be necessary, but were clearly not a sufficient requirement for anticomplementary activity of fucans. Taken together, these data illustrate the prospects for the use of fucans as potential anti-inflammatory agents.