A new low-molecular-weight 'heparin-like' component was obtained from an exopolysaccharide produced by a mesophilic strain found in deep-sea hydrothermal vents. Data concerning the structure of the native high-molecular-weight exopolysaccharide (10(6) g/mol, 10% sulfate content) are reported for the first time. Two depolymerization processes were used to obtain low-molecular-weight (24-35 x 10(3) g/mol) oversulfated fractions (sulfate content 20 or 40%), Nuclear magnetic resonance studies indicated that after sulfation (40%), the low-molecular-weight fraction obtained by free radical depolymerization was less sulfated in the 6-O-position than the fraction depolymerized by acid hydrolysis. The free radical depolymerized product also had sulfated residues in the 4-O-position and disulfated ones in the 2,3-O-positions. Moreover, the compounds generated by the free radical process were more homogeneous with respect to molecular mass. Also for the first time, the anticoagulant activity of the low-molecular-weight exopolysaccharide fractions is reported. When the fractions obtained after sulfation and depolymerization were compared with heparins, anticoagulant activity was detected in oversulfated fractions, but not in native exopolysaccharide. The free radical depolymerized fraction inhibited thrombin generation in both contact-activated and thromboplastin-activated plasma, showing a prolonged lag phase only in the contact-activated assay. Affinity co-electrophoresis studies suggested that a single population of polysaccharide chains binds to antithrombin and that only a subpopulation strongly interacts with heparin cofactor II. (C) 2001 Elsevier Science B.V. All rights reserved.