Effects of Ca2+ ions on the interactions between antithrombin III and factor Xa mediated by variously-sulfated semi-synthetic low-molecular-weight heparins

A homogeneous set of low-molecular weight heparins, chemically modified to yield different degrees of sulfation (SD), were investigated in their ability of interfering with the Antithrombin III (AT III)-Factor Xa (FXa) interaction process in the presence/absence of physiological concentrations of calcium ions. The heparin-AT III dissociation constants were not appreciably affected by the presence of the metal ion, whereas the catalytic process was strongly dependent upon Ca2+. Our data suggest that AT III binding to heparin represents the main factor driving the FXa inhibition process. In addition, the presence of the metal ion is likely to mask favorable AT III- heparin ionic contacts occurring with the highly sulfated material. These results help in assessing proper structure-activity relationships for glycosaminoglycans, a multi-target family of biologically active compounds.