Summary

Sodium plays an important role in modulating both the amidolytic and proteolytic activities of thrombin. By contrast, while the optimal amidolytic activity of factor Xa requires Na+ , the proteolytic activity of factor Xa in the prothrombinase complex is minimally affected by the monovalent cation. In this study, we analyzed the effect of Na+ on the amidolytic and proteolytic activity of factor IXa in the absence and presence of factor VIIIa. Factor IXa exhibited normal activity towards a fIXa-specific chromogenic substrate and antithrombin in the presence of physiological concentrations of Ca2+ with no obvious require- ment for Na + in either reaction. Further studies revealed that factor IXa binds to its cofactor factorVIIIa with a normal affinity in the absence of Na+ and that the catalytic function in the intrinsic Xase complex is also independent of Na+ in the presence of physiological concentrations of Ca2+.These results suggest that unlike the important role that Na+ plays in modulating the macromolecular substrate specificity of thrombin, the monovalent cation is not required for the physiological function of factor IXa in the intrinsic Xase complex.