Protection of factor Xa from neutralization by the heparin-antithrombin complex.

We have studied the accessibility of Factor Xa to neutralization by the heparin-antithrombin complex within plasma and whole blood. This serine protease was detected by measuring the concentrations of activation fragments (F2/F1+2) cleaved from prothrombin. The levels of F2/F1+2) were quantitated by means of a sensitive and specific radioimmunoassay. Our findings indicate that the binding of Factor Xa to "activated" platelets but not to phospholipid micelles results in the protection of the above enzyme from inactivation by the heparin-antithrombin complex. This sequestration of Factor Xa is not affected by the liberation of platelet release proteins or the molecular heterogeneity of the mucopolysaccharide preparations used. The magnitude of enzyme protection is strongly correlated with the extent of prothrombin activation at the time of heparin addition. On this basis, we suggest that high in vivo rates of thrombin generation may lead to the sequestration of Factor Xa on the platelet surface and hence allow this serine protease to resist the action of heparin until the complex is cleared from the circulation.