Role of ternary complexes, in which heparin binds both antithrombin and proteinase, in the acceleration of the reactions between antithrombin and thrombin or factor Xa.

Oligosaccharides (10-20 monosaccharide units) with high affinity for antithrombin, as well as larger high-affinity heparin fractions (having relative molecular masses between 6,000 and 21,500), all markedly accelerated the inhibition of Factor Xa by antithrombin. Moreover, all high-affinity oligosaccharides and heparins enhanced, to a similar extent, the amount of free proteolytically modified antithrombin cleaved at the reactive bond by Factor Xa. In contrast, a minimum high-affinity heparin size of approximately 18 monosaccharide units was required to significantly accelerate the inactivation of thrombin by antithrombin and to enhance the production of modified antithrombin by this enzyme. All high-affinity fractions studied had similar affinities for antithrombin, as determined by fluorescence titrations. In competition experiments, binary complexes of antithrombin with octadecasaccharide or larger high-affinity heparins, but not with smaller oligosaccharides, displaced inactivated 125I-thrombin from matrix-linked low-affinity heparin. Moreover, similar binary complexes with 3H-labeled octadecasaccharide or larger chains, but not with smaller oligosaccharides, were capable of binding to matrix-linked inactivated thrombin. These results indicate that simultaneous binding of antithrombin and thrombin to high-affinity heparin is a prerequisite to the acceleration of the antithrombin-thrombin reaction and that the minimum heparin sequence capable of binding both proteins comprises approximately 18 monosaccharide units. Similar complex formation apparently is not required for the acceleration of the antithrombin-Factor Xa reaction.