Binding of low-affinity and high-affinity heparin to antithrombin. Ultraviolet difference spectroscopy and circular dichroism studies

Evidence by chemical modification for the involvement of one or more tryptophanyl residues of bovine antithrombin in the binding of high-affinity heparin

Tryptophanyl residues of bovine antithrombin were modified with N-bromosuccinimide at near-neutral pH. The reaction was found to be specific for tryptophan at low levels of modification, i.e. when only up to 1--1.3 mol tryptophan/mol protein were oxidized. Further modification led to extensive side reactions. Modification of an average of about one tryptophanyl residue per protein molecule did not affect antithrombin activity measured in the absence of heparin, but decreased the activity assayed in the presence of heparin to about half the value given by unmodified antithrombin. Addition of an excess of high-affinity heparin to a similarly modified antithrombin sample resulted in much smaller circular dichroism, ultraviolet absorption and fluorescence changes than those observed with the intact protein. Modification experiments in the presence of excess high-affinity heparin gave a definitely lower extent of modification than when heparin was excluded. These studies thus reinforce the conclusion from previous spectroscopic analyses that one or more tryptophanyl residues of antithrombin are involved in the binding of high-affinity heparin, presumably by being located at or close to the heparin binding site.

Routes of thrombin action in the production of proteolytically modified, secondary forms of antithrombin-thrombin complex

The reaction between thrombin and antithrombin results in the formation of an inactive, stable, equimolar complex between the two proteins. However, under most reaction conditions several secondary complex forms, which have lower apparent molecular weights in dodecyl sulfate/polyacrylamide gel electrophoresis, appear concomitantly with or immediately following the production of the primary form of the complex. Purification of nascent, intact complex and treatment of this complex form with thrombin demonstrated that these subsidiary forms of antithrombin-thrombin complex may arise by proteolysis of the nascent complex by excess thrombin. Dissociation of such proteolytically modified complex preparations by hydroxylamine, and examination of the dissociation products by dodecyl sulfate/polyacrylamide gel electrophoresis suggested that degradation occurs primarily in the thrombin part of the complex, and only after prolonged proteolysis in its antithrombin moiety also. Incubation of antithrombin with several autolytically modified thrombin preparations showed that formation of subsidiary complex forms can also occur by an alternative route, i.e. between premodified thrombin forms and the inhibitor. In contrast, complex formation between thrombin and active forms of antithrombin, which have been modified by thrombin before complex formation, is unlikely, since no such active forms of antithrombin could be demonstrated.