Sucrose octasulfate selectively accelerates thrombin inactivation by heparin cofactor II

Abstract

Inactivation of thrombin (T) by the serpins heparin cofactor II (HCII) and antithrombin (AT) is accelerated by a heparin template between the serpin and thrombin exosite II. Unlike AT, HCII also uses an allosteric interaction of its NH2-terminal segment with exosite I. Sucrose octasulfate (SOS) accelerated thrombin inactivation by HCII but not AT by 2000-fold. SOS bound to two sites on thrombin, with dissociation constants (KD) of 10 ± 4 μM and 400 ± 300 μM that were not kinetically resolvable, as evidenced by single hyperbolic SOS concentration dependences of the inactivation rate (kobs). SOS bound HCII with KD 1.45 ± 0.30 mM, and this binding was tightened in the T·SOS·HCII complex, characterized by Kcomplex of ∼0.20 μM. Inactivation data were incompatible with a model solely depending on HCII·SOS but fit an equilibrium linkage model employing T·SOS binding in the pathway to higher order complex formation. Hirudin-(54-65)(SO-3) caused a hyperbolic decrease of the inactivation rates, suggesting partial competitive binding of hirudin-(54-65)(SO-3) and HCII to exosite I. Meizothrombin( des-fragment 1), binding SOS with KD = 1600 ± 300 μM, and thrombin were inactivated at comparable rates, and an exosite II aptamer had no effect on the inactivation, suggesting limited exosite II involvement. SOS accelerated inactivation of meizothrombin 1000-fold, reflecting the contribution of direct exosite I interaction with HCII. Thrombin generation in plasma was suppressed by SOS, both in HCIIdependent and -independent processes. The ex vivo HCII-dependent process may utilize the proposed model and suggests a potential for oversulfated disaccharides in controlling HCII-regulated thrombin generation. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.