Binding of heparin or dermatan sulfate to thrombin is essential for the sulfated polysaccharide-accelerated inhibition of thrombin by heparin cofactor II

Blood compatibility of heparin-inspired, lactose containing, polyureas depends on the chemistry of the polymer backbone

Sulfated glycopolymers were synthesized from diisocyanates and lactose containing diamines. Blood compatibility assays indicated highly sulfated glycopolymers with methylene bis(4-cyclohexyl isocyanate) backbones result in prolonged clotting times.

Long range communication between exosites 1 and 2 modulates thrombin function

Although exosites 1 and 2 regulate thrombin activity by binding substrates and cofactors and by allosterically modulating the active site, it is unclear whether there is direct allosteric linkage between the two exosites. To begin to address this, we first titrated a thrombin variant fluorescently labeled at exosite 1 with exosite 2 ligands, HD22 (a DNA aptamer), gamma'-peptide (an analog of the COOH terminus of the gamma'-chain of fibrinogen) or heparin. Concentration-dependent and saturable changes in fluorescence were elicited, supporting inter-exosite linkage. To explore the functional consequences of this phenomenon, we evaluated the capacity of exosite 2 ligands to inhibit thrombin binding to gamma(A)/gamma(A)-fibrin, an interaction mediated solely by exosite 1. When gamma(A)/gamma(A)-fibrinogen was clotted with thrombin in the presence of HD22, gamma'-peptide, or prothrombin fragment 2 there was a dose-dependent and saturable decrease in thrombin binding to the resultant fibrin clots. Furthermore, HD22 reduced the affinity of thrombin for gamma(A)/gamma(A)-fibrin 6-fold and accelerated the dissociation of thrombin from preformed gamma(A)/gamma(A)-fibrin clots. Similar responses were obtained when surface plasmon resonance was used to monitor the interaction of thrombin with gamma(A)/gamma(A)-fibrinogen or fibrin. There is bidirectional communication between the exosites, because exosite 1 ligands, HD1 (a DNA aptamer) or hirudin-(54-65) (an analog of the COOH terminus of hirudin), inhibited the exosite 2-mediated interaction of thrombin with immobilized gamma'-peptide. These findings provide evidence for long range allosteric linkage between exosites 1 and 2 on thrombin, revealing further complexity to the mechanisms of thrombin regulation.

Mechanism of thrombin inhibition by heparin cofactor II in the presence of dermatan sulphates, native or oversulphated, and a heparin-like dextran derivative

The kinetics of thrombin inhibition by heparin cofactor II (HC II) in the presence of dermatan sulphates, native (DS), or oversulphated (DSS 1 and DSS 2) and a biospecific dextran derivative substituted with carboxymethyl, carboxymethyl-benzylamide and carboxymethyl benzylamide-sulphonate functional groups (CMDBS), has been studied as a function of the sulphated polysaccharide concentration. The initial HC II and thrombin concentrations were set at equimolar levels. Analysis of the experimental data obtained for DS, DSS1 and DSS2 was performed using a previously described model which allows computation of the dissociation constant (KPS,HC) of the polysaccharide-HC II complex and the rate constant of thrombin inhibition by the polysaccharide-HC II complex (k). A KPS.HC of 9.6 x'10(-7) M and a k of 4.5 x 10(9) M-1 min-1 were found for DS, whereas KPS,HC 2.1 x 10(-6) M, k 1.1 x 10(10) M-1 min-1 and KPS,HC 4.3 x 10(-7) M, k 1.4 x 10(10) M-1 min-1 were found for DSS1 and DSS2, respectively. Knowing that DSS1 has a sulphur content per disaccharide of 7.8%, compared with 11.5% for DSS2, these results indicate that the polysaccharide affinity for HC II is increased only in the case of DSS 2, whereas the oversulphation increases the reactivities towards thrombin of both complexes DSS1-HC II and DSS2-HC II. A better conformation of these complexes may favour a faster interaction with the protease. Unlike heparin, DS at concentrations higher than 10(-5) M does not modify the reaction rate of thrombin inhibition, a fact which can be explained by the absence of complex formation between DS and thrombin. The experimental data obtained for CMDBS fit a kinetic model in which the biospecific dextran derivative rapidly forms a complex with thrombin which is more reactive towards HC II than the free protease. The reaction rate remained unchanged for CMDBS concentrations equal to or higher than 10(-5) M, whereas CMDBS was found to interfere strongly with the fibrinogen-thrombin interaction. These data suggest that CMDBS has a strong affinity for the protease and no affinity for HC II. The computed dissociation constant of the CMDBS-thrombin complex (KPS,E) was 2.4 x 10(-7) M and the rate constant of the reaction of this complex with HC II (k) was 1.7 x 10(8) M-1 min-1. These findings indicate that CMDBS exerts its catalytic effect through a unique mechanism of action and may constitute a new class of anticoagulant drugs.

Mesoglycan and sulodexide act as stabilizers and protectors of fibroblast growth factors (FGFs)

Heparin and heparan sulfate proteoglycans (HSPGs) stabilize FGFs which belong to heparin-binding growth factors (HBGFs) on active conformation. They also strongly potentiate their mitogenic activity on many cell types, and protect them against thermal denaturation and enzymatic degradation. In the present work we have tested two heparin-like substances named mesoglycan and sulodexide obtained from bovine intestinal mucosal extracts. These products are used as heparin, in various of therapeutic fields such as atherosclerosis or antithrombotic therapy. The compositions of mesoglycan and sulodexide are partially known and include chondroitin, dermatan and heparan sulfate. We have shown that mesoglycan and sulodexide potentiated the mitogenic activity of FGF1 and FGF2. The magnitude of this effect was identical with that of heparin used as a control substance but at double concentration. Mesoglycan and sulodexide also exerted stabilizing and protective effects on FGFs for heat denaturation and enzymatic degradation. The suppression of the protective properties after heparinase treatment of mesoglycan and sulodexide indirectly demonstrated the presence of heparan sulfate which was shown to represent about 60% of the commercial products.

Study of low molecular weight heparin effect on the relation between anticoagulant activity and antithrombin III affinity

Low molecular weight heparin (FR-860), and conventional unfractionated heparin (UF-heparin) were fractionated by rabbit antithrombin III (AT III)-Sepharose, and the effects of each affinity fraction on the coagulation and fibrinolytic activities were investigated. FR-860 was fractionated to no-affinity, low-affinity (LA) and high-affinity (HA) fractions, and UF-heparin to LA and HA fractions. The HA fractions showed higher activities regarding the prolongation of activated partial thromboplastin time, anti-factor Xa activity and antithrombin activity compared with those of LA. The HA and LA fractions exhibited the enhancement of heparin cofactor II (HC II) activity and fibrinolytic activity in a dose-dependent manner. These results suggest that the antithrombotic activity of FR-860 is exerted through AT III and other mechanism such as HC II-mediated system.

Immobilization of dermatan sulphate on a silica matrix and its possible use as an affinity chromatography support for heparin cofactor II purification

Dermatan sulphate (DS) is a glycosaminoglycan which catalyses specifically thrombin inhibition by a plasmatic inhibitor, Heparin cofactor II (HCII). DS was insolubilized on a silica matrix to study its interaction with HCII. The immobilization of DS was performed with a good yield on a silica previously coated with polysaccharides in order to neutralize the negatively charged silanol groups. The value of the affinity constant of insolubilized DS for HCII, measured by the adsorption isotherm, is consistent with the value obtained for soluble DS. The DS bound to the silica matrix was also tested as a chromatographic support for the purification of HCII from human plasma; the optimum conditions for HCII adsorption and desorption were determined. The eluted HCII was obtained with a good yield (21%) and with no contamination by antithrombin III, the other main plasmatic inhibitor of thrombin.

Anti-thrombin activities of heparin. Effect of saccharide chain length on thrombin inhibition by heparin cofactor II and by antithrombin

The interactions of two proteinase inhibitors, heparin cofactor II and antithrombin, with thrombin are potentiated by heparin. Using two methods, we have studied the potentiating effects of a series of heparin (poly)saccharides with high affinity for antithrombin and mean Mr ranging from approx. 1700 to 18,800. First, catalytic amounts of heparin (poly)saccharide were added to purified systems containing thrombin and either heparin cofactor II or antithrombin. Residual thrombin activity was determined with a chromogenic substrate. It was found that only the higher-Mr polysaccharides (Mr greater than 8000) efficiently catalysed thrombin inhibition by heparin cofactor II, there being a progressive catalytic effect with increasing Mr of the polysaccharide. Weak accelerating effects were noted with low-Mr saccharides (Mr less than 8000). This contrasted with the well-characterized interaction of heparin with antithrombin and thrombin, where heparin oligosaccharides of Mr less than 5400 had absolutely no ability to accelerate the reaction, while (poly)saccharides of Mr exceeding 5400 showed rapidly increasing catalytic activity with increasing Mr. Secondly, these and other heparin preparations were added in a wide concentration range to plasma with which 125I-labelled thrombin was then incubated for 30 s. Inhibited thrombin was determined from the distribution of labelled thrombin amongst inhibitor-thrombin complexes, predominantly antithrombin-thrombin and heparin cofactor II-thrombin complexes. In this situation, where the inhibitors competed for thrombin and for the (poly)saccharides, it was found that, provided the latter were of high affinity for antithrombin and exceeded a Mr of 5400, thrombin inhibition in plasma was mediated largely through antithrombin. Polysaccharides of Mr exceeding 8000 that were of low affinity for antithrombin accelerated thrombin inhibition in plasma through their interaction with heparin cofactor II. High concentrations of saccharides of Mr 1700-5400 exhibited a size-dependent acceleration of thrombin inhibition, not through their interaction with antithrombin, but through their interaction with heparin cofactor II.