Anticoagulant activities of heparin and fragments

Mechanism of factor IXa inhibition by antithrombin in the presence of unfractionated and low molecular weight heparins and fucoidan

Heparin exerts its anticoagulant activity by catalysing the inhibition of coagulation proteases by antithrombin (AT). Its main target is thrombin but it also catalyses the inhibition of the other serine-proteases of the coagulation cascade, such as factor IXa (fIXa). The aim of this study was to compare the catalysis of inhibition of blood fIXa by antithrombin in the presence of several sulfated polysaccharides with anticoagulant activity, i.e. heparin, three widely used in therapeutics low molecular weight heparins (LMWH) and fucoidan. Plots of the second-order rate constants of the fIXa-antithrombin reaction vs. the concentration of added heparin and LMWH are bell-shaped and fit the kinetic model established for thrombin-antithrombin reaction by Jordan R., Beeler D., Rosenberg R. (1979) J. Biol. Chem., 254, 2902-2913. In the ascending branch, the catalyst (C) binds quickly to the inhibitor (I) to form a catalyst-inhibitor (CI) complex which is more reactive towards the enzyme (E) than the free inhibitor, leading to the formation of an inactive enzyme-inhibitor complex (EI) and the release of free catalyst, in a rate-limiting second step. After a maximum corresponding to an optimal catalyst concentration, the decrease in the reaction rate was in keeping with the formation of a catalyst-enzyme (CE) complex, whose inactivation by the CI complex was slower than that of the free enzyme. Maximum second-order rate constants for the inhibition of fIXa by AT were 105, 6.8, 12.24 and 22 microM-1 min-1 with heparin, Enoxaparin, Fraxiparin and Fragmin, respectively, leading to 3500-, 225-, 405- and 728-fold increases in the inhibition rate in the absence of polysaccharide, respectively. Fucoidan yielded 23-fold increase in the fIXa-antithrombin interaction rate. The kinetic profiles obtained with this polysaccharide exhibited ascending branch which correlated well with the kinetic model based on the formation of binary complexes (CI or CE). Fucoidan was covalently conjugated with a fluorescent probe (DTAF) and used in conjunction with fluorescence anisotropy to follow its binding to antithrombin, heparin cofactor II (HCII), thrombin and fIXa. The binding of fucoidan to these proteins occurred with low affinities when compared to heparin and LMWH. Fucoidan had higher affinity for the inhibitor HCII compared to antithrombin and enzymes. These data suggest that binding of heparins and fucoidan to the inhibitor (CI) is required for the polysaccharide-dependent enhancement in the rate of neutralization of the enzyme by the inhibitor.

Immobilization of a lysine-terminated heparin to polyvinyl alcohol

Lysine terminated heparin, prepared by the nitrous acid partial depolymerization and reductive amination of heparin, failed to increase the active heparin content of a heparin-polyvinyl alcohol (heparin-PVA) hydrogel relative to the unmodified commercial heparin. The depolymerization of heparin resulted in a loss of biological activity which outweighed the increase in the terminal amine groups (produced by reductive amination), that were used for glutaraldehyde immobilization to the PVA. The loss in anti-thrombin activity (thrombin time or chromogenic substrate) paralleled the increase in anhydromannose end groups due to depolymerization making it necessary to optimize the loss of activity against the increase in terminal amine groups after amination. For example, depolymerization at a high sodium nitrite concentration (81 g/l) at pH4 and 25 degrees C for 20 min, resulted in a loss of 22-40% of the biological activity but achieved an anhydromannose content of 600 nmoles/mg (approximately 7 cleavage sites/molecule). After the anhydromannose groups were reductively aminated by lysine, the anhydromannose content was reduced to 190 nmol/mg indicating a terminal lysine content of 410 nmol/mg. This resulted in an increase in heparin content of the final hydrogel by 53% on mass terms. However, given the reduction in biological activity, it was not surprising that the modified heparin-PVA hydrogel coated on a polyethylene tube was no better than the hydrogel with unmodified heparin in inactivating thrombin in a flow circuit. These results point out the need for care in interpreting heparin immobilization results and for new strategies to increase the active heparin content of this hydrogel.

Anticoagulant properties of semisynthetic polysaccharide sulfates

Several naturally occurring polysaccharides were purified and subsequently sulfated by chlorosulfonic acid-pyridine complex. These were isolated as the sodium salt and further purified by ECTEOLA cellulose chromatography. Anticoagulant properties of the sulfated polysaccharides were compared with commercial heparin by measuring their in vitro effects on activated partial thromboplastin time (APTT), prothrombin time (PT) and thrombin time (TT) using pooled normal human plasma. In general, all the compounds exhibited antithrombic (anti-TT) properties similar to heparin but were less effective than heparin in inhibiting APTT or PT. The in vivo anticoagulant properties were also compared with commercial heparin by injecting rats a single intraperitoneal dose and measuring plasma APTT at 2, 4 and 6 hour intervals. All of the compounds including heparin increased APTT significantly at 2 hours and then gradually returned to near normal value after 6 hours. Larchwood xylan sulfate was almost as active as heparin in inhibiting APTT while the rest of the compounds were less active. Toxicological studies using rats showed wide variations in the LD50 of various compounds. Sulfated xylan and heparin were least toxic while sulfated polysaccharide of locust bean was most toxic. For most of the compounds the LD50 was 5-100 fold higher than the effective dosage.

Mechanism of potentiation of antithrombin III and heparin cofactor II inhibition by sulfated xylans

Kinetic analyses of antithrombin III (AT-III)-thrombin or heparin cofactor II (HC-II)-thrombin or AT-III-factor Xa interactions were carried out in the absence or in the presence of one of the sulfated xylans or unfractionated heparin or low molecular weight (LMW) heparin utilizing chromogenic substrates. These studies demonstrated that under pseudo first order conditions the inhibitions were proportional to the AT-III or HC-II concentrations used and the apparent second order rate constants determined from the slopes of the pseudo first order plots of log of thrombin or Xa remaining as a function of time were significantly elevated in presence of the sulfated compounds. On a molar basis oat spelts xylan sulfate was the most effective compound in accelerating the rate of thrombin-AT-III interaction followed by commercial heparin while the latter was most effective in accelerating the rate of thrombin-HC-II interaction. Heparin and LMW heparin were more effective in that order in accelerating the rate of Xa-AT-III interaction while oat spelts xylan sulfate, corn cob xylan sulfate, SP-54 were less effective than the heparins in that order. Studies were also conducted on the concentrations of the sulfated compounds required to inhibit by 50% the thrombin activity by AT-III or HC-II or that required to inhibit by 50% the factor Xa activity by AT-III. The results showed an inverse relationship between the increase in the rate of acceleration by the sulfated compound with the decrease in the amount required for 50% inhibition. SDS-polyacrylamide gel study of the reaction mixture containing thrombin, AT-III or HC-II along with heparin or oat spelts xylan sulfate showed that like heparin, oat spelts xylan sulfate potentiated the formation of thrombin-AT-III or thrombin-HC-II complexes which were stable in presence of denaturing or reducing agents. Chemical modification of arginine or lysine of AT-III significantly lowered its potentiation of thrombin or Xa inhibition by oat spelts xylan sulfate.