Heparin with fibrinogen

It is noteworthy that a qualitatively similar dependence was observed for heparin in solution116>. The complexes of immobilized heparin with fibrinogen and thrombin are rather stable (the corresponding association constants for the complexes with fibrinogen and thrombin are (1.4 0.5) x 105 M and (8 2) x 10s A/-1) and are not subjected to dissociation in physiological solution. 

Fig. 8. Kinetics of unstabilized fibrin hydrolysis with complexes of immobilized heparin with fibrinogen (1), thrombin (2), fibrinolysin (plasmin) (3), and serum albumin (4),06)...

The kinetics of the lytic effect displayed by the complexes of immobilized heparin with thrombin and fibrinogen, in distinction from those with plasmin, are described by their saturation curves. The observed slowing down of the dissolution of unstabilized fibrin is probably due to the inhibiton of the lytic activity of the complexes by the soluble products of the reaction. In fact, as it was shown in Ref. 106, further addition of immobilized heparin-protein complex to partially hydrolyzed fibrin results in a complete recovery of the dissolution rate. 

The more profound and extensive effect of immobilized heparin on the blood clotting system observed in the presence of immobilized trypsin is due to the tryptic lysis of the protein constituents of the complexes of immobilized heparin with the most thrombogenic plasma proteins (thrombin and fibrinogen) (Fig. 11). 

Benesch, J. Svedhem, S. Svensson, S. C. T. Valiokas, R. Liedberg, B. Tengvall, P., Protein adsorption to oligo(ethylene glycol) self-assembled monolayers experiments with fibrinogen, heparinized plasma, and serum, J. Biomat. Sci. 2001, 12, 581-597... 

Heparinoids and mucopolysaccharides react with, and modify, many of the plasma proteins. Heparin combines with fibrinogen, globulins and albumin. As judged by electrophoresis and various types of analysis and staining, the particular plasma protein components with which heparin combines are dependent upon the concentration of protein, concentration of heparin, pH value, and salts present. This explains the somewhat contradictory statements in literature about combinations of heparin with plasma proteins. The combination may result in change of solubility of the protein and reverse protein tests . Heparin can modify the murexide reaction for calcium in serum by affecting the calcium-protein-heparin complex. Many heparinoids... 

The interaction of heparin with human plasma low-density lipoprotein has been studied by gel permeation chromatography A fraction containing degradation products of fibrinogen formed in blood plasma after injection of thrombin also contains a complex of fibrinogen and heparin. The rate of inactivation of thrombin and Factor by anti-thrombin has been shown to be directly proportional to the chain-length of heparin.The effects of heparin on the reactions catalysed by Factor have been examined.Heparin inhibits the ability of phospholipid to accelerate prothrombin activation in a mechanism which appears to involve the displacement of Factor X from the surface of the phospholipid. 

Hirudin is produced by the medicinal leech and may be extracted from its saliva. It is a 65 amino acid peptide. It is now expressed recombinantly in yeast cells and the recombinant product is almost identical to the naturally occurring protein. It acts directly on thrombin, binding to the active site of thrombin, and preventing the enzymatic cleavage of fibrinogen. It will bind both to free thrombin and to thrombin involved in complexes with fibrinogen. It has the advantage that it does not have the same side effect profile as heparin and thus can be used in heparin-intolerant patients. Bivalimdin is low MW (2180) synthetic peptide which mimics hirudin and binds to thrombin at the same site. 

Heparin may be given intravenously or subcutaneously, and there is no universally accepted dose. Full-dose heparin therapy in adults is a bolus of 5000 units, followed by a continuous infusion of 1000 units/hour. Since the aPTT is already elevated in individuals with DIC, monitoring full-dose heparin therapy may be difficult, and D-dimer and fibrinogen levels maybe better markers of activity. Subcutaneous heparin at a dose of 80 to 100 units/kg every 4 to 6 hours and low-molecular-weight heparins are other, less studied options.25... 

The answer is d. (Hardman, p 1346.) A slow intravenous infusion of protamine sulfate will quickly reverse the bleeding. Protamine binds to heparin to form a stable complex with no anticoagulant activity It may also have its own anticoagulant effect by binding with platelets and fibrinogen. 

Schematic representation of Subsite Utilization in Thrombin Complexes (after Reference 8). Fibrinogen interacts with three thrombin subsites (here thrombin is represented by a large oval and the interconnected subsites by an irregular three-armed shape). Physiological effectors of thrombin and thrombin inhibitors form distinct interactions at these subsites. Additional subsites, such as the heparin-binding site, exist on the thrombin surface and are not indicated here.

The anticoagulant activity of heparin is endowed by its ability to form strong complexes with a variety of blood clotting factors and thus neutralize their action. For instance, heparin interacts with thrombin, fibrinogen, prothrombin, factors IX-XII... 

Thus, the increased platelet adhesion observed upon heparinization is a consequence of the enrichment of the surface of a HCP by fibrinogen 108,. Fibrinogen, as was already stated above, may specifically interact via its oligosaccharide fragments with the enzymes of the platelet membrane. 

Hence, concerning the interaction with plasma proteins, covalently immobilized heparin performs identically to heparin in solution, and this results in the enrichment of the HCP surface with the most thrombogenic plasma components fibrinogen and thrombin. 

The increased platelet adhesion onto HCP surfaces is mainly due to the high surfacial concentration of fibrinogen associated with immobilized heparin. Therefore one of the solutions to the problem could have been provided by simultaneous immobilization of heparin and proteolytic enzymes which could hydrolyze the adsorbed fibrinogen. The immobilized enzyme could also provide the lysis of the fibrin clot, in case it was formed, for instance in stagnation regions of the complicated devices. 

Fig. 12. Concentrations of fibrinogen (O) and serum albumin ( ) versus time of incubation with the polymers containing 0.46 mg of trypsin/g of the polymer (1,2) and 0.51 mg trypsin/g polymer + 1.43 mg heparin/g polymer (3,4)1361...

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