Fibrin-monomer-Sepharose was used to study thrombin binding to fibrin and the role of the enzyme active centre in this interaction. Binding properties of preformed enzyme-inhibitor complexes, as well as inhibition of thrombin already adsorbed to fibrin monomer, were investigated. No apparent difference was found in binding properties of phenylmethanesulphonyl fluoride-, D-Phe-Pro-Arg-CH2Cl- and dansylarginine NN-(3-ethylpentane-1,5-diyl)amide-inhibited thrombins. Also, the elution profile of phenylmethane-sulphonyl fluoride-inhibited thrombin from fibrinogen-Sepharose was identical with that of active thrombin from fibrin-monomer-Sepharose. Thus far the only low-Mr inhibitor that prevents thrombin from binding to fibrin monomer is pyridoxal 5′-phosphate. Preformed hirudin-thrombin complexes do not interact with fibrin. The extent to which the active centre of thrombin associated with fibrin is still accessible to substrates and inhibitors was also studied. Thrombin bound to fibrin hydrolyses a synthetic substrate at the same rate as the free enzyme. Water-soluble low-Mr inhibitors such as D-Phe-Pro-Arg-CH2Cl and dansylarginine NN-(3-ethylpentane-1,5-diyl)amide can readily modify the active centre of the fibrin-associated enzyme, and the active centre is exposed to the degree that displacement of dansylarginine NN-(3-ethylpentane-1,5-diyl)amide by D-Phe-Pro-Arg-CH2Cl is possible without disturbing the binding. Hirudin disrupts the affinity between thrombin and fibrin. These data indicate that the active centre of thrombin associated with fibrin through extended binding is fully exposed and freely accessible. It is possible that extended binding may play a regulatory role in the activation of Factor XIII by thrombin, as well as inactivation of this enzyme by antithrombin III.

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