Quantitation of plasma factor XIIIa activity using fibrin-coated microscopic latex beads

Reactivity of human platelets with immobilized fibrinogen is dictated by the chemical character of the surface

Quiescent platelets readily adhere to surface-immobilized fibrinogen. In contrast, platelets exposed to soluble fibrinogen do not demonstrate such activity. As part of an effort to characterize this phenomenon, a solid-phase reagent was prepared by adsorbing human fibrinogen to polystyrene-divinylbenzene microparticles. Using a suspension of human platelets, phase-contrast microscopy was used to quantitate directly platelets bound to fibrinogen-coated beads. This method is fast, straightforward, and requires minimal amounts of reagents and sample. An existing turbidimetric assay was modified to monitor optically the rate and extent of platelet-fibrinogen binding. When platelet-rich plasma was added to a stirred suspension of fibrinogen-coated beads, the rate of aggregation was related directly to the concentration of fibrinogen on the bead surface. This response could not be mitigated by the thrombin inhibitor, hirudin, indicating that any thrombin generated in the reaction has no role in bead aggregation. Conversely, the alpha(IIb)beta(3) antagonist, abciximab (ReoPro), completely prevented aggregation, implicating specific fibrinogen-alpha(IIb)beta(3) interactions as responsible for the observed effect. Beads coated with either albumin or a densely packed, pure film of the neutral phospholipid, phosphatidylcholine (lecithin), do not aggregate under identical conditions, nor do fibrinogen-coated beads aggregate when platelet-depleted plasma is added. When fibrinogen was coated to beads as a mixed film with lecithin, a striking increase in reactivity toward platelets was demonstrated, compared to unmodified beads. These studies indicate that the observed adhesion of platelets to beads is a direct result of platelet-fibrinogen interactions and platelets respond differently to fibrinogen when presented as a mixed film with lipid, compared to the protein alone at an interface.

A turbidimetric assay for quantitating functional fibrin(ogen) using polystyrene-divinylbenzene microparticles

A sensitive assay has been developed to quantitate fibrinogen in plasma or in other aqueous solutions. Microscopic latex particles, modified with a mixed monomolecular film of lecithin and fibrinogen, are used as a solid-phase reagent. These lecithin/fibrinogen-coated beads aggregate when stirred in the presence of thrombin and, when solution-phase fibrinogen is added, the increased rate of aggregation is proportional to the concentration of soluble fibrinogen. Using a sample volume of 200 microl, as little as 15 nM ( approximately 5 microg ml-1) fibrinogen can be measured. Fibrinogen determinations using the bead assay compared favorably with those derived from a standard clinical assay, with a correlation coefficient (r2) of 0.9710 over a range of 2.5 to 28.0 microM. Analytic precision was comparable to available assays, with typical coefficients of variation of 12.7 and 7.1% for fibrinogen concentrations of 30 nM and 15.0 microM, respectively. The method has a dynamic range of 15 nM to over 3.0 microM and offers the advantage of being sensitive to 20-fold lower concentrations of fibrinogen compared to routine clot-based methods. Unlike immunological assays, e.g., ELISA, it measures only the functional protein. This bead method should prove to be of greatest use to investigators measuring low levels of functional fibrin(ogen).

Adsorption and coagulability of fibrinogen on atheromatous lipid surfaces

Fibrinogen, the precursor of the blood clot matrix and a major constituent of atherosclerotic lesions, is shown to adsorb with high affinity to hydrophobic beads coated with cholesteryl oleate, cholesterol, or loosely packed lecithin. The quantity of fibrinogen that binds to cholesterol- or lecithin-coated beads decreases as the surface concentration of the lipid increases; densely packed films lecithin bind little,if any, if the protein. In sharp contrast, the appreciable quantity of fibrinogen that binds to cholesteryl oleate-coated beads is indifferent to the surface concentration of that lipid. Not unexpectedly, the quantity of fibrinogen that binds to beads coated with mixtures of cholesteryl oleate and lecithin increases with increasing concentration of the cholesteryl ester. When bound, fibrinogen can be converted by thrombin to fibrin and nucleate clot formation as manifested by the aggregation of stirred beads. These results indicate that hydrophobic, atheromatous lipid surfaces, particularly those rich in cholesteryl esters, may be predisposed to thrombosis by virtue of their inherent capacity to bind functional fibrinogen.

Preparation and characterization of fibrinogen-coated, reversibly adhesive, lecithin/cholesterol vesicles

We have developed a method for producing fibrinogen-coated, reversibly adhesive, lecithin/cholesterol vesicles. In this method, fibrinogen, which is acylated in the presence of preformed vesicles, spontaneously incorporates into vesicular membranes. The degree of incorporation is a function of the extent of acylation of the protein. Fibrinogen-coated vesicles aggregate in the presence of thrombin, a consequence of intervesicular fibrin formation. The rate and extent of thrombin-initiated aggregation depend on the fibrinogen surface concentration. Once aggregated, fibrin-coated vesicles can be dissociated by plasmin and by agents that disrupt intervesicular fibrin dimerization such as heparin and the tetrapeptide Gly-Pro-Arg-Pro. Fibrinogen-coated vesicles can be made to bind avidly to the surface of solution phase fibrin matrices and can be incorporated into solution phase fibrin clots. Fibrinogen-coated vesicles also bind to activated platelets. We propose that fibrinogen-coated vesicles will have practical applications as biomimetic hemostatic agents and as vehicles for the fibrin-specific targeting of drugs and other molecules.