Collagen-bound von Willebrand factor has reduced affinity for factor VIII

von Willebrand factor (vWf) is a multimeric adhesive glycoprotein that serves as a carrier for factor VIII in plasma. Although each vWf subunit displays a high affinity binding site for factor VIII in vitro, in plasma, only 2% of the vWf sites for factor VIII are occupied. We investigated whether interaction of plasma proteins with vWf or adhesion of vWf to collagen may alter the affinity or availability of factor VIII-binding sites on vWf. When vWf was immobilized on agarose-linked monoclonal antibody, factor VIII bound to vWf with high affinity, and neither the affinity nor binding site availability was influenced by the presence of 50% plasma. Therefore, plasma proteins do not alter the affinity or availability of factor VIII-binding sites. In contrast, when vWf was immobilized on agarose-linked collagen, its affinity for factor VIII was reduced 4-fold, with KD increasing from 0.9 to 3.8 nM. However, one factor VIII-binding site remained available on each vWf subunit. A comparable reduction in affinity for factor VIII was observed when vWf was a constituent of the subendothelial cell matrix and when it was bound to purified type VI collagen. In parallel with the decreased affinity for factor VIII, collagen-bound vWf displayed a 6-fold lower affinity for monoclonal antibody W5-6A, with an epitope composed of residues 78-96 within the factor VIII-binding motif of vWf. We conclude that collagen induces a conformational change within the factor VIII-binding motif of vWf that lowers the affinity for factor VIII.

Binding of factor VIII to von willebrand factor is enabled by cleavage of the von Willebrand factor propeptide and enhanced by formation of disulfide-linked multimers

von Willebrand factor (vWF) is a multimeric adhesive glycoprotein with one factor VIII binding site/subunit. Prior reports suggest that posttranslational modifications of vWF, including formation of N-terminal intersubunit disulfide bonds and subsequent cleavage of the propeptide, influence availability and/or affinity of factor VIII binding sites. We found that deletion of the vWF propeptide produced a dimeric vWF molecule lacking N-terminal intersubunit disulfide bonds. This molecule bound fluorescein-labeled factor VIII with sixfold lower affinity than multimeric vWF in an equilibrium flow cytometry assay (approximate KDs, 5 nmol/L v 0.9 nmol/L). Coexpression of propeptide-deleted vWF with the vWF propeptide in trans yielded multimeric vWF that displayed increased affinity for factor VIII. Insertion of an alanine residue at the N-terminus of the mature vWF subunit destroyed binding to factor VIII, indicating that the native mature N-terminus is required for factor VIII binding. The requirement for vWF propeptide cleavage was shown by (1) a point mutation of the vWF propeptide cleavage site yielding pro-vWF that was defective in factor VIII binding and (2) correlation between efficiency of intracellular propeptide cleavage and factor VIII binding. Furthermore, in a cell-free system, addition of the propeptide-cleaving enzyme PACE/furin enabled factor VIII binding in parallel with propeptide cleavage. Our results indicate that high-affinity factor VIII binding sites are located on N-terminal disulfide-linked vWF subunits from which the propeptide has been cleaved.

The effect of subcutaneous injection of unfractionated and low molecular weight heparin on thrombin generation in platelet rich plasma--a study in human volunteers

We administered a dose of unfractionated heparin (UFH) and two doses of a low molecular weight heparin (LMWH) to healthy volunteers by SC injection. The doses given were: a) UFH, 5000 IU, which represents 8.7 mg of > 5,400 MW active heparin (ACLM) and no < 5,400 active heparin (BCLM), b) enoxaparin 40 mg (3.4 mg ACLM, 2.2 mg BCLM) and c) enoxaparin 1 mg/kg body weight (on the mean 75 mg, containing 6.4 mg ACLM and 4.1 mg BCLM). We determined the effect on thrombin generation in platelet rich plasma (PRP) between 1 and 8 h after injection. UFH administration caused only a 5-8% inhibition of the thrombin potential (i.e. the area under the thrombin generation curve). Significantly higher inhibition of the thrombin potential was seen after administration of both doses of enoxaparin. To wit 9-26% at the low dose and 29-46% at the high dose. UFH injection caused a prolongation of the lag-time before the thrombin burst. Only with the high dose of enoxaparin the lag-times were significantly more prolonged with enoxaparin than with UFH. Excess amounts of platelet factor 4 (PF4) were able to neutralize completely the anti-thrombin activity in normal plasma spiked with enoxaparin as well as in plasma samples obtained after SC enoxaparin injection. With a large excess of PF4 the anti-factor Xa activity could be inhibited to a maximum of 50%. This indicates that ACLM (above critical length material, MW > 5400) is neutralized completely by PF4 whereas BCLM (below critical length material, MW < 5400) is not.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and pharmacodynamics of a low molecular weight heparin (enoxaparin) after subcutaneous injection, comparison with unfractionated heparin--a three way cross over study in human volunteers

We determined, in volunteers, the plasma levels of heparin above and below the critical chainlength necessary for thrombin inhibition (ACLM and BCLM), from 1 to 24 h after subcutaneous injection of 5000 IU unfractionated heparin (UFH), 40 mg enoxaparin and 1 mg/kg body weight of enoxaparin (LMWH) (n = 12 for each dose). The levels were calculated from the antithrombin- and anti-Xa activities using the specific activities of the materials injected. We also determined the course of thrombin- and of factor Xa generation after triggering the extrinsic system in the same samples. From the thrombin generation curves, we calculated the course of prothrombinase activity. When the ACLM and BCLM plasma-levels are plotted against the inhibition of thrombin- and factor Xa generation, it appears that: a) There is a unique dose response relationship between ACLM level and the inhibition of thrombin generation, independent of whether the ACLM is derived from UFH or LMWH. This relationship is not significantly altered by the BCLM appearing after LMWH injection. b) There is a similar unique relationship between ACLM level and the inhibition of factor Xa generation, again independent of BCLM. c) Inhibition of prothrombin activation hardly contributes to the overall effect on thrombin formation and is again independent of the source of ACLM. d) ACLM levels were significantly higher after injection of LMWH than after UFH injection, even though the amounts of ACLM injected with the highest dose of LMWH were smaller than those administered in the UFH injection. We conclude that the only functional difference between LMWH and UFH is the much higher bioavailability of the former.(ABSTRACT TRUNCATED AT 250 WORDS)

On the relationship between molecular mass and anticoagulant activity in a low molecular weight heparin (enoxaparin)

A low molecular weight heparin (enoxaparin, mean molecular weight approximately 4,400) was separated by gel chromatography into eight different fractions with a narrow distribution around the following mean molecular weights: 1,800, 2,400, 2,900, 4,200, 6,200, 8,600, 9,800 and 11,000. We compared the influence of enoxaparin on the generation of thrombin in plasma to that of the eight fractions. We determined: a) the % of material with high affinity to antithrombin III (HAM) and the % of HAM above the critical chain length necessary to allow for thrombin inhibition (ACLM), b) the specific catalytic activity on the decay of endogenous thrombin, and c) the inhibition of over-all thrombin formation in the extrinsic and the intrinsic pathway. From b and c we calculated the inhibition of prothrombin conversion in these pathways. We found that a) there is a gradual decrease of the HAM fraction with decreasing molecular weight; b) the specific catalytic activity for the inactivation of thrombin does not vary significantly between the fractions when expressed in terms of ACLM; c) the potency to inhibit prothrombin conversion does not vary significantly between the fractions when expressed in terms of HAM.

Determination of the levels of unfractionated and low-molecular-weight heparins in plasma: their effect on thrombin-mediated feedback reactions in vivo. Preliminary results after subcutaneous injection

We define a standard independent unit (SIU) of heparin as that amount that, in plasma containing 1 mumol of ATIII, raises the (pseudo-)first-order breakdown constant of factor Xa by 1 min-1. These units measure all material with a high affinity for ATIII (HAM); only material above the critical chain length of 17 monosaccharide units (above critical chain length material; ACLM) catalyzes the inactivation of thrombin. An SIU of ACLM is therefore analogously defined as the amount that, in plasma containing 1 mumol of ATIII, will raise the (pseudo-)first-order breakdown constant of thrombin by 1 min-1. Of any given heparin preparation one can determine the specific HAM and ACLM activities in terms of SIU/mg. On the basis of the factor Xa and thrombin breakdown constants found in a plasma sample one can then determine the levels of HAM and ACLM. Preliminary experiments were carried out in plasma samples obtained after subcutaneous injection of unfractionated heparin (UFH) and of two types of low-molecular-weight heparin (LMWH). About three times more of UFH activity than of LMWH activity has to be injected to obtain the same levels of ACLM in the plasma. Only with the LMWHs significant amounts of BCLM are found, which rises higher and persists longer than the ACLM. We determined the course of thrombin generation in platelet-rich plasma (PRP) and in platelet-poor plasma (PPP), as well as in the PPP factor Xa generation curve and the course of prothrombin conversion. The observed inhibitions correlated much better with the levels of ACLM than with those of below critical chain length material. The difference between UFH and LMWHs can therefore not be explained in terms of antithrombin and anti-factor-Xa activity. The essential difference between UFH and LMWH appears in the feedback effect of thrombin in PRP, where thrombin generation is both inhibited and retarded by LMWH, while it is only retarded but hardly inhibited by UFH.

Elements from in vitro studies that help understand the action of heparins

In determining heparin one has the choice to test a specific activity, such as the decay constant of thrombin or factor Xa on a global test such as the aPTT. The best test would be a global test that directly reflects the only important global effect, to wit antithrombotic efficiency. Such a test does not yet exist. We propose that the thrombin potential, i.e. the time concentration integral of thrombin activity appearing in plasma after triggering is a plausible candidate for such a test.

The inhibition of intrinsic prothrombinase and its generation by heparin and four derivatives in prothrombin poor plasma

The effect of different heparins and a synthetic pentasaccharide on the inhibition of intrinsic prothrombinase and of its generation was studied by a new technique, using a defibrinated prothrombin poor human plasma, supplemented with phospholipids and calcium. Prothrombinase activity was evaluated on purified prothrombin with a chromogenic substrate. This technique is designed to bypass the interference of massive endogenous thrombin generation on the measurement of prothrombinase activity. We first validated the specificity of the technique by using specific Xa and IIa inhibitors. Then, the inhibition of prothrombinase generation and the inhibition of generated prothrombinase were both studied. The results showed that anti-Xa activity measured on exogenous bovine factor Xa added to plasma was not correlated with the inhibition of prothrombinase generation or prothrombinase activity. The concentrations required for unfractionated heparin (the 4th International Standard: 4th IS UH), 1st International Standard Low Molecular Weight Heparins (1st IS LMWH), enoxaparin, Fraxiparine, and pentasaccharide in order to inhibit preformed prothrombinase were significantly higher than those necessary to inhibit prothrombinase generation. These data suggest that anti-Xa activity of unfractionated heparin and its derivatives does not completely reflect the extent of the inhibition of intrinsic prothrombinase generation by UH, LMWH, and pentasaccharide. On the other hand, anti-IIa activity of heparins could be responsible for the inhibition of prothrombinase generation. The action of pentasaccharide devoid of anti-IIa activity on prothrombinase generation appears related to its indirect effect on the formation of initial thrombin traces. This new technique provides a tool to study the essential role played by thrombin during the early steps of coagulation.