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Thrombin-activable fibrinolysis inhibitor (TAFI) is present in the circulation as an inactive zymogen. Thrombin converts TAFI to a carboxypeptidase B-like enzyme (TAFIa) by cleaving at Arg(92) in a process accelerated by the cofactor, thrombomodulin. TAFIa attenuates fibrinolysis. TAFIa can be inactivated by both proteolysis by thrombin and spontaneous temperature-dependent loss of activity. The identity of the thrombin cleavage site responsible for loss of TAFIa activity was suggested to be Arg(330), but site-directed mutagenesis of this residue did not prevent inactivation of TAFIa by thrombin. In this study we followed TAFI activation and TAFIa inactivation by thrombin/thrombomodulin in time and characterized the cleavage pattern of TAFI using matrix-assisted laser desorption ionization mass spectrometry. Mass matching of the fragments revealed that TAFIa was cleaved at Arg(302). Studies of a mutant R302Q-TAFI confirmed identification of this thrombin cleavage site and, furthermore, suggested that inactivation of TAFIa is based on its conformational instability rather than proteolytic cleavage at Arg(302).

Human group II secretory phospholipase A2 (hsPLA2) exhibits significant anticoagulant activity that does not require its enzymatic activity. We examined which coagulation factor was targeted by hsPLA2 and analyzed which region of the protein may be involved in this inhibition. Prothrombin time coagulation assays indicated that hsPLA2 did not inhibit activated factor V (FVa) activity, whereas activated factor X (FXa) one-stage coagulation assays suggested that FXa was inhibited. The inhibitory effect of hsPLA2 on prothrombinase activity of FXa, FV, phospholipids, and Ca2+ complex was markedly enhanced upon preincubation of hsPLA2 with FXa but not with FV. Prothrombinase activity was also strongly inhibited by hsPLA2 in the absence of PL. High concentrations of FVa in the prothrombinase generation assay reversed the inhibitory effect of hsPLA2. By using isothermal titration calorimetry, we demonstrated that hsPLA2 binds to FXa in solution with a 1:1 stoichiometry and a Kd of 230 nM. By using surface plasmon resonance we determined the rate constants, kon and koff, of the FXa/hsPLA2 interaction and analyzed the Ca2+ effect on these constants. When preincubated with FXa, synthetic peptides comprising residues 51-74 and 51-62 of hsPLA2 inhibited prothrombinase assays, providing evidence that this part of the molecule, which shares similarities with a region of FVa that binds to FXa, is likely involved in the anticoagulant interaction of hsPLA2 with FXa. In conclusion, we propose that residues 51-62 of hsPLA2 bind to FXa at a FVa-binding site and that hsPLA2 decreases the prothrombinase generation by preventing FXa.FVa complex formation.

Factor Va inactivation by activated protein C is associated with cleavages at Arg306, Arg506, and Arg679 with Arg306 cleavage causing the major activity loss. To study functional roles of the Arg306 region, overlapping 15-mer peptides representing the sequence of factor Va residues 271-345 were synthesized and screened for anticoagulant activities. The peptide containing residues 311-325 (VP311) noncompetitively inhibited prothrombin activation by factor Xa, but only in the presence of factor Va. Fluorescence studies showed that VP311 bound to fluorescence-labeled 5-dimethylaminonaphthalene-1-sulfonyl-Glu-Gly-Arg factor Xa in solution with a Kd of 70 microM. Diisopropylphosphoryl factor Xa and factor Xa but not factor VII/VIIa or prothrombin bound to immobilized VP311 with relatively high affinity. These results support the hypothesis that residues 311-325, which are positioned between the A1 and A2 domains of factor Va and likely exposed to solvent, contribute to the binding of factor Xa by factor Va. Based on this hypothesis, it is suggested that cleavage by activated protein C at Arg306 in factor Va not only severs the covalent connection between the A1 and A2 domains but also disrupts the environment and structure of residues 311-325, thereby down-regulating the binding of factor Xa to factor Va.

Protein S is a vitamin K-dependent non-enzymatic coagulation factor involved in the regulation of activated protein C (APC). In this paper we report an APC-independent anticoagulant function of protein S. We observed an inhibition of prothrombinase activity on endothelial cells and platelets which was half-maximal at physiological concentrations of free protein S in plasma. On endothelial cells, thrombin-cleaved protein S (PSt) as well as protein S in complex with C4b-binding protein (C4BP) inhibited prothrombinase activity to the same extent as protein S did. In solid-phase binding assays, direct binding of protein S and PSt to factor V and factor Va were observed. Protein S-C4BP complex did not bind to factor V or factor Va, implicating that the factor V(a) binding site on protein S is lost by the interaction with C4BP. A direct inhibition of factor Xa activity by protein S was also observed. Incubation of factor Xa with protein S revealed a noncompetitive inhibition of factor Xa by protein S with a Ki of (4.9 +/- 0.8) x 10(-7) M. Both protein S and protein S-C4BP complex were able to inhibit factor Xa to the same extent, whereas PSt had lost its inhibitory activity. This suggests that the conformational change induced by cleavage of the amino-terminal thrombin-sensitive loop results in a loss of a factor Xa binding site on protein S. The inhibitory effect of protein S involves interactions with both factor Va and factor Xa. The interaction of protein S with factor Xa is influenced by cleavage of the thrombin-sensitive loop, whereas C4BP blocks the interaction of protein S with factor Va. Inhibition of the prothrombinase complex by either forms of protein S might be an important mechanism in regulating thrombin generation in blood coagulation. The importance of the APC-independent anticoagulant action of protein S was emphasized by experiments in which the addition of protein S to normal plasma induced a prolongation of clotting time in a dilute activated partial thromboplastin time (dAPTT) assay. Furthermore, inhibition of endogenous protein S by the addition of monoclonal antibodies against protein S to normal plasma, induced a shortening of the clotting time in a dAPTT assay.

An important feedback mechanism in blood coagulation is supplied by the protein C/protein S anticoagulant pathway. In this study we demonstrate that the binding of human protein S to cultured human umbilical vein endothelial cells (HUVECs) is required for the expression of cofactor activity of protein S toward factor Va inactivation by activated protein C (APC). The initial rate of endothelial cell-mediated factor Va inactivation was 21.7 pM factor Va/50 pM APC min-1, which could be enhanced twice at a protein S concentration of 5 nM. This increase appeared to be specific for protein S because it could be inhibited by C4b-binding protein and polyclonal antibodies against protein S. Furthermore, thrombin-cleaved protein S did not accelerate factor Va inactivation by APC on endothelial cells. The binding of 125I-protein S to endothelial cells was time-dependent, specific, saturable, and required the presence of calcium ions. Scatchard analysis revealed (8.0 +/- 0.3) x 10(5) binding sites per cell with an apparent Kd of 24.4 +/- 2.2 nM. To study the physiological importance of the binding of protein S to human endothelial cells, seven monoclonal antibodies were examined for their ability to influence the protein S cofactor activity and binding capacity. Monoclonal antibodies directed against the gamma-carboxyglutamic acid domain and the thrombin-sensitive region of protein S completely inhibited the protein S cofactor function in factor Va inactivation by APC on HUVECs. These monoclonal antibodies also inhibited 125I-protein S binding to HUVECs. Another monoclonal antibody, directed against an epitope on the third and/or fourth epidermal growth factor-like region, did not influence either protein S cofactor activity or binding of protein S to HUVECs. We conclude that binding of protein S to HUVECs is essential for the expression of its cofactor activity for APC. At least two regions in protein S, the gamma-carboxyglutamic acid domain and the thrombin-sensitive region, are involved in the expression of cofactor activity.