Thrombin generation and inactivation in the presence of antithrombin III and heparin

We have determined the rate constants of inactivation of factor Xa and thrombin by antithrombin III/heparin during the process of prothrombin activation. The second-order rate constant of inhibition of factor Xa alone by antithrombin III as determined by using the synthetic peptide substrate S-2337 was found to be 1.1 X 10(6) M-1 min-1. Factor Xa in prothrombin activation mixtures that contained prothrombin, and either saturating amounts of factor Va or phospholipid (20 mol % dioleoylphosphatidylserine/80 mol % dioleoylphosphatidylcholine, 10 microM), was inhibited by antithrombin III with a second-order rate constant that was essentially the same: 1.2 X 10(6) M-1 min-1. When both factor Va and phospholipid were present during prothrombin activation, factor Xa inhibition by antithrombin III was reduced about 10-fold, with a second-order rate constant of 1.3 X 10(5) M-1 min-1. Factor Xa in the prothrombin activation mixture that contained both factor Va and phospholipid was even more protected from inhibition by the antithrombin III-heparin complex. The first-order rate constants of these reactions at 200 nM antithrombin III and normalized to heparin at 1 microgram/mL were 0.33 and 9.5 min-1 in the presence and absence of factor Va and phospholipid, respectively. When the prothrombin concentration was varied widely around the Km for prothrombin, this had no effect on the first-order rate constants of inhibition. It is our conclusion that factor Xa when acting in prothrombinase on prothrombin is profoundly protected from inhibition by antithrombin III in the absence as well as in the presence of heparin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of factor IXa and factor Xa by antithrombin III/heparin during factor X activation

We investigated the kinetics of the inhibitory action of antithrombin III and antithrombin III plus heparin during the activation of factor X by factor IXa. Generation and inactivation curves were fitted to a three-parameter two-exponentional model to determine the pseudo first-order rate constants of inhibition of factor IXa and factor Xa by antithrombin III/heparin. In the absence of heparin, the second-order rate constant of inhibition of factor Xa generated by factor IXa was 2.5-fold lower than the rate constant of inhibition of exogenous factor Xa. It appeared that phospholipid-bound factor X protected factor Xa from inactivation by antithrombin III. It is, as yet, unclear whether an active site or a nonactive site interaction between factor Xa and factor X at the phospholipid surface is involved. The inactivation of factor IXa by antithrombin III was found to be very slow and was not affected by phospholipid, calcium, and/or factor X. With unfractionated heparin above 40 ng/ml and antithrombin III at 200 nM, the apparent second-order rate constant of inhibition of exogenous and generated factor Xa were the same. Thus, in this case phospholipid-bound factor X did not protect factor Xa from inhibition. In the presence of synthetic pentasaccharide heparin, however, phospholipid-bound factor X reduced the rate constant about 5-fold. Pentasaccharide had no effect on the factor IXa/antithrombin III reaction. Unfractionated heparin (1 micrograms/ml) stimulated the antithrombin III-dependent inhibition of factor IXa during factor X activation 400-fold. In the absence of reaction components this stimulated was 65-fold. We established that calcium stimulated the heparin-dependent inhibition of factor IXa.     

Functional properties of factor Va subunits after proteolytic alterations by activated protein C

The two-subunit structure of the factor Va molecule is essential to its function in the prothrombinase complex. In the presence of phospholipids, the cleavage of the light chain of bovine factor Va by activated protein C proceeded at the same rate as the cleavage of the heavy chain. The limited proteolysis of factor Va is accompanied by a parallel loss of factor Va activity. Evidence that loss of activity was solely the result of the cleavage of the heavy chain, was obtained from reconstitution experiments utilizing cleaved and intact chains. The pseudo first-order rate constant of factor Va inactivation by activated protein C was found to be dependent on the amount of phospholipid-bound activated protein C and not on the amount of phospholipid-bound factor Va. However, phospholipids enhance the rate of proteolysis of the phospholipid-binding subunit, i.e. the light chain, and not the cleavage of the heavy chain. Cleavage of the heavy chain and as a consequence the inactivation of factor Va by activated protein C is mediated by phospholipid-bound light chain. After cleavage of the light chain, the 'two-subunit' structure, as well as the phospholipid-binding properties of factor Va were found to be conserved.     

Identification, cloning, expression, and characterization of the extracellular acarbose-modifying glycosyltransferase, AcbD, from Actinoplanes sp. strain SE50.

An extracellular enzyme activity in the culture supernatant of the acarbose producer Actinoplanes sp. strain SE50 catalyzes the transfer of the acarviosyl moiety of acarbose to malto-oligosaccharides. This acarviosyl transferase (ATase) is encoded by a gene, acbD, in the putative biosynthetic gene cluster for the alpha-glucosidase inhibitor acarbose. The acbD gene was cloned and heterologously produced in Streptomyces lividans TK23. The recombinant protein was analyzed by enzyme assays. The AcbD protein (724 amino acids) displays all of the features of extracellular alpha-glucosidases and/or transglycosylases of the alpha-amylase family and exhibits the highest similarities to several cyclodextrin glucanotransferases (CGTases). However, AcbD had neither alpha-amylase nor CGTase activity. The AcbD protein was purified to homogeneity, and it was identified by partial protein sequencing of tryptic peptides. AcbD had an apparent molecular mass of 76 kDa and an isoelectric point of 5.0 and required Ca(2+) ions for activity. The enzyme displayed maximal activity at 30 degrees C and between pH 6.2 and 6.9. The K(m) values of the ATase for acarbose (donor substrate) and maltose (acceptor substrate) are 0.65 and 0.96 mM, respectively. A wide range of additional donor and acceptor substrates were determined for the enzyme. Acceptors revealed a structural requirement for glucose-analogous structures conserving only the overall stereochemistry, except for the anomeric C atom, and the hydroxyl groups at positions 2, 3, and 4 of D-glucose. We discuss here the function of the enzyme in the extracellular formation of the series of acarbose-homologous compounds produced by Actinoplanes sp. strain SE50.     

The adsorption of prothrombin to phospholipid monolayers quantitated by ellipsometry

We investigated by means of an automated ellipsometer the calcium-dependent binding of prothrombin from a buffer solution to monolayers of dioleoylphosphatidylserine (DOPS) and dioleoylphosphatidylcholine (DOPC) deposited on chromium slides. This technique allows direct measurements of bound and free protein concentrations and is not hampered by calcium-induced aggregation of vesicles. For pure DOPS a dominant class of binding sites exists with a dissociation constant, Kd = (6 +/- 2) X 10(-10) M (mean +/- S.D.) and maximal binding of prothrombin, gamma max = 0.26 +/- 0.03 micrograms/cm2. Incorporation of a small fraction of DOPC in the monolayer causes a large decrease in the binding affinity with a pronounced biphasic behavior of the binding curve. For monolayers consisting of 20% DOPS and 80% DOPC the binding curve becomes monophasic with Kd = (1.6 +/- 0.6) X 10(-7) M and gamma max = 0.22 +/- 0.03 micrograms/cm2. The procoagulant activity of the monolayers was tested by measuring the generation of thrombin after addition of prothrombin and activated coagulation factors X and V. The thrombin-generating capacity of monolayers and single-bilayer vesicles is comparable but is apparently diffusion limited in the monolayer system. The calcium-dependent formation of stacked multilayers according to the Blodgett technique appeared to be strongly influenced by the DOPS/DOPC ratio in the phospholipid monolayer. From these results it is concluded that for pure DOPS monolayers high-affinity prothrombin-phospholipid and phospholipid-phospholipid interactions exist which are radically disturbed when the monolayer contains more than 20-30% of DOPC.     

In vitro prothrombin synthesis from a purified precursor protein III. Preparation of an acid-soluble substrate for vitamin K-dependent carboxylase by limited proteolysis of bovine descarboxyprothrombin

Bovine descarboxyprothrombin and descarboxyfragment-1 can be used as substrates for rat and bovine vitamin K-dependent carboxylase. In both enzyme systems, however, these substrates have a high K_m (0.3–0.4 mM). A better substrate (K_m = 0.001–0.003 mM) was prepared from bovine descarboxyprothrombin by limited proteolysis with subtilisin Carlsberg. This substrate is called Fragment-Su and is composed of the amino acids 13–29 of descarboxyprothrombin.     

The role of gamma-carboxyglutamyl residues in the positive cooperative binding of Ca2+ to blood coagulation factor X

1. The calcium binding properties of factor X and its analogous decarboxyprotein have been compared with the aid of flow rate dialysis and ultraviolet difference spectroscopy. 2. Factor X binds approx. 20 mol of calcium per mol of protein. The first four sites exhibit positive cooperativity. 3. Changes in the ultraviolet difference spectrum when Ca2+ is bound suggest a conformational change. 4. In decarboxyfactor X low affinity of Ca2+ and no ligand-induced conformational change was observed. It is concluded that the presence of gamma-carboxyglutamate residues is a prerequisite for positive cooperative Ca2+ binding.     

In vitro prothrombin synthesis from a purified precursor protein. II. Partial purification of bovine carboxylase

In this paper, we describe the isolation and partial purification of an enzyme system that converts bovine decarboxyfactor II (PIVKA-II) into prothrombin (factor II). It is shown that the increase in factor II activity occurs in parallel with 14CO2 incorporation into BaSO4 adsorbable proteins. The system is not strictly vitamin K-dependent because it is obtained from the livers of normal healthy cows. By preincubating the enzyme(s) with an excess of warfarin, an absolute vitamin K1-dependence can be obtained. The reaction is inhibited by its own product, factor II.     

Thrombin-dependent Incorporation of von Willebrand Factor into a Fibrin Network

Attachment of platelets from the circulation onto a growing thrombus is a process involving multiple platelet receptors, endothelial matrix components, and coagulation factors. It has been indicated previously that during a transglutaminase reaction activated factor XIII (FXIIIa) covalently cross-links von Willebrand factor (VWF) to polymerizing fibrin. Bound VWF further recruits and activates platelets via interactions with the platelet receptor complex glycoprotein Ib (GPIb). In the present study we found proof for binding of VWF to a fibrin monomer layer during the process of fibrinogen-to-fibrin conversion in the presence of thrombin, arvin, or a snake venom from Crotalus atrox. Using a domain deletion mutant we demonstrated the involvement of the C domains of VWF in this binding. Substantial binding of VWF to fibrin monomers persisted in the presence of the FXIIIa inhibitor K9-DON, illustrating that cross-linking via factor XIII is not essential for this phenomenon and suggesting the identification of a second mechanism through which VWF multimers incorporate into a fibrin network. Under high shear conditions, platelets were shown to adhere to fibrin only if VWF had been incorporated. In conclusion, our experiments show that the C domains of VWF and the E domain of fibrin monomers are involved in the incorporation of VWF during the polymerization of fibrin and that this incorporation fosters binding and activation of platelets. Fibrin thus is not an inert end product but partakes in further thrombus growth. Our findings help to elucidate the mechanism of thrombus growth and platelet adhesion under conditions of arterial shear rate.     

Cholesterol sensor ORP1L contacts the ER protein VAP to control Rab7–RILP–p150Glued and late endosome positioning

Late endosomes (LEs) have characteristic intracellular distributions determined by their interactions with various motor proteins. Motor proteins associated to the dynactin subunit p150^Glued bind to LEs via the Rab7 effector Rab7-interacting lysosomal protein (RILP) in association with the oxysterol-binding protein ORP1L. We found that cholesterol levels in LEs are sensed by ORP1L and are lower in peripheral vesicles. Under low cholesterol conditions, ORP1L conformation induces the formation of endoplasmic reticulum (ER)–LE membrane contact sites. At these sites, the ER protein VAP (VAMP [vesicle-associated membrane protein]-associated ER protein) can interact in trans with the Rab7–RILP complex to remove p150^Glued and associated motors. LEs then move to the microtubule plus end. Under high cholesterol conditions, as in Niemann-Pick type C disease, this process is prevented, and LEs accumulate at the microtubule minus end as the result of dynein motor activity. These data explain how the ER and cholesterol control the association of LEs with motor proteins and their positioning in cells.