Circular dichroic, infrared and other studies on the protein component of pig brain thromboplastin

The mechanism of inhibition of factor III (thromboplastin) activity by apolipoprotein B-100. Protein-protein interactions

Factor III (thromboplastin) activity is inhibited by apoB-100, but the mechanism of inhibition is unknown. By examining the effect of purified apoB-100 on factor III activity, we showed that apoB-100 can inhibit factor III via a different mechanism from that caused by the issue-factor pathway-inhibitor, which is mainly carried on the surface of lipoproteins. Although the presence of calcium ions and factors X and VII may enhance the rate of inhibition, they are not a prerequisite for the inhibition of factor III by apoB-100. In addition, by investigating the changes in the UV spectra of apoB-100 on interaction with factor III and factors X and VII and by assigning the shifts in absorption spectra to particular amino acids, we showed that these interactions involve negative and positive residues within these proteins. By following the rates of interactions between apoB-100 and either factors III, X, VII, a two-step mechanism for the inhibition process involving factors X and VII was postulated. In this mechanism, the primary interaction of apoB-100 with factor III is followed by a rate-limiting step that can be accelerated by the presence of either factor X or VII and leads to the inhibition of factor III. Furthermore, a computer-based analysis of the sequences of factor III revealed a possible binding site for apoB-100.

The effect of lipid peroxidation and lipolysis on the ability of lipoproteins to influence thromboplastin activity

High, low and very low density lipoproteins and lipoprotein (a) were prepared from porcine serum. The apolipoprotein components of the lipoproteins were then isolated and resuspended in soybean lecithin. Apolipoprotein B was also resuspended in lipids more representative of those found in LDL and VLDL. Lipid peroxidation was induced in samples of all the lipoproteins and reconstituted apolipoproteins by incubation with either Cu2+ ions or hedgehog 15-lipoxygenase. Furthermore, aliquots of the samples were incubated with a mixture of lipases. The effect of native preparations and the treated samples on the procoagulant activity of thromboplastin was examined. Native HDL, apo A-II, native LDL, reconstituted LDL and apo B inhibited thromboplastin activity, whereas native VLDL and reconstituted VLDL enhanced this activity. While the ability of HDL and apolipoprotein A-II to inhibit thromboplastin was unaltered by either Cu2+ oxidation, lipoxygenase oxidation or lipolysis, VLDL and particles resembling VLDL, which acted cooperatively with thromboplastin lost their activating potential. On the other hand, LDL and particles resembling LDL changed from being inhibitory to enhancing the thromboplastin activity following oxidation, but not after lipolysis. Apolipoprotein B fragments obtained by mild digestion of this protein, expressed an inhibitory effect towards thromboplastin, while extensive degradation of the protein reduced its inhibitory potential. It is suggested that modifications of lipoproteins in vivo can lead to a hypercoagulable state by modulation of the cofactor activity of thromboplastin to factor VII.

The inhibition of thromboplastin by apolipoprotein-B and the effect of various associated lipids

Human apolipoprotein B was purified by barium sulphate adsorption, subsequent delipidation and gel filtration. The protein was then reconstituted in soya bean lecithin and its inhibitory effect towards thromboplastin was assayed by incubation with rabbit brain thromboplastin. The use of an antibody against human apolipoprotein B diminished this inhibition. The level of inhibition of thromboplastin by the reconstituted apolipoprotein was found to be dependent on the concentration of the phospholipids with which it was reconstituted, reaching a maximum inhibition at a lipid: protein ratio of 1:1 (w/w). However, higher phospholipid concentrations or inclusion of cholesterol esters or triglycerides diminished and at certain concentrations reversed the inhibitory effect of the apolipoprotein. These results point towards apolipoprotein B as an inhibitor whose activity towards thromboplastin could be dependent on the complexes it forms with the surrounding lipids.

Two sites in the tissue factor extracellular domain mediate the recognition of the ligand factor VIIa

Tissue factor (TF) binds the serine protease coagulation factor VIIa and initiates the coagulation protease cascade by forming a catalytic cofactor-enzyme complex. Using a photoactivatable crosslinking reagent coupled to factor VIIa, we have identified interactive sites in the amino-terminal (residues 44-84) and the carboxyl-terminal (residues 129-169) aspect of the extracellular domain of TF. Epitopes of inhibitory antibodies have previously indicated participation of these regions in TF function. The presence of the gamma-carboxyglutamic acid domain in factor VIIa appears to facilitate the interaction with the negatively charged, amino-proximate site, whereas crosslinking of TF with VIIa or des-(1-38)-VIIa at the positively charged carboxyl-proximate site was similar. Lack of alpha-helical secondary structure in the TF extracellular domain is consistent with the proposed structural similarity of TF with the cytokine receptor family. The interactive sites identified for TF are located in sequence spans that demonstrate a low degree of sequence conservation among the members of this receptor family. Regions with highly conserved residues, such as sequences encoded by exon 2 and 5 in TF, were not implicated in ligand recognition, suggesting that conserved residues in the receptor family may maintain the common beta-strand architecture, and variable regions provide a pair of nonidentical motifs for oriented ligand recognition.

Structural design and molecular evolution of a cytokine receptor superfamily

A family of cytokine receptors comprising molecules specific for a diverse group of hematopoietic factors and growth hormones has been principally defined by a striking homology of binding domains. This work proposes that the approximately 200-residue binding segment of the canonical cytokine receptor is composed of two discrete folding domains that share a significant sequence and structural resemblance. Analogous motifs are found in tandem approximately 100-amino acid domains in the extracellular segments of a receptor family formed by the interferon-alpha/beta and -gamma receptors and tissue factor, a membrane tether for a coagulation protease. Domains from the receptor supergroup reveal clear evolutionary links to fibronectin type III structures, approximately 90-amino acid modules that are typically found in cell surface molecules with adhesive functions. Predictive structural analysis of the shared receptor and fibronectin domains locates seven beta-strands in conserved regions of the chain; these strands are modeled to fold into antiparallel beta-sandwiches with a topology that is similar to immunoglobulin constant domains. These findings have strong implications for understanding the evolutionary emergence of an important class of regulatory molecules from primitive adhesive modules. In addition, the resulting double-barrel design of the receptors and the spatial clustering of conserved residues suggest a likely binding site for cytokine ligands.

Purification of glycosylated apoprotein of tissue factor from human brain and inhibition of its procoagulant activity by a specific antibody

The glycosylated component of tissue factor from human brain which totally retained on Concanavalin A-Sepharose was isolated by a four step procedure. The final product (6-7 S) exhibited an apparent molecular weight of approximately 45,000 under reducing conditions on SDS-gels corresponding with procoagulant activity of tissue factor after detergent removal. The optimal ratio of lipid to protein in reconstituting tissue factor activity was 500:1 (w/w) amounting to 240,000 U/mg. Peptidase activity was not detectable in the purified apoprotein or in the relipidated protein. The procoagulant activity of tissue factor was dependent on Factor VII but not on Factors VIII or IX, since in the absence of Factor VII clotting times were significantly prolonged. The activity of the reconstituted tissue factor could be reduced by a factor of 15 upon incubation with an antibody against the purified glycosylated apoprotein that was raised in the rabbit. The IgG-fraction of the antiserum neutralized the procoagulant activity of tissue factor in saline extracts from human brain and placenta in a time- and concentration-dependent manner indicating common antigenic determinants on the cofactor protein from both tissues.