A new member of the plasma protease inhibitor gene family

The complete N-terminal extension of heparin cofactor II is required for maximal effectiveness as a thrombin exosite 1 ligand

Background

Heparin cofactor II (HCII) is a circulating protease inhibitor, one which contains an N-terminal acidic extension (HCII 1-75) unique within the serpin superfamily. Deletion of HCII 1-75 greatly reduces the ability of glycosaminoglycans (GAGs) to accelerate the inhibition of thrombin, and abrogates HCII binding to thrombin exosite 1. While a minor portion of HCII 1-75 can be visualized in a crystallized HCII-thrombin S195A complex, the role of the rest of the extension is not well understood and the affinity of the HCII 1-75 interaction has not been quantitatively characterized. To address these issues, we expressed HCII 1-75 as a small, N-terminally hexahistidine-tagged polypeptide in E. coli.

Results

Immobilized purified HCII 1-75 bound active α-thrombin and active-site inhibited FPR-ck- or S195A-thrombin, but not exosite-1-disrupted γ_T-thrombin, in microtiter plate assays. Biotinylated HCII 1-75 immobilized on streptavidin chips bound α-thrombin and FPR-ck-thrombin with similar K_D values of 330-340 nM. HCII 1-75 competed thrombin binding to chip-immobilized HCII 1-75 more effectively than HCII 54-75 but less effectively than the C-terminal dodecapeptide of hirudin (mean K_i values of 2.6, 8.5, and 0.29 μM, respectively). This superiority over HCII 54-75 was also demonstrated in plasma clotting assays and in competing the heparin-catalysed inhibition of thrombin by plasma-derived HCII; HCII 1-53 had no effect in either assay. Molecular modelling of HCII 1-75 correctly predicted those portions of the acidic extension that had been previously visualized in crystal structures, and suggested that an α-helix found between residues 26 and 36 stabilizes one found between residues 61-67. The latter region has been previously shown by deletion mutagenesis and crystallography to play a crucial role in the binding of HCII to thrombin exosite 1.

Conclusions

Assuming that the K_D value for HCII 1-75 of 330-340 nM faithfully predicts that of this region in intact HCII, and that 1-75 binding to exosite 1 is GAG-dependent, our results support a model in which thrombin first binds to GAGs, followed by HCII addition to the ternary complex and release of HCII 1-75 for exosite 1 binding and serpin mechanism inhibition. They further suggest that, in isolated or transferred form, the entire HCII 1-75 region is required to ensure maximal binding of thrombin exosite 1.

Glycosaminoglycan-binding properties and kinetic characterization of human heparin cofactor II expressed in Escherichia coli

Irreversible inactivation of alpha-thrombin (T) by the serpin, heparin cofactor II (HCII), is accelerated by ternary complex formation with the glycosaminoglycans (GAGs) heparin and dermatan sulfate (DS). Low expression of human HCII in Escherichia coli was optimized by silent mutation of 27 rare codons and five secondary Shine-Dalgarno sequences in the cDNA. The inhibitory activities of recombinant HCII, and native and deglycosylated plasma HCII, and their affinities for heparin and DS were compared. Recombinant and deglycosylated HCII bound heparin with dissociation constants (K(D)) of 6+/-1 and 7+/-1 microM, respectively, approximately 6-fold tighter than plasma HCII, with K(D) 40+/-4 microM. Binding of recombinant and deglycosylated HCII to DS, both with K(D) 4+/-1 microM, was approximately 4-fold tighter than for plasma HCII, with K(D) 15+/-4 microM. Recombinant HCII, lacking N-glycosylation and tyrosine sulfation, inactivated alpha-thrombin with a 1:1 stoichiometry, similar to plasma HCII. Second-order rate constants for thrombin inactivation by recombinant and deglycosylated HCII were comparable, at optimal GAG concentrations that were lower than those for plasma HCII, consistent with its weaker GAG binding. This weaker binding may be attributed to interference of the Asn(169)N-glycan with the HCII heparin-binding site.

Vascular dermatan sulfate and heparin cofactor II

Heparin cofactor II (HCII) is a plasma protease inhibitor of the serpin family that inactivates thrombin by forming a covalent 1:1 complex. The rate of complex formation increases more than 1000-fold in the presence of dermatan sulfate (DS). Endothelial injury allows circulating HCII to enter the vessel wall, where it binds to DS and presumably becomes activated. Mice that lack HCII develop carotid artery thrombosis more rapidly than wild-type mice after oxidative damage to the endothelium. These mice also have increased arterial neointima formation following mechanical injury and develop more extensive atherosclerotic lesions when made hypercholesterolemic. Similarly, low plasma HCII levels appear to be a risk factor for atherosclerosis and in-stent restenosis in human subjects. These observations suggest that a major function of the HCII-DS system is to regulate the physiologic response to arterial injury.

Heparin cofactor II modulates the response to vascular injury

Heparin cofactor II (HCII) has several biochemical properties that distinguish it from other serpins: (1) it specifically inhibits thrombin; (2) the mechanism of inhibition involves binding of an acidic domain in HCII to thrombin exosite I; and (3) the rate of inhibition increases dramatically in the presence of dermatan sulfate molecules having specific structures. Human studies suggest that high plasma HCII levels are protective against in-stent restenosis and atherosclerosis. Studies with HCII knockout mice directly support the hypothesis that HCII interacts with dermatan sulfate in the arterial wall after endothelial injury and thereby exerts an antithrombotic effect. In addition, HCII deficiency appears to promote neointima formation and atherogenesis in mice. These results suggest that HCII plays a unique and important role in vascular homeostasis.

Control of the coagulation system by serpins. Getting by with a little help from glycosaminoglycans

Members of the serine protease inhibitor (serpin) superfamily play important roles in the inhibition of serine proteases involved in complex systems. This is evident in the regulation of coagulation serine proteases, especially the central enzyme in this system, thrombin. This review focuses on three serpins which are known to be key players in the regulation of thrombin: antithrombin and heparin cofactor II, which inhibit thrombin in its procoagulant role, and protein C inhibitor, which primarily inhibits the thrombin/thrombomodulin complex, where thrombin plays an anticoagulant role. Several structures have been published in the past few years which have given great insight into the mechanism of action of these serpins and have significantly added to a wealth of biochemical and biophysical studies carried out previously. A major feature of these serpins is that they are under the control of glycosaminoglycans, which play a key role in accelerating and localizing their action. While further work is clearly required to understand the mechanism of action of the glycosaminoglycans, the biological mechanisms whereby cognate glycosaminoglycans for each serpin come into contact with the inhibitors also requires much further work in this important field.

Reformable intramolecular cross-linking of the N-terminal domain of heparin cofactor II

The crystal structure of a heparin cofactor II (HCII)-thrombin Michaelis complex has revealed extensive contacts encompassing the N-terminal domain of HCII and exosite I of the proteinase. In contrast, the location of the N-terminal extension in the uncomplexed inhibitor was unclear. Using a disulfide cross-linking strategy, we demonstrate that at least three different sites (positions 52, 54 and 68) within the N terminus may be tethered in a reformable manner to position 195 in the loop region between helix D and strand s2A of the HCII molecule, suggesting that the N-terminal domain may interact with the inhibitor scaffold in a permissive manner. Cross-linking of the N terminus to the HCII body does not strongly affect the inhibition of alpha-chymotrypsin, indicating that the reactive site loop sequences of the engineered inhibitor variants, required for interaction with one of the HCII target enzymes, are normally accessible. In contrast, intramolecular tethering of the N-terminal extension results in a drastic decrease of alpha-thrombin inhibitory activity, both in the presence and in the absence of glycosaminoglycans. Treatment with dithiothreitol and iodoacetamide restores activity towards alpha-thrombin, suggesting that release of the N terminus of HCII is an important component of the multistep interaction between the inhibitor and alpha-thrombin.

Homozygous Deficiency of Heparin Cofactor II

Background— Heparin cofactor II (HCII) is a hepatic serpin with significant antithrombin activity that has been implicated in coagulation, inflammation, atherosclerosis, and wound repair. Recent data obtained in mice lacking HCII suggest that this serpin might inhibit thrombosis in the arterial circulation. However, the clinical relevance and molecular mechanisms associated with deficiency of HCII in humans are unclear.

Methods and Results— We studied the first family with homozygous HCII deficiency, identifying a Glu428Lys mutation affecting a conserved glutamate at the hinge (P17) of the reactive loop. No carrier reported arterial thrombosis, and only 1 homozygous HCII-deficient patient developed severe deep venous thrombosis, but she also had a de novo Glu100Stop nonsense truncation in the antithrombin gene.

Conclusions— Our results confirm the key structural role of the P17 glutamate in serpins. The same mutation causes conformational instability and polymerization in 3 serpins: Drosophila necrotic, human α1-antitrypsin, and human HCII, which explains their plasma deficiency. In the family under study here, however, plasma HCII deficiency was not associated with a significant clinical phenotype.

Human antithrombin III-derived heparin-binding peptide, a novel heparin antagonist

In the blood coagulation cascade, human antithrombin III (hAT III) acts as an inhibitor of serine proteases such as thrombin and factor Xa, and this anticoagulatory glycoprotein requires the binding of heparin for its activation. In this study, we synthesized the polypeptides corresponding to the proposed heparin-binding sites including the (41-49), (286-301) and (123-139) regions of hAT III, and examined their interactions with heparin by means of physicochemical and biochemical methods. All the synthetic peptides had a high affinity toward heparin, evidenced by the fact that they were eluted from a heparin-agarose column at the high salt concentration range of 520-700 mM. In addition, hAT III (123-139) attenuated the effect of heparin on the activation of hAT III, whereas other HBPs did not, suggesting that only hAT III (123-139) could interact with the active site of heparin. On the basis of these results, we prepared novel hAT III (123-139)-related derivatives as potent heparin antagonist candidates, and examined the influence of several modifications on their activity in vitro. The results provided new findings about the structure-activity relationship of hAT III (123-139), and led us to the successful development of a potent antagonist for heparin.

Novel approach for preparation of heparins specific to factor Xa using affinity chromatography coupled with synthetic antithrombin III-related peptides

In the blood coagulation cascade, heparin activates human plasma antithrombin III (hAT III), resulting in the inhibition of factor Xa. This polysaccharide also exhibits hemorrhagic tendency mediated by the inhibition of thrombin in heparinotherapy. Therefore, attention has focused on the development of low molecular weight heparins (LMW-heparins) that inhibit factor Xa but not thrombin. In this investigation, we examined the biochemical and physicochemical properties of hAT III-derived heparin-binding peptides (HBPs). Of all the tested HBPs, hAT III (123-139) exhibited the highest affinity with heparin and showed an inhibitory effect on the heparin-induced enhancement of hAT III activity toward factor Xa, indicating that hAT III (123-139) specifically interacts with the active region in heparin. We prepared a synthetic hAT III (123-139)-coupled affinity chromatography system, and demonstrated that this novel affinity chromatography is useful for fractionation of highly active moieties in LMW-heparins.

Aspartic acid residues 72 and 75 and tyrosine-sulfate 73 of heparin cofactor II promote intramolecular interactions during glycosaminoglycan binding and thrombin inhibition

We used site-directed mutagenesis to investigate the role of Glu(69), Asp(70), Asp(71), Asp(72), Tyr-sulfate(73), and Asp(75) in the second acidic region (AR2) of the serpin heparin cofactor II (HCII) during formation of the thrombin.HCII complex with and without glycosaminoglycans. E69Q/D70N/D71N recombinant (r)HCII, D72N/Y73F/D75N rHCII, and E69Q/D70N/D71N/D72N/Y73F/D75N rHCII were prepared to localize acidic residues important for thrombin inhibition. Interestingly, D72N/Y73F/D75N rHCII had significantly enhanced thrombin inhibition without glycosaminoglycan (4-fold greater) and with heparin (6-fold greater), showing maximal activity at 2 microg/ml heparin compared with wild-type recombinant HCII (wt-rHCII) with maximal activity at 20 microg/ml heparin. The other rHCII mutants had lesser-enhanced activities, but they all eluted from heparin-Sepharose at significantly higher ionic strengths compared with wt-rHCII. Neutralizing and reversing the charge of Asp(72), Tyr-sulfate(73), and Asp(75) were done to characterize their individual contribution to HCII activity. Only Y73K rHCII and D75K rHCII have significantly increased heparin cofactor activity compared with wt-rHCII; however, all of the individual rHCII mutants required substantially less glycosaminoglycan at maximal inhibition than did wt-rHCII. Inhibition of either alpha-thrombin/hirugen or gamma(T)-thrombin (both with an altered anion-binding exosite-1) by the AR2 rHCII mutants was similar to wt-rHCII. D72N/Y73F/D75N rHCII and D75K rHCII were significantly more active than wt-rHCII in a plasma-based thrombin inhibition assay with glycosaminoglycans. These results indicate that improved thrombin inhibition in the AR2 HCII mutants is mediated by enhanced interactions between the acidic domain and anion-binding exosite-1 of thrombin and that AR2 may be a "molecular rheostat" to promote thrombin inhibition in the presence of glycosaminoglycans.

Tyrosine sulfation and N-glycosylation of human heparin cofactor II from plasma and recombinant Chinese hamster ovary cells and their effects on heparin binding

The structure of post-translational modifications of human heparin cofactor II isolated from human serum and from recombinant Chinese hamster ovary cells and their effects on heparin binding have been characterized. Oligosaccharide chains were found attached to all three potential N-glycosylation sites in both protein preparations. The carbohydrate structures of heparin cofactor II circulating in blood are complex-type diantennary and triantennary chains in a ratio of 6 : 1 with the galactose being > 90% sialylated with alpha 2-->6 linked N-acetylneuraminic acid. About 50% of the triantennary structures contain one sLe(x) motif. Proximal alpha 1-->6 fucosylation of oligosacharides from Chinese hamster ovary cell-derived HCII was detected in > 90% of the diantennary and triantennary glycans, the latter being slightly less sialylated with exclusively alpha 2-->3-linked N-acetylneuraminic acid units. Applying the ESI-MS/ MS-MS technique, we demonstrate that the tryptic peptides comprising tyrosine residues in positions 60 and 73 were almost completely sulfated irrespective of the protein's origin. Treatment of transfected Chinese hamster ovary cells with chlorate or tunicamycin resulted in the production of heparin cofactor II molecules that eluted with higher ionic strength from heparin-Sepharose, indicating that tyrosine sulfation and N-linked glycans may affect the inhibitor's interaction with glycosaminoglycans.

Heparin cofactor II, antithrombin-beta and their complexes with thrombin in human tissues

In the presence of glycosaminoglycans, thrombin is rapidly inactivated by two natural inhibitors secreted from liver: antithrombin (AT) is presumed to be the principal thrombin inhibitor in circulating blood, while for heparin cofactor II (HCII), a role outside circulation has been proposed. In this study, we show that HCII and AT differ with respect to their association with human tissues. Aside from brain, each of these inhibitors was found in sodium dodecyl sulphate (SDS) soluble extracts of various human organs, with a preponderance of HCII in placenta. AT levels, however, predominated in liver. Compared to plasma, the beta-variant of AT was found to be strongly enriched in human organs, while tissue-resident HCII did not differ in its electrophoretic mobility from the circulating form. In placenta, comparable amounts of HCII/thrombin and AT/thrombin complexes were detected, indicating that HCII may exert a thrombin regulating role in that organ under conditions of tissue or blood vessel damage. Transcripts coding for HCII and AT were detected in all tissues examined. The low levels of their mRNAs suggest that most of the tissue-associated thrombin inhibitor molecules originate from circulation and are retained in organs, possibly by specific receptors. The differential presence of HCII and AT in organs is in accordance with individual physiological roles of these inhibitors.

Effect of oligodeoxynucleotide thrombin aptamer on thrombin inhibition by heparin cofactor II and antithrombin

'Thrombin aptamers' are based on the 15-nucleotide consensus sequence of d(GGTTGGTGTGGTTGG) that binds specifically to thrombin's anion-binding exosite-I. The effect of aptamer-thrombin interactions during inhibition by the serine protease inhibitor (serpin) heparin cofactor II (HCII) and antithrombin (AT) has not been described. Thrombin inhibition by HCII without glycosaminoglycan was decreased approximately two-fold by the aptamer. In contrast, the aptamer dramatically reduced thrombin inhibition by >200-fold and 30-fold for HCII-heparin and HCII-dermatan sulfate, respectively. The aptamer had essentially no effect on thrombin inhibition by AT with or without heparin. These results add to our understanding of thrombin aptamer activity for potential clinical application, and they further demonstrate the importance of thrombin exosite-I during inhibition by HCII-glycosaminoglycans.

Coagulation and Bleeding Disorders: Review and Update

Hemostasis is initiated by injury to the vascular wall, leading to the deposition of platelets adhering to components of the subendothelium. Platelet adhesion requires the presence of von Willebrand factor and platelet receptors (IIb/IIIa and Ib/IX). Additional platelets are recruited to the site of injury by release of platelet granular contents, including ADP. The “platelet plug” is stabilized by interaction with fibrinogen. In this review, I consider laboratory tests used to evaluate coagulation, including prothrombin time, activated partial thromboplastin time, thrombin time, and platelet count. I discuss hereditary disorders of platelets and/or coagulation proteins that lead to clinical bleeding as well as acquired disorders, including disseminated intravascular coagulation and acquired circulating anticoagulants.

Structure-based design of a potent chimeric thrombin inhibitor

Using the three-dimensional structures of thrombin and the leech-derived tryptase inhibitor (LDTI), which does not inhibit thrombin, we were able to construct three LDTI variants inhibiting thrombin. Trimming of the inhibitor reactive site loop to fit thrombin's narrow active site cleft resulted in inhibition constants (Ki) in the 10 nM concentration range; similar values were obtained by the addition of an acidic C-terminal peptide corresponding to hirudin's tail to LDTI. Combination of both modifications is additive, resulting in very strong inhibition of thrombin (Ki in the picomolar range). On the one hand, these results confirm the significance of the restricted active site cleft of thrombin in determining its high cleavage specificity; on the other, they demonstrate that sufficient binding energy at the fibrinogen recognition exosite can force thrombin to accept otherwise unfavorable residues in the active site cleft. The best inhibitor thus obtained is as effective as hirudin in plasma-based clotting assays.

The molecular genetics of familial venous thrombosis

This study of naturally occurring mutations predisposing to venous thrombosis has led to a number of important advances in our understanding of protein structure and function relationships and the molecular basis of gene mutation. It has also potentiated the accurate and reliable presymptomatic and antenatal detection of predisposing gene lesions. Perhaps the major challenge facing us is the probabilistic nature of thromboembolism; only a certain proportion of patients with recognized gene defects predisposing to thrombosis will actually suffer from thrombotic episodes. Environmental insults of various kinds, and perhaps epistatic effects resulting from the influence of other loci, are likely to be contributory factors and will help to determine whether a thrombotic event occurs in individuals already compromised by a defect in a gene whose malfunction is known to predispose to thrombosis. Since molecular genetic techniques allow us to dissect the allelioheterogeneity of the different deficiency states by characterizing the wide spectrum of gene mutations giving rise to thrombosis, it may eventually prove possible to relate specific gene lesions to the probability of thromboembolism as well as to the severity and frequency of thrombotic episodes. The multifactorial nature of thrombosis demands a multidisciplinary approach to the analysis of its causation, early detection, treatment and prevention. The application of the new and powerful techniques of molecular genetics promises to make a substantial contribution to all aspects of thrombosis research.

The interaction of glycosaminoglycans with heparin cofactor II

The binding sites for dermatan sulfate and heparin in HCII overlap but are not identical. This may explain the observation that HCII binds nonspecifically to heparin oligosaccharides, but preferentially binds to a minor hexasaccharide isolated from dermatan sulfate. The tissue distribution of dermatan sulfate molecules containing the high-affinity HCII binding site may regulate HCII activity in vivo. Finally, in the presence of dermatan sulfate or heparin, the N-terminal acidic region of HCII may interact with the hirudin-binding site of thrombin to produce maximal stimulation of the thrombin-HCII reaction.

Secondary thrombin-binding site, glycosaminoglycan binding domain and reactive center region of leuserpin-2 are strongly conserved in mammalian species

Using a combination of conventional and inverse polymerase chain reaction the cDNA structure coding for the thrombin inhibitor leuserpin-2 from rat has been determined. The rat genome specifies leuserpin-2 mRNAs differing in their 5'-untranslated regions probably due to alternative splicing of the primary transcript. Comparisons of partial sequences from five different mammalian species show that the secondary thrombin binding site, the glycosaminoglycan binding domain and the reactive center region of the inhibitor are strongly conserved.

Glycosaminoglycans and the regulation of blood coagulation

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The interaction of glycosaminoglycans with heparin cofactor II: structure and activity of a high-affinity dermatan sulfate hexasaccharide

The binding sites for dermatan sulfate and heparin in HCII overlap but are not identical. This may explain the observation that HCII binds non-specifically to heparin oligosaccharides but preferentially binds to a minor hexasaccharide isolated from dermatan sulfate having the structure shown in Fig. 4B. The tissue distribution of dermatan sulfate molecules containing the high-affinity HCII binding site may regulate HCII activity in vivo. Finally, in the presence of dermatan sulfate or heparin, the N-terminal acidic domain of HCII may interact with the hirudin-binding site of thrombin to produce maximal stimulation of the thrombin-HCII reaction.

The molecular genetics of familial venous thrombosis

Inherited defects of antithrombin III, protein C, protein S, heparin cofactor II, plasminogen and the fibrinogens are thought to be responsible for between 10 and 15% of all patients presenting with recurrent venous thrombosis. The structure, function and expression of these genes and the nature of the gene lesions underlying the deficiency states are reviewed in detail.

Complete nucleotide sequence of the gene for human heparin cofactor II and mapping to chromosomal band 22q11

Heparin cofactor II (HCII) is a 66-kDa plasma glycoprotein that inhibits thrombin rapidly in the presence of dermatan sulfate or heparin. Clones comprising the entire HCII gene were isolated from a human leukocyte genomic library in EMBL-3 lambda phage. The sequence of the gene was determined on both strands of DNA (15,849 bp) and included 1749 bp of 5'-flanking sequence, five exons, four introns, and 476 bp of DNA 3' to the polyadenylation site. Ten complete and one partial Alu repeats were identified in the introns and 5'-flanking region. The HCII gene was regionally mapped on chromosome 22 using rodent-human somatic cell hybrids, carrying only parts of human chromosome 22, and the chronic myelogenous leukemia cell line K562. With the cDNA probe HCII7.2, containing the entire coding region of the gene, the HCII gene was shown to be amplified 10-20-fold in K562 cells by Southern analysis and in situ hybridization. From these data, we concluded that the HCII gene is localized on the chromosomal band 22q11 proximal to the breakpoint cluster region (BCR). Analysis by pulsed-field gel electrophoresis indicated that the amplified HCII gene in K562 cells maps at least 2 Mbp proximal to BCR-1. Furthermore, the HCII7.2 cDNA probe detected two frequent restriction fragment length polymorphisms with the restriction enzymes BamHI and HindIII.

Type II hereditary angioneurotic edema that may result from a single nucleotide change in the codon for alanine-436 in the C1 inhibitor gene

Identical single-base changes in the C1 inhibitor gene that may result in dysfunctional inhibitor proteins are described in two different families with type II hereditary angioneurotic edema. Initially, a restriction fragment length polymorphism was defined that resulted from loss of a Pst I site within exon VIII, which encodes the region containing the reactive center. Exon VIII from the normal and abnormal allelles was amplified by the polymerase chain reaction. Amplified DNA product was cloned into plasmid pUC18; clones representing normal and mutant allelles were distinguished by the presence and absence, respectively, of the Pst I restriction site. DNA sequence analysis revealed a G----A mutation in the codon for alanine-436, which would result in replacement with a threonine residue. This position is nine amino acid residues amino-terminal to the reactive-center arginylthreonine peptide bond. In contrast, previously defined mutations in type II hereditary angioneurotic edema result in replacement of the reactive-center arginine.

Conformational lability of vitronectin: induction of an antigenic change by alpha-thrombin-serpin complexes and by proteolytically modified thrombin

We previously showed that the alpha-thrombin-antithrombin III complex causes antigenic change in vitronectin as monitored by the monoclonal anti-vitronectin antibody 8E6 (Tomasini & Mosher, 1988). We have extended these studies to other protease-serpin complexes and to gamma-thrombin, a proteolytic derivative of alpha-thrombin. In the presence of heparin, recognition of vitronectin by 8E6 was increased 64- or 52-fold by interaction with the complex of alpha-thrombin and heparin cofactor II or the Pittsburgh mutant (Met358----Arg) of alpha 1-protease inhibitor, respectively. This was comparable to the value obtained with the alpha-thrombin-antithrombin III complex. Factor Xa-serpin complexes were approximately 4-fold less effective than the corresponding thrombin complexes. alpha-Thrombin-serpin complexes but not Xa-serpin complexes formed disulfide-bonded complexes with vitronectin. Antigenic changes and disulfide-bonded complexes were not detected when trypsin- or chymotrypsin-serpin complexes were incubated with vitronectin. gamma-Thrombin caused 7- and 34-fold increases in recognition of vitronectin by MaVN 8E6 in the absence and presence of heparin, respectively. In contrast, alpha-thrombin by itself had no effect. The antigenic change induced by gamma-thrombin was maximal when gamma-thrombin and vitronectin were equimolar, was not dependent on cleavage of vitronectin, and was abolished by inhibition of gamma-thrombin with Phe-Pro-Arg-chloromethyl ketone but not with diisopropyl fluorophosphate. These data indicate that alpha-thrombin is the component in alpha-thrombin-serpin complexes that induces the antigenic change in vitronectin, probably via a region that is preferentially exposed in gamma-thrombin.

Anti-thrombin activities of heparin. Effect of saccharide chain length on thrombin inhibition by heparin cofactor II and by antithrombin

The interactions of two proteinase inhibitors, heparin cofactor II and antithrombin, with thrombin are potentiated by heparin. Using two methods, we have studied the potentiating effects of a series of heparin (poly)saccharides with high affinity for antithrombin and mean Mr ranging from approx. 1700 to 18,800. First, catalytic amounts of heparin (poly)saccharide were added to purified systems containing thrombin and either heparin cofactor II or antithrombin. Residual thrombin activity was determined with a chromogenic substrate. It was found that only the higher-Mr polysaccharides (Mr greater than 8000) efficiently catalysed thrombin inhibition by heparin cofactor II, there being a progressive catalytic effect with increasing Mr of the polysaccharide. Weak accelerating effects were noted with low-Mr saccharides (Mr less than 8000). This contrasted with the well-characterized interaction of heparin with antithrombin and thrombin, where heparin oligosaccharides of Mr less than 5400 had absolutely no ability to accelerate the reaction, while (poly)saccharides of Mr exceeding 5400 showed rapidly increasing catalytic activity with increasing Mr. Secondly, these and other heparin preparations were added in a wide concentration range to plasma with which 125I-labelled thrombin was then incubated for 30 s. Inhibited thrombin was determined from the distribution of labelled thrombin amongst inhibitor-thrombin complexes, predominantly antithrombin-thrombin and heparin cofactor II-thrombin complexes. In this situation, where the inhibitors competed for thrombin and for the (poly)saccharides, it was found that, provided the latter were of high affinity for antithrombin and exceeded a Mr of 5400, thrombin inhibition in plasma was mediated largely through antithrombin. Polysaccharides of Mr exceeding 8000 that were of low affinity for antithrombin accelerated thrombin inhibition in plasma through their interaction with heparin cofactor II. High concentrations of saccharides of Mr 1700-5400 exhibited a size-dependent acceleration of thrombin inhibition, not through their interaction with antithrombin, but through their interaction with heparin cofactor II.

Molecular modeling of protein-glycosaminoglycan interactions

Forty-nine regions in 21 proteins were identified as potential heparin-binding sites based on the sequence organizations of their basic and nonbasic residues. Twelve known heparin-binding sequences in vitronectin, apolipoproteins E and B-100, and platelet factor 4 were used to formulate two search strings for identifying potential heparin-binding regions in other proteins. Consensus sequences for glycosaminoglycan recognition were determined as [-X-B-B-X-B-X-] and [-X-B-B-B-X-X-B-X-] where B is the probability of a basic residue and X is a hydropathic residue. Predictions were then made as to the heparin-binding domains in endothelial cell growth factor, purpurin, and antithrombin-III. Many of the natural sequences conforming to these consensus motifs show prominent amphipathic periodicities having both alpha-helical and beta-strand conformations as determined by predictive algorithms and circular dichroism studies. The heparin-binding domain of vitronectin was modeled and formed a hydrophilic pocket that wrapped around and folded over a heparin octasaccharide, yielding a complementary structure. We suggest that these consensus sequence elements form potential nucleation sites for the recognition of polyanions in proteins and may provide a useful guide in identifying heparin-binding regions in other proteins. The possible relevance of protein-glycosaminoglycans interactions in atherosclerosis is discussed.

Synthetic peptide inhibitors of complement serine proteases--I. Identification of functionally equivalent protease inhibitor sequences in serpins and inhibition of C1s and D

Sequence homology comparisons between serum serine protease inhibitors led to the prediction that the C-terminal sequences are functionally equivalent and represent an essential protease binding domain. Inhibition of complement serine protease D cleavage of factor B and of C1s cleavage of C4 by synthetic peptides containing sequences from the C-termini of three serum serine protease inhibitors supports this prediction. These functionally equivalent peptides represent a new class of inhibitors of D and C1s as well as other serum serine proteases.

Studies on the structural requirements of heparin for the catalysis of thrombin inhibition by heparin cofactor II

The structural requirements of heparin for the catalysis of thrombin inhibition by heparin cofactor II (HC II) were investigated. A series of well characterized heparin derivatives were prepared and their activities were measured using human thrombin in the presence of an excess of purified human HC II and, for comparison, antithrombin III (AT III). The 50% inhibitory concentrations of each derivative were calculated and compared with those of unmodified heparin. Heparin activity was strongly dependent on molecular weight (Mr) in a manner grossly comparable for the two inhibitors. High-Mr fractions were the most active. Below 10 kDa, the activity dropped rapidly. A minimum size of 26 residues appeared to be required for HC II activation (against 16-18 for AT III). Below 5 kDa, a residual activity two orders of magnitude lower than that of high-Mr species remained with HC II (but not with AT III). Heparin was selectively desulfated or oversulfated in the O- and/or N-position. When an N-acetyl group was substituted for the original N-sulfate in the glucosamine and the derivative was oversulfated in the O-position, a strong activity with HC activities with both inhibitors decreased when the overall sulfate content (i.e., the charge density) was reduced, and vice-versa. Carboxyl-reduced heparin was also inactive but activity could be restored after O-sulfation. Our results thus suggest that, unlike the case of AT III, no functional group in heparin is critical for optimal thrombin inhibition by HC II. Sulfate and carboxylate are important in as much as they contribute to the global charge of the molecule.

Molecular cloning of the mouse angiotensinogen gene

We describe here the cloning, restriction mapping, and sequencing of the mouse angiotensinogen gene. The 5' flanking region contains consensus sequences for several hormone-responsive elements and viral-like enhancers within 750 bp of the cap site. The deduced amino acid sequence shows 87% identity with rat angiotensinogen, but there is a discrepancy in the number of cysteine residues in the mature protein among rat (n = 3), human (n = 4), and mouse (n = 4). Because angiotensinogen is homologous to other members of the serine protease inhibitor family, we aligned the putative reactive center of angiotensinogens from various species. This alignment shows that the inhibitor site in human angiotensinogen is different from its rodent counterpart, but the role of this sequence divergence in the pathogenesis of human disease remains to be established.

Structure-function relationships in heparin cofactor II: spectral analysis of aromatic residues and absence of a role for sulfhydryl groups in thrombin inhibition

This study characterizes the structural and functional significance of sulfhydryl residues in human plasma heparin cofactor II (HCII). For quantification of sulfhydryl groups, the extinction coefficient of HCII was redetermined and found to be 0.593 ml mg-1 cm-1 using second-derivative spectroscopy and multicomponent analysis assuming 4, 10, and 2 residues of tryptophan, tyrosine, and tyrosine-O-sulfate per mole of protein, respectively. The results show that tyrosine-O-sulfate residues in HCII and in cholecystokinin peptide fragments (as model compounds) do not significantly contribute to the absorbance spectrum from 280 to 300 nm. A total of three sulfhydryl groups per mole of HCII was detected by Ellman's reagent titration, with or without treatment with dithioerythritol, indicating the absence of intramolecular disulfide bonds. Incubation of HCII with 0.1-10 mM dithioerythritol did not diminish its heparin-enhanced thrombin inhibition activity. Treatment with various sulfhydryl-specific reagents, including p-mercuribenzoate, HgCl2, and N-substituted maleimide derivatives, inactivated HCII. Titration with Ellman's reagent after these reactions identified the modification site as a cysteinyl residue(s). However, complete methanethio derivatization of the sulfhydryl groups of HCII using methyl methanethiosulfonate did not alter heparin-catalyzed thrombin inhibition. These results indicate that the sulfhydryl groups of HCII are not essential for thrombin inhibition. HCII differs from antithrombin III, which contains an essential disulfide bond for heparin-dependent thrombin inhibition (Longas, M. O., et al. (1980) J. Biol. Chem. 255, 3436). Furthermore, within the "serpin" (serine proteinase inhibitor) superfamily, HCII resembles chicken ovalbumin in occurrence of sulfhydryl residues and reactivity with various sulfhydryl group-directed compounds.

Amino-acid sequence of human alpha 2-antiplasmin

The amino-acid sequence of human alpha 2-antiplasmin was determined by Edman degradation of peptides purified from CNBr, tryptic and chymotryptic digests. Of the total sequence of 452 amino acids of mature alpha 2-antiplasmin, as deduced from the cDNA sequence [Holmes et al. (1987) J. Biol. Chem. 262, 1659-1664], 444 residues were identified by amino-acid sequencing. Two differences were found between the peptide and cDNA analyses (Gly instead of Leu at position 10 and Gly instead of Ser at position 369). alpha 2-Antiplasmin contains two disulfide bridges (Cys64-Cys104 and Cys31-Cys113) and four glucosamine-based carbohydrate chains attached to Asn87, Asn256, Asn270 and Asn277. alpha 2-Antiplasmin is homologous with 12 other proteins belonging to the serine protease inhibitor (serpin) superfamily.

Expression of human antithrombin III in Saccharomyces cerevisiae and Schizosaccharomyces pombe

Recombinant plasmids were constructed that direct the synthesis of human antithrombin III in baker's yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe. The signal sequence of antithrombin III was recognized by both yeast species, and antithrombin III was secreted into the medium. When the signal sequence was replaced by a sequence of ten arbitrary amino acids, the product expressed from such a construct stayed inside the cell. Antithrombin III was glycosylated by the baker's and fission yeast and was immunologically identical to antithrombin III isolated from human plasma. Antithrombin III isolated from the culture media of recombinant yeasts was biologically active, as could be shown by progressive inhibitor activity and heparin cofactor activity.