Abstract:
A modified heparin cofactor II is disclosed, HCII(Leu 444  →Arg), which has substantially improved inhibitory activity against thrombin compared to native HCII or antithrombin.

Description:
This is a division of application Ser. No. 07/424,440, filed Oct. 20, 1989. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to heparin cofactor II and, more particularly, to a modified heparin cofactor II that has substantially improved inhibitory activity against thrombin compared to native heparin cofactor II or antithrombin. 
     The anticoagulant activities of glycosaminoglycans are mediated by antithrombin (AT) and heparin cofactor II (HCII), which are members of the `serpin` family of serine protease inhibitors [Tollefsen in Heparin: chemical and biological properties, clinical applications (eds. Lane and Lindahl), Edward Arnold, London, 1989, pp. 257-273]. HCII inhibits thrombin but has no activity against other proteases involved in coagulation or fibrinolysis [Parker and Tollefsen, J. Biol. Chem. 260, 3501-3505 (1985)]. In contrast, AT inhibits all of the proteases of the intrinsic coagulation pathway as well as the fibrinolytic protease plasmin [Rosenberg in The molecular basis of blood diseases (eds. Stamatoyannopoulos et al.), W. B. Saunders Co., Philadelphia, 1987, pp. 534-574]. Heparin and dermatan sulfate increase the rate of thrombin inhibition by HCII˜1000-fold by providing a catalytic template to which both the inhibitor and the protease bind [Tollefsen et al., J. Biol. Chem. 258, 6713-6716 (1983); Griffith, Proc. Natl. Acad. Sci. USA 80, 5460-5464 (1983)]. Although heparin catalyzes protease inhibition by both HCII and AT, the anticoagulant effect of dermatan sulfate is mediated exclusively by HCII. 
     HCII is a 65,600 dalton plasma glycoprotein. Using conventional recombinant DNA gene splicing methods, the cDNA sequence and corresponding amino acid sequence of HCII were determined and the protein was expressed in E. coli by Blinder et al., Biochemistry 27, 752-759 (1988). The cDNA consisted of 2215 base pairs (bp), including an open-reading frame of 1525 bp, a stop codon, a 3&#39;-noncoding region of 654 bp, and a poly(A) tail. The deduced amino acid sequence contained a signal peptide of 19 amino acid residues and a native protein of 480 amino acids. The published cDNA and protein sequences are also shown hereinbelow. 
     HCII forms a stable 1:1 complex with thrombin or chymotrypsin in which the protease is inactive. During complex formation, both thrombin and chymotrypsin attack the reactive site Leu 444  -Ser 445  peptide bond (designated P1--P1&#39;) near the C-terminal end of HCII [Church et al., Proc. Natl. Acad. Sci. USA 82, 6431-6434 (1985); Griffith et al., J. Biol. Chem. 260, 2218-2225 (1985); Griffith et al., Biochemistry 24, 6777-6782 (1985)]. The resulting complex does not dissociate when heated at 100° C. with sodium dodecyl sulfate, suggesting that under these conditions the two proteins become linked by a covalent bond. The bond is presumed to be an ester linkage between the serine hydroxyl group in the active site of the protease and the carbonyl group of Leu 444  in HCII. Therefore, HCII can be thought of as a pseudosubstrate for these proteases. 
     The P1 residue in the reactive site of a `serpin` appears to play a major role in determining the relative rates of inhibition of various proteases [Carrell and Travis, TIBS 10, 20-24 (1985)]. Evidence for this was derived from analysis of the variant α1-antitrypsin Pittsburgh, which was discovered in a child who had a fatal bleeding disorder. The variant contained an arginine residue in place of Met 358  at the P1 position of the inhibitor [Owen et al., N. Engl. J. Med. 309, 694-698 (1983)]. This substitution resulted in markedly increased rates of inhibition of several coagulation proteases, including thrombin, factor Xa, factor XIa, and kallikrein, which cleave Arg-X peptide bonds in their natural substrates [Schapira et al., J. Clin. Invest. 77, 635-637 (1986); Scott et al., Ibid. 77, 631-634 (1986); Travis et al., Biol. Chem. Hoppe-Seyler 367, 853-859 (1986)]. A reciprocal decrease in the rate of inhibition of neutrophil elastase, the principal target protease of native α1-antitrypsin, was also noted. 
     Heparin is commonly employed in the prophylaxis and treatment of venous thrombosis and pulmonary embolism, but its use is sometimes complicated by severe bleeding or thrombocytopenia. Therefore, alternatives to heparin for short-term anticoagulation would be desirable. In this regard, a modified HCII which could inhibit thrombin rapidly in the absence of heparin would have significant value and be useful for the treatment of thrombotic disorders. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with the present invention a novel modified HCII has been developed which has substantially improved inhibitory activity against thrombin compared to native HCII or antithrombin. These improved results of about 100-fold increased inhibitory activity were unexpected in view of the single site mutation of the native HCII which characterizes the novel modified HCII. For convenience, the modified HCII of this invention can be represented as HCII(Leu 444  →Arg). In this mutation, the leucine residue at position 444 is replaced by a positively charged arginine in the published 480 amino acid sequence of native HCII. 
     The modified HCII(Leu 444  →Arg) of this invention inhibited thrombin at a 98-fold higher rate (k 2  =6.2×10 6  M -1  min -1 ) than native HCII (k 2  =6.3×10 4  M -1  min -1 ). Dermatan sulfate accelerated thrombin inhibition by both the native and modified forms of HCII, but the maximum rate constant in the presence of dermatan sulfate was only 2-fold higher for HCII(Leu 444  →Arg)(k 2  =5.3×10 8  M -1  min -1 ) than for native HCII (k 2  =2.2×10 8  M -1  min -1 ). Heparin was less effective than dermatan sulfate in stimulating both forms of HCII. Factor Xa and kallikrein were inhibited more rapidly and chymotrypsin more slowly by HCII(Leu 444  →Arg) than by native HCII. The rapid rate of inhibition of thrombin by HCII(Leu 444  →Arg) in the absence of heparin or dermatan sulfate suggests that this variant may be useful as a therapeutic agent. 
     The single site mutation of HCII as defined herein is illustrated in a recombinant HCII (rHCII). However, the amino acid sequence of rHCII is identical to that of HCII purified from human plasma. Thus, as defined herein the single site mutation HCII(Leu 444  →Arg) can be made on a recombinant HCII or a synthetically produced HCII. 
     Conventional recombinant DNA procedures can be used to prepare the novel HCII mutant of this invention. The starting material can be a conventional DNA construct or vector comprising a DNA sequence encoding for the entire sequence of native HCII. The reactive site mutant rHCII(Leu 444  →Arg) is then prepared by replacing the codon for leucine at site 444 with the codon for arginine. 
     Thus, the DNA polymer can be prepared by the enzymatic ligation of chemically synthesized fragments. The DNA polymer which encodes the modified HCII may be prepared by site-directed mutagenesis of the cDNA which codes for HCII by conventional methods such as those described by Winter et al., Nature 299, 756-758 (1982) or Zoller and Smith, Nucl. Acids Res. 10, 6487-6500 (1982). Expression of the modified HCII can be carried out by conventional methods. 
     A suitable starting material for preparing the HCII(Leu 444  →Arg) of this invention is the known plasmid expression vector pMON-5840 as described by Blinder et al., J. Biol. Chem. 264, 5128-5133 (1989). This plasmid is a variant of pMON5527 which, in addition to an irrelevant sequence, contains the recA promoter (P rec  A) and the ribosome-binding site, T7 phage gene 10 leader RNA (G10-L RBS), and is suitable for enhanced expression of foreign genes in E. coli, as further described by Olins et al., Gene 73, 227-235 (1988); Olins and Rangwala, J. Biol. Chem. 264, 16973-16976 (1989). The recA promoter in pMON-5840 is derived as the HpaII fragment and corresponds to nucleotides 63-210 of the sequence published by Horii et al., Proc. Natl. Acad. Sci. USA 77, 313-317 (1980). The cDNA for HCII(Leu 444  →Arg) can be ligated into the parent vector which can then be used for expression of the rHCII(Leu 444  →Arg) by conventional procedures, e.g. in E. coli JM101 cells. 
     DETAILED DESCRIPTION OF THE INVENTION 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as forming the present invention, it is believed that the invention will be better understood from the following detailed description of preferred embodiments of the invention taken in conjunction with the appended drawings in which: 
    
    
     FIG. 1 is a schematic diagram of the expression vector pMON-HCII. HCII cDNA (shaded rectangle) was altered in M13 to remove a SalI site and create a BstEII site. Cassette mutagenesis between BstEII and SalI was performed to create the reactive site mutant pMON-HCII(Leu 444  →Arg). The open rectangle denotes the 3&#39;-untranslated sequence from the HCII cDNA. The RecA promoter (P recA) and G10L translational control element, derived from the T7 gene, are indicated, along with the deletion of a SalI site in the parent vector pMON-5840. The codons and amino acids at the translation initiation site and in the reactive site cassette are shown. 
     FIG. 2 shows the immunoblot of pMON-HCII, pMON-HCII(Leu 444  →Arg) pMON-5840 lysates with anti-HCII antibody. pMON-HCII, pMON-HCII(Leu 444  →Arg) and pMON-5840 were expressed in E. coli and subjected to heparin-Sepharose® chromatography, as described below under &#34;Procedures.&#34; The partially purified rHCII, rHCII(Leu 444  →Arg) and control lysate were immunoblotted and probed with  125  I-labelled rabbit anti-HCII antibodies. An autoradiograph of the immunoblot is shown. The amount (μl) of partially purified lysate loaded in each lane is indicated. Molecular weight markets are shown on the right hand side. 
     FIG. 3 is a graphical representation which shows the inhibition of thrombin, chymotrypsin, factor Xa and kallikrein by rHCII and rHCII(Leu 444  →Arg). Heparin-Sepharose column eluates (90 μl) of rHCII (142 nM), rHCII(Leu 444  →Arg) (76 nM) or pMON-5840 control were incubated with thrombin (15 nM), chymotrypsin (21 nM), factor Xa (16 nM), or kallikrein (10 nM) for the indicated amount of time at the stated final concentrations. The % protease activity was calculated from the ratio of final to initial protease activity, determined by adding the appropriate chromogenic substrate and measuring Δ405/min as described below under &#34;Procedures.&#34;  , Control; O, Native rHCII;  , rHCII(Leu 444  →Arg). 
     FIG. 4 is a graphical representation which shows the effect of glycosaminoglycan concentration on the second-order rate constant of thrombin inhibition by rHCII and rHCII(Leu 444  →Arg). Second-order rate constants were determined by incubating heparin-Sepharose column eluates (90 μl) of rHCII (142 nM) or rHCII(Leu 444  →Arg) (76 nM) with thrombin (15 nM) and either heparin or dermatan sulfate at the indicated final concentrations. The rate constants were calculated as described below under &#34;Procedures.&#34; After times ranging from 5 seconds to 5 minutes remaining thrombin activity was determined by adding Chromozym TH and measuring Δ405/min. The rate constants with dermatan sulfate are the average of three similar tests. The heparin-stimulated rate constants are from a single determination. O, rHCII plus heparin;  , rHCII(Leu 444  →Arg) plus heparin; □,rHCII plus dermatan sulfate;  , rHCII(Leu 444  →Arg) Arg) plus dermatan sulfate. 
     In order to illustrate the invention in greater detail, the following exemplary laboratory preparative work was carried out and the results obtained as described and shown in Table 1 and the accompanying drawings. 
    
    
     EXAMPLES 
     Procedures 
     Materials--Chromogenic substrates were purchased from the following sources: tosyl-Gly-Pro-Arg-p-nitroanilide (Chromozym TH), Boehringer Mannheim; N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilde, Sigma; methoxycarbonyl cyclohexylglycyl-Gly-Arg-p-nitroanilde (Spectrozyme FXa), American Diagnostica; and Pro-Phe-Arg-p-nitroanilide (S-2302), KabiVitrum. Thrombin and HCII derived from human plasma were prepared and assayed by conventional procedures as previously described by Blinder et al., J Biol. Chem. 264, 5128-5133 (1989). Human coagulation factor X a  was provided by Dr. George Broze, Jr., Washington University, St. Louis, Mo. Human chymotrypsin and plasma kallikrein were purchased from Sigma. Affinity purified rabbit anti-HCII antibodies were prepared and labeled with Na[ 125  I] by conventional procedure as described previously by Blinder et al., Biochemistry 27, 752-759 (1988). Bovine lung heparin and porcine skin dermatan sulfate were purchased from Sigma; the dermatan sulfate was treated with nitrous acid to remove contaminating heparin prior to use [Teien et al., Thromb. Res. 8, 859-867 (1976)]. Heparin-Sepharose was prepared by commonly known procedure as described previously by Tollefsen et al., J Biol. Chem. 257, 2162-2169 (1982). 
     The reagents used for oligonucleotide directed mutagenesis were purchased from Amersham. Restriction enzymes were obtained from Promega, Amersham, New England Biolabs, and Bethesda Research Laboratories. Reagents for the Taq polymerase chain reaction were purchased from Perkin Elmer Cetus. DNA sequencing was performed using T7 DNA polymerase from U.S. Biochemical Corporation. Deoxyadenosine [α 35  S]-thiotriphosphate([α 35  S]dATP) was obtained from Dupont-New England Nuclear. The Protein Chemistry Facility of Washington University, St. Louis, Mo., provided the synthetic oligonucleotides. 
     Construction of the pMON-HCII expression vector--Previously, the PstI-HindIII fragment of the HCII cDNA was inserted into pMON-5840 to yield a construct in which the N-terminal 18 amino acid residues of mature plasma HCII were replaced by the sequence Met-Ala- as described by Blinder et al., J. Biol. Chem. 264, 5128-5133 (1989). Although the resulting inhibitor was active by several criteria, it was desired to obtain a full-length rHCII for the purposes of the present invention. In addition, it was decided to construct unique SalI and BstEII restriction sites for cassette mutagenesis of the reactive site. The SalI site in the parent vector pMON-5840 was deleted to form pMON-5840(ΔSalI) by digestion with SalI, treatment with mung bean exonuclease, and religation. A full-length cDNA for HCII was isolated previously from a human fetal liver library in λgt11 as described by Blinder et al., Biochemistry 27, 752-759 (1988). Oligonucleotide-directed mutagenesis of the cDNA was performed in M13mp18 phage vector containing the non-coding strand by the published method of Nakamaye and Eckstein, Nucl. Acids Rev. 14, 9679-9698 (1986). The codon for Val 77  was altered (GTC→GTT) to remove a SalI site at that position, and the codon for Thr 434  was altered (ACT→ACG) to construct a BstEII site. Using the polymerase chain reaction with appropriate primers [Saiki et al., Science 239, 487-491 (1988)], an NcoI site (CCATGG) was incorporated into the cDNA so that the ATG of the NcoI site immediately preceded the codon for Gly 1  of mature plasma HCII. The amplified cDNA was then ligated into pMON-5840(ΔSalI). The final construct, designated pMON-HCII, is diagrammed in FIG. 1. 
     Cassette mutagenesis of pMON-HCII--pMON-HCII was digested with SalI and BstEII to remove a fragment of DNA containing the codons for Val 436  -Val 452 , which span the reactive site. The vector was then treated with alkaline phosphatase. Complementary oligonucleotides (48- and 49-mers) containing the codon for arginine at position 444 were phosphorylated with T4 kinase, preannealed, and then ligated to the vector. E. coli strain JM101 cells were transformed with the vector, and colonies were selected for ampicillin resistance. The sequence of the final plasmid was verified using the dideoxy chain termination method of Sanger et al., Proc. Natl. Acad. Sci. USA 74, 5463-5467 (1977). 
     Expression and quantification of native rHCII and rHCII(Leu 444  →Arg)--For each rHCII preparation, transformed E. coli were grown to an optical density (550 nm) of 1.0-1.2 in 500 ml of medium and expression was induced with nalidixic acid according to Li et al., J. Biol. Chem. 262, 13773-13779 (1987). The cells were then sedimented, rinsed once in 0.05M NaCl, 0.05M Tris-HCl, pH 7.5, resuspended in 25 ml of the same buffer, and lysed by sonication [Blinder et al., Biochemistry 27, 752-759 (1988)]. To partially purify the rHCII, the cell lysate was applied to a 30 ml column of heparin-Sepharose equilibrated with the same buffer. After the column was washed, the rHCII was eluted with 1.0M NaCl, 0.05M Tris-HCl, pH 7.5. The eluate was dialyzed against 0.05M NaCl, 0.05M Tris-HCl, pH 7.5, and stored frozen. 
     The concentration of rHCII was determined by a slot blot immunoassay. Various amounts of the partially purified rHCII were blotted onto nitrocellulose, and rHCII was detected with  125  I-labeled anti-HCII antibodies as described previously by Blinder et al., Ibid. After autoradiography, the bands were scanned with a densitometer and the areas under each peak were determined (LKB Ultroscan). A standard curve constructed with known amounts of plasma HCII was linear from 2.5 to 30 ng. The standard deviation of multiple determinations of plasma HCII using this method was 20%. 
     Inhibition of proteases by rHCII--Protease inhibition by rHCII was determined by incubating 90 μl of the partially purified rHCII (or the corresponding heparin-Sepharose fractions of a control lysate from cells transformed with pMON-5840) with 5 μl of the protease and 5 μl of either glycosaminoglycan or water in a disposable polystyrene cuvette at room temperature. Final concentrations of the proteases were as follows: thrombin, 15 nM; chymotrypsin, 21 nM; factor Xa, 16 nM; and kallikrein, 10 nM. After a specified period of time from 5 seconds (s) to 150 min, 300-500 μl of the appropriate chromogenic substrate was added, and the absorbance at 405 nm was recorded continuously for 100 s. The rate of change of absorbance was proportional to the concentration of active protease remaining in the incubation. The amounts added and concentrations of each substrate were as follows: Chromozym TH (thrombin), 500 μl, 0.1 mM; N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilde (chymotrypsin), 300 μl, 0.5 mM; Spectrozyme FXa (factor Xa), 400 μl, 0.125 mM; and S-2302 (kallikrein), 300 μl, 0.4 mM. 
     Results 
     Expression vectors for rHCII--A diagram of the HCII expression vector (pMON-HCII) is shown in FIG. 1. The mRNA produced by this vector contains an initiator Met codon followed by the nucleotide sequence encoding the mature polypeptide (i.e., minus signal peptide) of native HCII. Edman degradation of rHCII purified from a cell lysate revealed that the sequence begins with Gly 1 , indicating that the initiating Met residue is removed during biosynthesis in E. coli. Thus, the amino acid sequence of native rHCII is identical to that of HCII purified from human plasma. The reactive site mutant rHCII(Leu 444  →Arg) was prepared by insertion of the appropriate synthetic oligonucleotide cassette into pMON-HCII digested with BstEII and SalI as indicated in FIG. 1. 
     Immunoblot and quantification of rHCII--FIG. 2 shows the autoradiograph of an immunoblot of lysates of E. coli transformed with pMON-5840 (parent vector lacking the HCII cDNA), pMON-HCII, or pMON-HCII(Leu 444  →Arg). Each lysate was subjected to heparin-Sepharose chromatography as described above under &#34;Procedures&#34; prior to immunoblotting. The immunoblot was probed with  125  I-labeled rabbit anti-HCII antibodies. The figure indicates that the rHCII expressed by pMON-HCII(Leu 444  →Arg) is identical in size to that expressed by pMON-HCII and has an apparent molecular weight of ˜60,000. A minor band (MW=43,000) containing 5-10% of the HCII antigen is also present in both lysates and may represent a degraded form of the protein. No HCII antigen is present in a lysate of E. coli transformed with pMON-5840. The level of expression of rHCII was determined by densitometry of immunoblots standardized with known amounts of purified plasma HCII. The rHCII concentration in the pMON-HCII and pMON-HCII(Leu 444  →Arg) lysates (after heparin-Sepharose chromatography) were 158 nM and 84 nM, respectively. 
     Protease inhibition by rHCII in the absence of a glycosaminoglycan--The time courses of inhibition of thrombin, factor Xa, kallikrein, and chymotrypsin by native rHCII(Leu 444  →Arg) in the absence of a glycosaminoglycan are indicated in FIG. 3. The incubations were performed under pseudo-first-order conditions, such that the initial concentration of rHCII was greater than the concentration of protease. The pseudo-first-order rate constants (k 1 ) for protease inhibition were determined by fitting the data to the following equation: k 1  t=1n([P] 0  /[P] t ), in which [P] 0  =initial prolease activity and [P] t  =protease activity at time=t. The second-order rate constants (k 2 ) were then calculated by dividing k by the initial rHCII concentration. The rate constants are summarized in Table 1. It was observed that rHCII(Leu 444  →Arg) inhibits thrombin almost 100 times more rapidly than native rHCII in the absence of a glycosaminoglycan Furthermore, rHCII(Leu 444  →Arg) inhibits factor Xa and kallikrein, while inhibition of these proteases by native rHCII is undetectable. In contrast to native rHCII, rHCII(Leu 444  →Arg) does not inhibit chymotrypsin at a detectable rate. 
     Inhibition of thrombin by rHCII in the presence dermatan sulfate or heparin--The time course of inhibition of thrombin was determined in the presence of various concentrations of dermatan sulfate or heparin. The second-order rate constants derived from these tests are shown in FIG. 4. Maximum rate constants were observed at 25-100 μg/ml of either glycosaminoglycan (Table 1). Dermatan sulfate produces approximately a 3500-fold increase in the rate constant for thrombin inhibition by native rHCII but has a much smaller effect on the rate constant obtained with rHCII(Leu 444  →Arg). Thus, rHCII(Leu 444  →Arg) inhibits thrombin only about twice as fast as native rHCII in the presence of dermatan sulfate. Heparin increases the rate constant for thrombin inhibition by native rHCII about 40-fold but has a minimal effect on the rate constant obtained with rHCII(Leu 444  →Arg). 
     Inhibition of chymotrypsin, factor Xa and kallikrein by rHCII in the presence of dermatan sulfate or heparin--Dermatan sulfate does not significantly increase the rate of inhibition of chymotrypsin by native rHCII, nor does it increase the rate of inhibition of factor Xa or kallikrein by rHCII(Leu 444  →Arg) (Table 1). Heparin (100 μg/ml) appears to protect factor Xa and kallikrein from inhibition by rHCII(Leu 444  →Arg). 
     
                                           TABLE 1__________________________________________________________________________Second-order rate constants (k.sub.2) for inhibition of proteases byrHCII  native rHCII        rHCII(Leu.sub.444 →Arg)         + Dermatan          + DermatanProtease  No GAG Sulfate                + Heparin                      No GAG Sulfate                                    + Heparin__________________________________________________________________________  k.sub.2 (M.sup.-1 min.sup.-1)Thrombin  6.3 × 10.sup.4 a         2.2 × 10.sup.8 b                2.4 × 10.sup.6 c                      6.2 × 10.sup.6 a                             5.3 × 10.sup.8 b                                    1.6 × 10.sup.7 cChymotrypsin  2.2 × 10.sup.5 a         3.7 × 10.sup.5 c                2.4 × 10.sup.5 c                      &lt;1 × 10.sup.4 b                             &lt;1 × 10.sup.4 c                                    N.D.Factor Xa  &lt;1 × 10.sup.3 b         &lt;1 × 10.sup.3 b                N.D.  4.4 × 10.sup.4 c                             6.2 × 10.sup.4 b                                    2.1 × 10.sup.4 bKallikrein  &lt;3 × 10.sup.3 c         N.D.   N.D.  1.3 × 10.sup.5 c                             1.4 × 10.sup.5 c                                    1.0 × 10.sup.4__________________________________________________________________________                                    c The rate constants were determined as described above under &#34;Procedures&#34; and are derived from the data in FIGS. 2 and 3. The maximum rate constant obtained in the presence of 25-100 μg/ml of dermatan sulfate or hepari are given. GAG, glycosaminoglycan. N.D., not determined. .sup.a Average of three determinations. .sup.b Average of two determinations. .sup.c Single determination. ##STR1## 
    
     Various other examples will be apparent to the person skilled in the art after reading the present disclosure without departing from the spirit and scope of the invention. It is intended that all such other examples be included within the scope of the appended claims.