Thrombomodulin-independent activation of protein C and specificity of hemostatically active snake venom serine proteinases: crystal structures of native and inhibited Agkistrodon contortrix contortrix protein C activator

Protein C activation initiated by the thrombin-thrombomodulin complex forms the major physiological anticoagulant pathway. Agkistrodon contortrix contortrix protein C activator, a glycosylated single-chain serine proteinase, activates protein C without relying on thrombomodulin. The crystal structures of native and inhibited Agkistrodon contortrix contortrix protein C activator determined at 1.65 and 1.54 A resolutions, respectively, indicate the pivotal roles played by the positively charged belt and the strategic positioning of the three carbohydrate moieties surrounding the catalytic site in protein C recognition, binding, and activation. Structural changes in the benzamidine-inhibited enzyme suggest a probable function in allosteric regulation for the anion-binding site located in the C-terminal extension, which is fully conserved in snake venom serine proteinases, that preferentially binds Cl(1-) instead of SO(4)(2-).     

Primary structure of a protein C activator from Agkistrodon contortrix contortrix venom

The amino acid sequence of a protease, protein C activator, from Agkistrodon contortrix contortrix venom was determined. Peptide fragments obtained by chemical or enzymatic cleavage of the S-carboxymethylated protein were purified by gel filtration and reverse-phase high-performance liquid chromatography. The present study demonstrates that protein C activator from A. contortrix contortrix venom is a trypsin-type serine protease that is composed of 231 residues with a molecular weight of 25,095 for the polypeptide portion of the molecule. By analogy to the mammalian serine proteases, the catalytic triad in venom protein C activator consists of His-40, Asp-85, and Ser-177. The protein also contains three N-linked glycosylation sites at Asn-21, Asn-78, and Asn-129. The amino acid sequence of protein C activator exhibits a high degree of sequence identity with other snake venom proteases: 73% with batroxobin, 68% with flavoxobin, and 55% with Russell's viper venom factor V activator.     

Purification of a protein C activator from the venom of the southern copperhead snake (Agkistrodon contortrix contortrix)

A protease has been purified by ion-exchange chromatography from the venom of Agkistrodon contortrix contortrix (Southern copperhead snake) that can activate the vitamin K dependent protein, protein C. The apparent molecular weight of this protease, determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was 20,000 under nonreducing conditions. Incubation of this protease with plasma resulted in a prolongation of the clotting time and a time-dependent increase in amidolytic activity. Incubation of the protease with purified protein C resulted in an increase in both amidolytic and anticoagulant activity. The protease had no inhibitory effect on thrombin, factor V, fibrinogen, or factor X. It had slight clotting activity toward fibrinogen. The apparent Km of the protease for protein C was 0.28 microM. Calcium ions were observed to inhibit protein C activation with an apparent Ki of 0.2 mM. Ethylenediaminetetraacetic acid, diisopropyl fluorophosphate, and soybean trypsin inhibitor were observed to inhibit the venom protease. These results suggest that the venom of the Southern copperhead snake contains a protease that is a specific activator of protein C.     

Characterization of a protein C activator from the venom of Agkistrodon contortrix contortrix

An enzyme capable of activating protein C has been purified 60-fold from the venom of the Southern copperhead snake (Agkistrodon contortrix) by ion-exchange and gel filtration chromatography. The purified enzyme consists of a single polypeptide with an apparent molecular weight of 37,000. The isoelectric point of the protein C activator was determined to be 6.3 when measured by chromatofocusing. The enzyme was inhibited by p-nitrophenyl p-guanidinobenzoate, phenylmethanesulfonyl fluoride, and D-Phe-Pro-Arg-CH2Cl but was not affected by cysteine-directed reagents or by metal chelators. These results suggest that the enzyme is a serine protease. Protein C activator was capable of hydrolyzing the thrombin substrate tosyl-Gly-Pro-Arg-p-nitroanilide (TGPRpNA), and steady-state kinetic studies determined that the Km for amidolysis of this substrate was 1.1 mM while the Vmax was 66 s-1. The activator demonstrated considerable substrate specificity since the amidolysis of D-Phe-Pip-Arg-pNA, D-Ile-Pro-Arg-pNA, Bz-Ile-Glu-Gly-Arg-pNA, D-Val-Leu-Arg-pNA, and pyrGlu-Pro-Arg-pNA was less than 10% of that of TGPRpNA when measured under identical conditions using 1.0 mM substrate concentrations. The enzyme appears to be thrombin-like in its preference for arginyl as compared to lysyl chloromethyl ketones as well as by its inhibition by benzamidine and p-aminobenzamidine. However, the substrate specificity of the activator is distinguished from alpha-thrombin in that it does not clot fibrinogen and does not react with antithrombin III or hirudin.(ABSTRACT TRUNCATED AT 250 WORDS)     

HPLC-based two-step purification of fibrinolytic enzymes from the venom of Agkistrodon contortrix contortrix and Agkistrodon piscivorus conanti

In investigations aimed at characterizing snake venom blood clot-dissolving enzymes, we have developed a rapid two-step high-performance chromatography method for the isolation of these fibrinolytic enzymes from the venoms of Agkistrodon contortrix contortrix and Agkistrodon piscivorus conanti. The first step consisted of hydrophobic interaction chromatography on a propyl-aspartamide column. Fractions containing the fibrinolytic activity were then concentrated and applied to a hydroxylapatite column. The resulting preparation, assessed for purity by reverse-phase chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was homogeneous. The molecular weight of both venom fibrinolytic enzymes was approximately 23,000 and amino acid analysis, immunological cross-reaction, cyanogen bromide, and tryptic digestion indicate a significant degree of structural similarity. However, the general proteolytic activity of the A. p. conanti venom enzyme was significantly lower than the corresponding activity of the A. c. contortrix venom, whereas their fibrinolytic activities were quite similar.     

Isolation and characterization of a protein C activator from the moccasin snake venom

The protein C activator detectable in the venom of the Southern Copperhead snake (Agkistrodon contortrix contortrix) was isolated by a combination of chromatofocusing on PBE-94 in the range pH9-7 and gel-filtration on Sephadex G-100 column. The peak protein C activator from Sephadex G-100 column appeared as double diffuse bands with apparent molecular weight of 37,700 and 31,400 after electrophoresis in the presence of sodium dodecylsulfate and 2-mercaptoethanol. The isolated enzyme does not clot human fibrinogen and when mixed with normal plasma generates activity of Protein C. It can be used for the measurement of protein C functional activity.     

Practical application of the protein C activator Protac from Agkistrodon contortrix venom

The protein C activator Protac from A. contortrix venom is being investigated as a potential antithrombotic agent and as a tool for the preparation of activated protein C. Its established major application is the zymogen activation in functional protein C determinations based on either a clotting assay or a chromogenic substrate technique. The sensitivity of the activated partial thromboplastin time as an indicator reaction for Protac activated protein C depends on the contact activator component of the reagent. Protein C dose-response increased in the following order: kaolin greater than ellagic acid greater than sulfatide. This phenomenon is due to a competition of molecular affinities between Protac, plasma components and the different activating surfaces.     

Isolation and structural characterization of a cytotoxic L-amino acid oxidase from Agkistrodon contortrix laticinctus snake venom: preliminary crystallographic data.

We have purified a cytotoxic L-amino acid oxidase (LAO) from Agkistrodon contortrix laticinctus snake venom by means of Superdex-200 gel filtration, followed by phenyl-Sepharose CL-4B chromatography. The purified enzyme (ACL LAO) is a dimer on gel filtration, with a M(r) of 60,000 for the monomer as estimated by SDS-PAGE. LAO activity was tested against 15 amino acids, but only 9 were oxidized by the enzyme, suggesting that it presents some degree of specificity. ACL LAO has apoptosis-inducing activity in an HL-60 cell culture assay. After 24 h treatment with 25 micrograms/ml of ACL LAO, the typical DNA fragmentation pattern of apoptotic cells was observed on agarose gel electrophoresis. NMR analysis showed the presence of a flavin mononucleotide prosthetic group. To solve its 3-D structure, crystals of the purified protein were grown in 0.1 M Tris-HCl, pH 8.5, and 2 M (NH(4))(2)SO(4). Diffraction data collected to 3.5 A showed that the protein crystallized in the tetragonal system, with unit cell a = b = 103.22 A, c = 183.45 A. This is the first report of preliminary crystallization data for a snake venom L-amino acid oxidase.     

Isolation of a protein C activator from southern copperhead venom

A protease from the venom of the Southern Copperhead snake (Agkistrodon contortrix contortrix) that activates protein C was purified to homogeneity by a combination of sulfopropyl (SP)-Sephadex C-50, Sephadex G-150 and Mono-S column chromatography. The purified enzyme is a glycoprotein, and migrated as a single band in sodium dodecylsulfate-polyacrylamide gel electrophoresis with an apparent molecular weight of 37,000 under non-reducing conditions. Upon reduction with 2-mercaptoethanol, the enzyme exhibited a Mr of 40,000. The purified enzyme prolonged the clotting time of human plasma in a dose- and temperature-dependent manner. Purified bovine protein C was completely activated within 10 minutes upon incubation with the purified protease at a 1:500 enzyme: substrate ratio. This reaction was markedly inhibited by calcium ions. The purified venom protein C activator had no effect on human fibrinogen.     

Characterization of hyaluronidase isolated from Agkistrodon contortrix contortrix (Southern Copperhead) venom

Snake venoms are a rich source of enzymes including many hydrolytic enzymes. Some enzymes such as phospholipase A2, proteolytic enzymes, and phosphodiesterases are well characterized. However many enzymes, such as the glycosidase, hyaluronidase, have not been studied extensively. Here we describe the characterization of snake venom hyaluronidase. In order to determine which venom was the best source for isolation of the enzyme, the hyaluronidase activity of 19 venoms from Elapidae, Viperidae, and Crotalidae snakes was determined. Since Agkistrodon contortrix contortrix venom showed the highest activity, this venom was used for purification of hyaluronidase. Molecular weight was determined by matrix-assisted laser desorption ionization mass spectroscopy and was found to be 59,290 Da. The molecular weight value as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 61,000 Da. Substrate specificity studies indicated that the snake venom enzyme was specific only for hyaluronan and did not hydrolyze similar polysaccharides of chondroitin, chondroitin sulfate A (chondroitin 4-sulfate), chondroitin sulfate B (dermatan sulfate), chondroitin sulfate C (chondroitin 6-sulfate), chondroitin sulfate D, chondroitin sulfate E, or heparin. The enzyme is an endo-glycosidase without exo-glycosidase activity, as it did not hydrolyze p-nitrophenyl-beta-D-glucuronide or p-nitrophenyl-N-acetyl-beta-D-glucosaminide. The main hydrolysis products from hyaluronan were hexa- and tetrasaccharides with N-acetylglucosamine at the reducing terminal. The cleavage point is at the beta1,4-glycosidic linkage and not at the beta1,3-glycosidic linkage. Thus, snake venom hyaluronidase is an endo-beta-N-acetylhexosaminidase specific for hyaluronan.     

Purification and characterization of a fibrinolytic enzyme from venom of the southern copperhead snake (Agkistrodon contortrix contortrix)

A fibrinolytic enzyme present in Agkistrodon contortrix contortrix (southern copperhead) venom has been purified by combination of CM-cellulose chromatography, molecular sieve chromatography on Sephadex G-100, p-aminobenzamidine-agarose affinity chromatography, and DEAE-cellulose chromatography. The enzyme, fibrolase, has a molecular weight of 23,000-24,000 and an isoelectric point of pH 6.8. It is composed of approximately 200 amino acids, possesses a blocked NH2-terminus and contains little or no carbohydrate. The enzyme shows no activity against a series of chromogenic p-nitroanilide substrates and is not inhibited by diisopropylfluorophosphate, soybean trypsin inhibitor, Trasylol, or p-chloromercuribenzoate. However, the enzyme is a metalloproteinase since it is inhibited by EDTA, o-phenanthroline and tetraethylenepentamine (a specific zinc chelator). Metal analysis revealed 1 mol of zinc/mol of protein. Study of cleavage site preference of the fibrinolytic enzyme using the oxidized B chain of insulin revealed that specificity is similar to other snake venom metalloproteinases with cleavage primarily directed to an X-Leu bond. Interestingly, unlike some other venom fibrinolytic metalloproteinases, fibrolase exhibits little if any hemorrhagic activity. The enzyme exhibits direct fibrinolytic activity and does not activate plasminogen. In vitro studies revealed that fibrolase dissolves clots made either from purified fibrinogen or from whole blood.     

Assessment of a complex mixture of interacting proteins in the coagulant active venom from Agkistrodon contortrix contortrix

The crude venom of Agkistrodon contortrix contortrix was characterized by means of 2D-PAGE (using various separation principles in the respective directions) and high performance gel filtration chromatography. It was found that the venom presents a rich and remarkably stable mixture of proteins, mostly glycoproteins, which may interact each other. High stability of the venom in spite of the presence of many proteolytic enzymes, must most likely be attributed to the sugar moieties of venom proteins. Carbohydrate composition also causes considerable heterogeneity in charge and the presence of wide range of charge isomers. The intricate complexity of the venom makes it a real difficult-to-separate mixture.     

Detection of specific proenzyme activators in snake venoms by a new immunoabsorbant-chromogenic substrate method

In separate experiments, antibodies to plasminogen, factor X and protein C were applied to microtitre trays as commonly used in enzyme-linked immunoassays. After incubation with dilute normal human plasma as a source of the corresponding proenzyme antigen, the wells were exposed to dilutions of various snake venoms. After thorough washing, the microtitre tray wells were tested overnight with chromogenic tripeptide substrates known to be relatively specific for the activated forms of the above factors, i.e., plasmin, factor Xa and activated protein C. The immunochromometric assay described detected two new activators of protein C in Agkistrodon piscivorus and Agkistrodon contortrix venoms and a new factor X activator in Agkistrodon rhodostoma venom. Gel filtration of the latter venom indicated that the factor X activator eluted with high molecular weight, was clearly distinct from the peak fibrinogen clotting activity (Ancrod) and appeared to have no procoagulant activity. Although several Bothrops venoms appeared to contain plasminogen activator by this technique, the observed strong chromogenic activity was observed in microtitre wells independently of plasminogen and represented nonspecific amidase activity.     

Simultaneous isolation of protein C activator,fibrin clot promoting enzyme (fiprozyme) and phospholipase A2 from the venom of the southern copperhead snake

The simultaneous isolation of three enzymes from the southern copperhead snake venom (Agkistrodon contortrix contortrix; ACC) is described. The first step is a chromatography of crude venom on a Mono S cation-exchange column at pH 6.5. A fibrin clot promoting enzyme (fiprozyme) that preferentially releases fibrinopeptide B from fibrinogen is isolated from the fraction not binding to the Mono S by a further three-step process. The procedure involves affinity chromatography on Blue Sepharose, gel chromatography on Sephacryl S-200 and metal–chelate chromatography on Chelating Sepharose. Protein C activator and phospholipase coelute from the Mono S column. They are separated by a gel chromatography on Sephacryl S-200. After this step two enzymes are obtained: a highly purified protein C activator applicable in methods for determination of functional level of protein C (a plasma regulator of hemostasis) and an electrophoretically pure enzyme with the activity of phospholipase A₂.     

蛇毒丝氨酸蛋白酶多样性──底物专一性免疫化学及序列比较研究

本文报道烙铁头（Ｔｒｉｍｅｒｅｓｕｒｕｓｍｕｃｒｏｓｑｕａｍａｔｕｓ）蛇毒纤维蛋白原溶酶（ＴＭＶＦｇ），眼镜王蛇（Ｏｐｈｉｏｐｈａｇｕｓｈａｎｎａｈ）蛇毒纤维蛋白原溶酶（ｏｈＳ１），竹叶青（Ｔｒｉｍｅｒｅｓｕｒｕｓｓｔｅｊｎｅｇｅｒｉ）蛇毒专一纤溶酶原激活剂（ｓｖ－ｐＡ）对５种小分子多肽底物的底物专一性，及这些蛇毒丝氨酸蛋白酶对各种凝血因子（第Ｘ因子、凝血酶原、纤溶酶原、蛋白Ｃ）的作用，并和其它蛇毒丝氨酸蛋白酶如矛头蝮（Ｂｏｔｈｒｏｐｓａｔｒｏｘ）蛇毒凝血酶样酶（Ｂａｔｒｏｘｏｂｉｎ）、铜头蝮（Ａｇｋｉｓｔｒｏｄｏｎｃｏｎｔｏｒｔｒｉｘｃｏｎｔｏｒｔｒｉｘ）蛇毒蛋白Ｃ激活剂ＡＣＣ－Ｃ、蝰蛇（Ｖｉｐｅｒａｒｕｓｓｅｌｌｉ）毒第Ⅴ因子激活剂ＲＶＶ－Ｖ进行比较研究。通过酶标偶联免疫反应研究了抗ｓｖ－ＰＡ抗体与各种丝氨酸蛋白酶的免疫交叉反应，并对蛇毒丝氨酸蛋白酶及相应功能的哺乳动物蛋白酶进行了序列比较分析。从底物专一性多样性及已知序列结构分化上对这一类蛇毒丝氨酸蛋白酶的结构与功能进行了探讨和研究。     

Sporocysts isolated from the southern copperhead (Agkistrodon contortrix contortrix) produce Sarcocystis montanaensis-like sarcocysts in prairie voles (Microtus ochrogaster)

Sporulated oocysts and free sporocysts of a Sarcocystis sp. were isolated from the feces of a southern copperhead (Agkistrodon contortrix contortrix) collected in Arkansas (USA). Twenty sporocysts measured 11.2 by 8.5 microns, lacked a Stieda body, and had four sporozoites and a granular sporocyst residuum. Sarcocysts similar to those of Sarcocystis montanaensis were present in the tongues of prairie voles (Microtus ochrogaster) inoculated orally with 800 sporocysts 128 days previously. Sarcocysts were thin-walled, divided into compartments by septa, and had electron dense projections (0.14 microns) on the primary cyst wall. Infection was not pathogenic for prairie voles under the conditions of this study. No infections were observed in ICR strain laboratory mice (Mus musculus) or white-footed mice (Peromyscus leucopus) following oral inoculation of 800 sporocysts.     

Amino acid sequence of fibrolase, a direct-acting fibrinolytic enzyme from Agkistrodon contortrix contortrix venom.

The complete amino acid sequence of fibrolase, a fibrinolytic enzyme from southern copperhead (Agkistrodon contortrix contortrix) venom, has been determined. This is the first report of the sequence of a direct-acting, nonhemorrhagic fibrinolytic enzyme found in snake venom. The majority of the sequence was established by automated Edman degradation of overlapping peptides generated by a variety of selective cleavage procedures. The amino-terminus is blocked by a cyclized glutamine (pyroglutamic acid) residue, and the sequence of this region of the molecule was determined by mass spectrometry. Fibrolase is composed of 203 residues in a single polypeptide chain with a molecular weight of 22,891, as determined by the sequence. Its sequence is homologous to the sequence of the hemorrhagic toxin Ht-d of Crotalus atrox venom and with the sequences of two metalloproteinases from Trimeresurus flavoviridis venom. Microheterogeneity in the sequence was found at both the amino-terminus and at residues 189 and 192. All six cysteine residues in fibrolase are involved in disulfide bonds. A disulfide bond between cysteine-118 and cysteine-198 has been established and bonds between cysteines-158/165 and between cysteines-160/192 are inferred from the homology to Ht-d. Secondary structure prediction reveals a very low percentage of alpha-helix (4%), but much greater beta-structure (39.5%). Analysis of the sequence reveals the absence of asparagine-linked glycosylation sites defined by the consensus sequence: asparagine-X-serine/threonine.     

Molecular mechanism analysis of Gloydius shedaoensis venom gloshedobin interaction with inhibitors by homology modeling

Snake venom thrombin-like enzymes (SVTLEs) are widely applied in the treatment of thrombotic diseases, however, the molecular mechanism of its inhibition by synthetic and natural proteinaceous inhibitors is not yet understood. Here we investigated effects of protease inhibitors including phenylmethylsulfonil fluoride (PMSF), benzamidine (BMD) and its derivates on the activity of recombinant gloshedobin, a SVTLE from the snake Gloydius shedaoensis. The molecular inhibition mechanism was postulated by separately docking inhibitors into three-dimensional model of gloshedobin using protein C activator from Agkistrodon contortrix contortrix venom (ACC-C, which bear 78% identity with gloshedobin) as template. The analysis indicated that the strongest inhibitor, PMSF, was via a covalent bond with the catalytic Ser195, while other inhibitors showing weaker inhibitory activity were via hydrogen bond with Ser195 or non-catalytic residues.     

Subspecific variations in Agkistrodon contortrix venoms.

1. Commercially available preparations of venoms of three subspecies of copperhead snake (Agkistrodon contortrix) were compared as to toxicity, enzymatic activities, effect on a nerve-muscle preparation and capacity to induce clotting of a fibrinogen solution or plasma. 2. Northern copperhead venom contained apparent neurotoxic activities that were not present in broadbanded copperhead venom and only partially present in southern copperhead venom. 3. Procoagulant activity was demonstrated in whole northern copperhead venom in the absence of exogenous calcium. Procoagulant activity was present in certain isolated fractions of southern and broadbanded copperhead venoms, but was not apparent in the whole venoms. 4. Differences were noted in the levels of enzyme activities and electrophoretic patterns of the three venoms.     

A novel venombin B from agkistrodon contortrix contortrix: evidence for recognition properties in the surface around the primary specificity pocket different from thrombin

A novel thrombin-like enzyme (named contortrixobin) has been purified to homogeneity from the venom of Agkistrodon contortrix contortrix by affinity chromatography on arginine-Sepharose, anionic exchange chromatography, and HPLC. The complete amino acid sequence has been determined by Edman degradation and by mass spectral analysis of peptides generated by enzymatic cleavage. A microheterogeneity at the level of residue 234 has been detected, as demonstrated by peptides differing for the occurrence of Pro234 ( approximately 85%) or Asp234 ( approximately 15%). Contortrixobin (i) has six disulfide bonds whose sequence positions have been determined by mass spectrometry and (ii) does not contain carbohydrates in its structure. As expected, the 234 residue sequence of contortrixobin exhibits strong homology with snake venom serine proteases acting on either fibrinogen or other blood coagulation components. The interaction of contortrixobin with chromogenic substrates indicates a higher specificity for arginine over lysine in the primary subsite and a faster attack to ester than amides. The hydrolytic activity of contortrixobin is strongly inhibited by diisopropyl fluorophosphate and to a less extent by phenylmethylsulfonyl fluoride, benzamidine, and 4', 6-diamidino-2-phenylindole; hirudin (a specific alpha-thrombin inhibitor) as well as basic pancreatic trypsin inhibitor has a small effect on contortrixobin's catalytic properties. Contortrixobin (i) preferentially releases fibrinopeptide B from human fibrinogen, (ii) activates blood coagulation Factors V and XIII with a rate 250-500-fold lower than human alpha-thrombin, and (iii) does not induce thrombocyte aggregation, intracytoplasmatic calcium ion increase in platelets, and activation of Factor VIII. Evidence for biorecognition properties different from thrombin is also reported.