Isoelectric focusing in immobilized pH gradients of a snake venom fibrinolytic enzyme

Resolution of isoforms of natural and recombinant fibrolase, the fibrinolytic enzyme from Agkistrodon contortrix contortrix snake venom, and comparison of their EDTA sensitivities

Fibrolase, the fibrinolytic enzyme from Agkistrodon contortrix contortrix snake venom, is a zinc metalloproteinase with a molecular mass of 23 kDa. We report a method to isolate two isoforms of natural fibrolase (fib1 and fib2) and three isoforms of recombinant fibrolase (r-fib1, r-fib2 and r-fib3) using CM 300 cation-exchange high-performance liquid chromatography. Utilizing mass spectrometry we characterized differences in molecular masses of the isoforms of r-fibrolase. These findings suggest that the isoforms differ by minor sequence variations at their amino-termini. Since the stability of fibrolase is exquisitely sensitive to the removal of zinc, we examined the EDTA sensitivity of the isoforms of fibrolase and r-fibrolase to determine if their different chromatographic behavior is related to differences in their zinc affinities. All of the isoforms examined appear to have similar zinc binding affinities. Thus, the IC50 (concentration of EDTA to produce 50% inhibition of enzymatic activity) for fib1 is 160 microM. For the closely related r-fib1, the IC50 is 180 microM. Similarly, r-fib3 has an IC50 of 140 microM.

Purification and characterization of fibrolase isoforms from venom of individual southern copperhead (Agkistrodon contortrix Contortrix) snakes

Fibrolase, a zinc metalloproteinase possessing direct-acting fibrinolytic activity, has been previously purified from southern copperhead (Agkistrodon contortrix contortix) snake venom. We recently reported that a pool of southern copperhead venom from different geographical locations possesses two isoforms of fibrolase (fib1 and fib2) [Loayza, S. L. et al. (1994) J. Chromat. B, in press]. We now report that venom from individual southern copperhead snakes contains the two isoforms which can be separated by a three-step high performance liquid chromatography (HPLC) procedure consisting of hydrophobic interaction chromatography, hydroxylapatite chromatography and weak cation exchange chromatography. Utilizing mass spectrometry we determined that fib1 has a molecular mass of 22,879 atomic mass units (amu) compared to 22,753 amu for fib2. These results support earlier observations during amino acid sequence analysis that a truncated version of the enzyme is produced which is missing the amino-terminal amino acid (< Glu-Arg-Phe-Pro vs. the intact enzyme < Glu-Gln-Arg-Phe-Pro, where < Glu is cyclized glutamine). The truncated version of fibrolase (fib2) has full fibrinolytic activity compared to fib1. EC50 values (concentration of enzyme required to degrade 50% of fibrin in a micro-fibrin plate assay) are 6.4 (+/- 1.0) microM and 5.2 (+/- 0.8) microM for fib 1 and fib2, respectively. Therefore, loss of the amino-terminal amino acid does not appear to influence enzymatic activity. We conclude that the two isoforms of fibrolase arise from variations in the molecular processing of the enzyme by the snake venom gland rather than being caused by the pooling of southern copperhead venoms from different geographical locations.

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.

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.