The preparation of 3-nitrotyrosyl derivatives of three elapid venom cardiotoxins

Snake venom cardiotoxins-structure, dynamics, function and folding

Snake cardiotoxins are highly basic (pI > 10) small molecular weight (approximately 6.5 kDa), all beta-sheet proteins. They exhibit a broad spectrum of interesting biological activities. The secondary structural elements in these toxins include antiparallel double and triple stranded beta-sheets. The three dimensional structures of these toxins reveal an unique asymmetric distribution of the hydrophobic and hydrophilic amino acids. The 3D structures of closely related snake venom toxins such as neurotoxins and cardiotoxin-like basic proteins (CLBP) fail to show similar pattern(s) in the distribution of polar and nonpolar residues. Recently, many novel biological activities have been reported for cardiotoxins. However, to-date, there is no clear structure-function correlation(s) available for snake venom cardiotoxins. The aim of this comprehensive review is to summarize and critically evaluate the progress in research on the structure, dynamics, function and folding aspects of snake venom cardiotoxins.

Role of Tyr and Trp in membrane responses of Pyrularia thionin determined by optical and NMR spectra following Tyr iodination and Trp modification

Pyrularia thionin is a strongly basic and bioactive 47 amino acid peptide which contains two Tyr residues at positions 13 and 45 and one Trp at position 8. Limited iodination does not have a significant effect, but prolonged iodination of the peptide leads to progressive inactivation for all known cellular responses (Evans, J. et al. (1989) Proc. natn. Acad. Sci. U.S.A. 86, 5849-5853). 1H NMR spectra of the native Pyrularia thionin show four Tyr bands, two arising from each Tyr residue. One resonance band for the epsilon hydrogens of Tyr 45 disappears early during limited iodination and the band from the delta hydrogens shifts to low field. The two bands corresponding to Tyr 13 remain during limited iodination, but both decrease in intensity during prolonged iodination, with the epsilon hydrogen band decreasing somewhat more. The resonance bands arising from Trp disappear during prolonged iodination. This sequence of reactions is verified by the optical absorbance properties of two small peptide fragments obtained by Staphylococcal V8 protease hydrolysis of thionin which had been iodinated to varying degrees. Limited iodination did not significantly inhibit the thionin's biological activity, yet the fragment from the -COOH terminus showed the conversion of Tyr 45 to diiodoTyr. This treatment did not significantly modify the Tyr 13 or Trp 8 located in the -NH2 terminal fragment. More extensive iodination resulted in a disappearance of Trp 8 absorbance with an accompanying conversion of Tyr 13 to the monoiodo form. Extensive iodination yielded two atoms of iodine in the Tyr 45-containing fragment, and only one atom in the Tyr 13 fragment. The data indicate that Tyr 45 of the native thionin is more readily iodinated, proceeding to the diiodo form without significant loss of activity. Prolonged iodination does not lead to the formation of any diiodoTyr 13, but does lead to modification of Trp 8 and probably formation of monoiodoTyr 13. Modification of Trp 8 with N-bromosuccinimide inhibits the hemolytic activity of the thionin, showing that Trp 8 is necessary for Pyrularia thionin activity. It is most likely Trp 8 modification during prolonged iodination which results in the loss of biological activity.

Correlation between the surface hydrophobicities and elution orders of elapid neurotoxins and cardiotoxins on hydrophobic-interaction high-performance liquid chromatography

Hydrophilic and hydrophobic regions were predicted for Elapid neuro- and cardiotoxins. The contribution of these regions to the retention times of neuro- and cardiotoxins on hydrophobic-interaction HPLC was assessed from the known surface accessibilities of amino acid side-chains within these regions. Differences in retention times between neuro- and cardiotoxins on hydrophobic-interaction HPLC could be attributed to differences in hydrophobicity of regions 6-12 and 22-26 between these two types of toxins. Smaller differences in retention times between cardiotoxins were due to the variable hydrophobicities of regions 1-4 and 26-36.

Role of tyrosine and tryptophan residues in the structure-activity relationships of a cardiotoxin from Naja nigricollis venom

This paper is an attempt to localize the critical area determining toxicity in a snake cardiotoxin. Toxin gamma is a single-chain polypeptide of 60 amino acids, which has been isolated from the venom of the African spitting cobra, Naja nigricollis. Three aromatic residues, namely, Trp-11, Tyr-22, and Tyr-51, have been individually modified by chemical means. The structure of the native toxin and of each derivative has been carefully investigated by circular dichroism, fluorescence, proton magnetic resonance spectroscopy, and two specific monoclonal antibodies. None of the chemical modifications alters the overall structure of the toxin, which in all cases remains folded into three adjacent loops (I, II, and III) rich in beta-pleated sheet emerging from a small globular region containing four disulfide bridges. A number of subtle changes, however, have been detected in the structure of each derivative compared with that of the native toxin. In particular, nitration of Tyr-51 provoked a structural perturbation in the globular region. Nitration of Tyr-22 induces a more substantial change in the beta-sheet area of the molecule. Thus, the strong inter-ring NOE that is observed in the native toxin between Tyr-22 and Tyr-51 vanishes in the Tyr-22 derivative, and significant changes are observed in the globular region. In contrast, no alteration of the beta-sheet structure of loops II and III has been detected after modification of Trp-11. All changes observed for this derivative remain located in the vicinity of the indole side chain of Trp-11 in loop I. The biological consequences of the modifications were measured: the lethal potency in vivo in mice and the cytotoxic activities in vitro on FL-cells. Lethal activities correlate with cytotoxicity: Tyr-51 modified toxin is equally potent as native toxin, whereas Tyr-22 and Trp-11 derivatized toxins are characterized by substantially lesser activities, the Trp-11 derivatized toxin being the least potent. We conclude that (1) Tyr-51 is not involved in the functional site of the toxin, although it is in interaction with the core of the molecule, (2) Tyr-22 may play a dual structural and functional role, and (3) Trp-11 is in, or in close proximity to, the functional site of the toxin. These data indicate the importance of loop I in determining toxicity of the cardiotoxin.

The selective S-alkylation of a methionine residue in an elapid venom cardiotoxin

1. The reaction of cardiotoxin with iodoacetamide or iodomethane at pH 3.0 afforded the corresponding methionine sulphonium derivatives. The major products were S-alkylated at Met-26 whilst the minor products were S-alkylated at both Met-24 and -26. 2. Reaction with iodoacetamide under denaturing conditions led to a reversal of the relative abundances of the two derivatives in the respective reaction mixtures. 3. The derivative S-methylated at Met-26 was about 5-fold less toxic than the parent cardiotoxin. That derivatised at both Met-24 and -26 was non-toxic, indicating the importance of Met-24. 4. The results are discussed in the light of a structural model, previous chemical modifications and 1H-NMR data. It appeared that Met-24 is important for the integrity of an important structural feature of cardiotoxin.

Properties of some 3-nitrotyrosyl elapid venom cardiotoxins

Nitration of the invariant Tyr-22 in Hemachatus haemachates cardiotoxin 12B did not greatly decrease lethality, and the haemolytic potency towards guinea-pig erythrocytes remained unchanged. This residue is thus non-essential for cardiotoxin to exert its biological action. Nitration of Naja haje annulifera and Naja melanoleuca cardiotoxins VII1 decreased but did not abolish the lethalities and haemolytic potencies. Thus Tyr-25 and Tyr-51 were concluded to have no direct functional role in cardiotoxin lethality. The pKa values of the phenolic hydroxyl groups of the tyrosine residues appeared to be important for certain properties of cardiotoxin in solution. No evidence could be produced to show that Tyr-51 is unreactive to nitration under normal (non-denaturing) conditions.

The binding of snake venom cardiotoxins to heart cell membranes

Cobra venom cardiotoxins have the effect, inter alia, of causing systolic arrest of the heart. We have observed significant binding in vitro of 35S-labelled cardiotoxins to mouse heart cell membranes. Part of the binding was saturable and could be displaced with homologous unlabelled cardiotoxins but not by neurotoxins or cardiotoxins inactivated by chemical modification. The specifically bound component represented more than 70% of total binding at saturation. Inclusion of Triton X-100 and NaCl in the phosphate-buffered incubation medium prevented nonspecific adsorption to centrifuge tube walls, and gave lower but more reproducible specific binding results, respectively. An apparent dissociation constant of 5 . 10(-7) M and a binding density of 500 pmol toxin/mg membrane protein were derived from the saturation isotherms.