alpha-cobratoxin and 2-nitrobenzenesulfenyl-chloride

Two alpha-neurotoxins, Oh-4 and Oh-7, from the king cobra (Ophiophagus hannah) venom were subjected to Trp modification with 2-nitrophenylsulfenyl chloride (NPS-Cl). One major NPS derivative was isolated from the modified mixtures of Oh-4 and two from Oh-7 by HPLC. Amino acid analysis and sequence determination revealed that Trp-27 in Oh-4, and Trp-30 and Trp-26 and 30 in the two Oh-7 derivatives, were modified, respectively. Sulfenylation of Trp-27 in Oh-4 caused about 70% drop in lethal toxicity and nicotinic acetylcholine receptor-binding activity. Modification of Trp-30 in Oh-7 resulted in the decrease of lethal toxicity by 36% and binding activity by 61%. The activities were further lost when the conserved Trp-26 in Oh-7 was modified. Sulfenylation of the Trp residues did not significantly affect the secondary structure of the toxins as revealed by the CD spectra. These results indicate that the Trp residues in these two long alpha-neurotoxins may be involved in the receptor binding.

Topics: Amino Acid Sequence; Animals; Binding, Competitive; Bungarotoxins; Chromatography, High Pressure Liquid; Chymotrypsin; Circular Dichroism; Cobra Neurotoxin Proteins; Lethal Dose 50; Mice; Molecular Sequence Data; Nitrobenzenes; Peptides; Receptors, Nicotinic; Sequence Analysis; Spectrometry, Fluorescence; Tryptophan

Raman spectroscopy was used to determine structural features of the native toxin alpha from Naja nigricollis, which contains only one Trp and one Tyr, and of chemically modified toxins having chromophores added to these two conserved aromatic amino acids. The percentages of secondary structure were determined by using amide I polypeptidic vibration analysis and are in agreement with X-ray structure [Low et al. (1976) Proc. Natl. Acad Sci. U.S.A. 73, 2991-2994] as well as with the geometry of the disulfide bridges estimated by using the v(S-S) vibrations. In the native toxin alpha, the single invariant tyrosine 25 appears to be buried in the structure and involved in a strong hydrogen bond. We have chemically modified these two invariant aromatic side chains by addition of chromophores. The presence of a (nitrophenyl)sulfenyl (NPS) chromophore bound to the Trp does not perturb the secondary structure of the toxin as shown by the analysis of the polypeptidic amide I vibrations; however, the environment of this Trp and the geometry of a disulfide bridge seem to be modified. The secondary structure is not affected by the presence of the NPS chromophore; therefore, the decrease in binding affinity observed after modification of Trp-29 by the reagent NPS-Cl [Faure et al. (1983) Biochemistry 22, 2068-2076] is due to an alteration of the environment of this aromatic amino acid and/or a steric hindrance and not to an overall modification of the toxin structure. The binding assays of [nitrotyrosyl]toxin show that after nitration the affinity toward the monoclonal antibody M alpha 1 is unchanged and that the affinity toward the cholinergic receptor (AcChR) from Torpedo marmorata remains high. We concluded that the structure of toxin alpha after adding the NO2 chromophore to Tyr-25 is the same as it is in native toxin.

Topics: Amino Acid Sequence; Cobra Neurotoxin Proteins; Models, Molecular; Molecular Sequence Data; Nitrobenzenes; Protein Conformation; Spectrum Analysis, Raman; Tryptophan