Chemical modification of the histidine residue in basic phospholipase A2 from the venom of Naja nigricollis

Phospholipase A2 from Naja nigricollis venom was separated into three fractions by chromatography on a column of CM-Sephadex C-25. The pI values of fractions CMS-5, CMS-6 and CMS-9 were determined to be 7.6, 8.3 and 10.6, respectively. Fraction CMS-9 was further purified on a DEAE-Sephacel column and the homogeneity was verified. The specific activity of CMS-9 was found to be 1300 units per mg and lethal toxicity 0.3 mg per kg mouse. The most basic and toxic fraction, CMS-9, was subjected to chemical modification with p-bromophenzcyl bromide. The enzyme lost both the enzyme activity and lethal toxicity, however, the antigenicity remained unchanged. Although both 8-anilinonaphthalenesulfonate and Ca2+ showed pronounced protection on the inactivation process, the mechanism of 8-anilinonaphthalene-sulfonate protection is different from that of Ca2+. Amino acid analysis showed that only one (His-47) out of three histidine residues was modified. Although both native and His-modified CMS-9 were perturbed by the presence of Ca2+, the modified enzyme lost the characteristic tryptophan blue shift suggesting that the modified enzyme is unable to exert a charge effect upon Ca2+ binding in the vicinity of the tryptophan group. Scatchard plots revealed only one type of binding sites for 8-anilinonaphthalenesulfonate in the presence of Ca2+. On the other hand, the modified enzyme lost the ability to bind 8-anilinonaphthalene. It is suggested tentatively that the hydrophobic pocket in which 8-anilinonaphthalenesulfonate is bound may be the site of the enzyme that interacts with phospholipid.

Laser Raman studies on cobrotoxin

Laser Raman spectra of cobrotoxin under various conditions have been obtained. Comparison of the spectra of native cobrotoxin in lyophilized form and in aqueous solution indicates that the secondary structures of cobrotoxin are not significantly affected by the removal of the aqueous solvent. On going from the native to the partially reduced and the completely reduced, carboxy-methylated forms, characteristic peaks of the C-S-S-C and tyrosine ring in the region of 500--900 cm-1 showed definite changes in structure. The partially reduced form gave two peaks at 502 and 524 cm-1, suggesting difference in the conformation of the remaining disulfide bonds. As indicated by the present work, the conformation of the main chain of cobrotoxin in the native unperturbed state, in the partially reduced and in the completely reduced forms are the coexistence of beta-pleated sheet with random-coil structure, predominantly random coil, and predominantly random coil with the existence of an alpha-helix type structure, respectively. The effect of pH on the conformation of cobrotoxin in solution appeared to give rise to the change of the local structure of two aromatic residues common to all snake neurotoxins.

Purification and properties of non-precipitating antibodies to cobrotoxin

The heterogeneity of precipitating and non-precipitating antibodies to cobrotoxin was demonstrated by their elution pattern on cobrotoxin-Sepharose and chromatography on DEAE-cellulose column. Gel filtration patterns on a Sepharose 6B column revealed that the soluble complexes formed from non-precipitating antibody and HNB-cobrotoxin at a different molar ratio all emerged in the void volume, indicating that the molecular weight of the soluble complex is around 4,000,000 or larger. Unreacted free non-precipitating antibody coincided with the peak of IgG and was proved to be free from HNB-cobrotoxin. The molar ratio of antibody to antigen for the soluble complex was found to be 0.79 to 0.97 indicating that 1.58--1.94 molecules of non-precipitating antibody are bound to HNB-cobrotoxin instead of three molecules as in the case of precipitating antibody.

Molecular evolution of snake venom toxins

Phylogenetic trees were constructed for 62 venom toxins of snakes of Proteroglyphae suborder using matrix method. The resulting tree from Minimum Spanning Tree-Cluster Analysis technique had the lowest "percent deviation" (8.55). The taxonomic relationship of these toxins agrees very well with zoological opinions. However, the appearance of the tree did not directly provide a plausible evolutionary model for the toxins. A model was derived from nodal ancestral sequence calculations, comparisons between intra- and intergenerical rates of amino acid change, and generally held ideas about protein evolution. According to the model, short neurotoxin is the ancient form of snake venom toxins. The courses of evolution leading to the present intraspecific homologous toxins are explained by gene duplication and allelomorphism.

Conformational prediction for snake venom toxins and laser Raman scattering of a cardiotoxin from Taiwan cobra (Naja naja atra) venom

Secondary structure regions in snake venom toxins were predicted using the prediction method of Chou and Fasman (Chou, P. Y., and Fasman, G. D. (1974), biochemistry 13,222) and an averaging scheme assuming structural homology in each type of toxins. The results indicate that, in general, snake toxins contain only some beta-sheet regions and beta bends. The content of secondary structures thus predicted does vary to some extent. The predicted results correlate well with conclusions from physicochemical studies. Interestingly, beta-bend regions predicted for the two types of neurotoxins, short-neurotoxin-type and long-neurotoxin-type, are primarily located in the middle of disulfide loops in spite of large differences in primary sequences. Comparisons between predicted results and the crystal structure of erabutoxin b determined at 2.75 A resolution suggest that the two types of neurotoxin are both sequencely and conformationally related while cardiotoxins could have an entirely different molecular topology. The Raman spectrum of a Taiwan cobra cardiotoxin indicates that the content of beta-pleated-sheet structure could be greater than that in neurotoxins.

Studies on the status of free amino and carboxyl groups in cobrotoxin

The status of free amino groups in cobrotoxin was studied by stepwise modification with trinitrobenzene sulfonate. Lys-27 was selectively modified without altering the activity of cobrotoxin. However, complete loss of activity was observed when Lys-27 and Lys-47 were trinitrophenylated, suggesting that the epsilon-amino group of Lys-47 is essential for the activity of cobrotoxin. The alpha-amino group of N-terminal leucine had no correlation with activity, demonstrated by the guanidination of the lysine residues with O-methylisourea followed by trinitrophenylation of the alpha-amino group. The carboxyl groups in cobrotoxin were modified with glycine methyl ester after activation with water-soluble carbodiimide. Six out of seven free carboxyls reacted in the absence of guanidine.HCl without altering the biological activity. When the remaining carboxyl was modified in the presence of 5 M guanidine.HCl, the resulting toxin was devoid of activity. This "buried" carboxyl is essential for activity and was identified as the gamma-carboxyl group of Glu-21.