The importance of protein structure and conformation in the preparation of phospholipase-free cardiotoxin from snake venom

Cardiotoxin 1 from cobra (Naja naja atra) venom causes necrosis of skeletal muscle in vivo

Cardiotoxin 1 from cobra (Naja naja atra) venom was tested for its ability to cause necrosis of skeletal muscle cells after i.m. injection into mice. Light and electron microscopic examination of tissue indicated that the toxin caused necrosis of skeletal muscle as early as 30 min after injection. The plasma membranes of affected cells were ruptured in the area of delta lesions, and the myofibrils were condensed into dense clumps alternating with clear areas containing elements of the sarcotubular system and damaged mitochondria. By 24 hr the affected cells appeared as empty 'bags' containing only remnants of myofibrils and swollen mitochondria. To eliminate the possibility that the necrosis was due to contaminating phospholipase A2 (PLA2) activity of the sample, the sample was treated with p-bromophenacyl bromide (p-BPB), a known inhibitor of PLA2 activity. The p-BPB-treated preparation caused myonecrosis in vivo in mice, and the treatment caused a significant decrease in the release of fatty acids and no detectable lysophospholipid in human muscle cell cultures treated in vitro with the preparation, indicating the lack of PLA2 activity. Additionally, purified PLA2 from the same venom failed to cause myonecrosis in vivo at doses equal to or ten times the estimated contaminating concentration. Thus, it is concluded that cardiotoxin 1 from Naja naja atra venom causes necrosis of skeletal muscle cells in vivo upon i.m. injection.

Isolation of nuclei from melittin-destabilized cells

Melittin, the hydrophobic polypeptide from bee venom, sufficiently destabilizes the plasma membrane of cultured cells to allow cell disruption in the absence of detergents with minimal homogenization. Nuclei are thus isolated in high yield with intact nuclear membranes and high transcriptional activity.

Reversed-phase and hydrophobic-interaction high-performance liquid chromatography of elapid cardiotoxins

The separation of proteins by hydrophobic-interaction HPLC and reversed-phase HPLC depends upon differences in the hydrophobicity of accessible surface groups. The elution order of a group of snake venom cardiotoxins was found to vary between these two HPLC methods. Circular dichroism spectroscopy showed that the eluant acetonitrile-trifluoroacetic acid used for reversed-phase HPLC altered the conformation of the toxins, whereas the salt-buffer eluting medium used for hydrophobic-interaction HPLC did not affect toxin conformation. The retention times of cardiotoxins on reversed-phase HPLC are therefore influenced by their conformational instability in the eluting medium which causes partial or complete unfolding. Hydrophobic interaction is clearly the preferred method with which to correlate the "surface hydrophobicity" of cardiotoxins and their biological effects.

Cobra cardiotoxins. Purification, effects on skeletal muscle and structure/activity relationships [published errtum appears in Eur J Biochem 1988 Feb 1;171(3):727]

A new preparative method for isolating homogeneous cardiotoxins from cobra venoms is described. The technique, based on reverse-phase high-pressure liquid chromatography, was used to isolate eight cardiotoxins of known sequence from four different venoms. In each case the method was found to be particularly efficient at removing trace quantities of contaminating phospholipase. Cardiotoxins isolated in this manner were found to retain their full biological activity. Without exception the purified cardiotoxins lacked powerful haemolytic activity at concentrations up to 0.01 mM (about 100 micrograms ml-1), although some lysis of human erythrocytes was induced at higher concentrations. The cardiotoxins displayed a wide range of depolarizing activity on cultured skeletal muscle, the lowest activity being associated with the highest LD50 value. Correlating variations in amino acid sequence and variations in depolarization potency revealed the importance of residues in the second and third loops, especially lysine-46, serine-48 and lysine-52, together with a number of hydrophobic residues. Further modifications of pharmacological activity were associated with the presence of additional basic residues in the first and second loops and to minor differences in secondary structure.

Rapid purification of a phospholipase-free alpha-bungarotoxin: maintenance of cation barriers of acetylcholine receptor membranes upon preincubation with purified toxin

The purification of highly homogeneous, phospholipase-free alpha-bungarotoxin (alpha-Bgt) from the venom of the elapid Bungarus multicinctus or from commercial samples of alpha-Bgt is described. The method combines a conventional procedure for the purification of alpha-Bgt [D. Mebs, K. Narita, S. Iwanaga, Y. Samejima, and C. Y. Lee (1972) Hoppe-Seyler's Z. Physiol. Chem. 353, 243-262] with high-resolution gel-filtration and cation-exchange chromatography steps to remove membrane-damaging, contaminating phospholipase activity. The procedure also removes contaminating radioactive peptides from commercial preparations of 125I-alpha-Bgt. Apparent homogeneity of the purified alpha-Bgt (referred to as fraction D in the text), as well as the absence of contaminating phospholipase A2 activity, is assessed by (i) polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, (ii) gel-filtration and cation-exchange high-performance liquid chromatography, (iii) direct measurements of phospholipase A2 activity under conditions where very low enzymatic levels should be detected, (iv) lack of interference with the passive cation permeability properties of acetylcholine receptor membranes, (v) competitive inhibition of 125I-alpha-Bgt binding to the acetylcholine receptor membranes, and (vi) amino acid analysis and end-group (C- and N-terminus) determination. alpha-Bgt preparations subjected to these criteria do not exert the increase in membrane passive permeability to cations detected with other laboratory or commercial samples of alpha-Bgt. Availability of the new alpha-Bgt preparation allows for an assessment of the inertness of alpha-Bgt on lipid membrane properties while preventing cholinergic ligand binding to nicotinic acetylcholine receptor-rich membranes. These conditions are necessary for experiments requiring maintenance of the physical and phospholipid integrity of membranes.

Cobra venom factor: improved method for purification and biochemical characterization

A method to purify cobra venom factor (CVF) from cobra venom by sequential column chromatography is described which yields a product virtually free of phospholipase A2, a common contaminant of CVF preparations. The separation of phospholipase A2 from CVF was achieved by chromatography on Cibacron blue-agarose, a resin that tightly binds cobra venom phospholipase A2. A rapid and simple hemolytic assay for the qualitative and quantitative determination of CVF based on its ability to induce bystander lysis of erythrocytes has been devised. CVF was isolated from the venom of the Naja naja kaouthia subspecies and some of its biochemical properties and physicochemical parameters were delineated.

Effect of phospholipase A on actions of cobra venom cardiotoxins on erythrocytes and skeletal muscle

The actions of two phospholipase-free cardiotoxins from the venom of the cobra Naja naja siamensis were compared to phospholipase-contaminated cardiotoxins in terms of their ability to lyse human erythrocytes and to depolarize and contract skeletal muscle. The presence of 3-5% (w/w) phospholipase caused a 20-30-fold increase in the haemolytic activity of the two cardiotoxins, the pure cardiotoxins being virtually without haemolytic activity at 10(-7)-10(-6) M. Phospholipase contamination did not enhance the ability of the cardiotoxins to cause contracture of chick biventer cervicis muscles and it caused less than a 2-fold increase in the depolarizing activity of the cardiotoxins on cultured skeletal muscle. Phospholipase-free cardiotoxins were about 10-20 times more active on cultured skeletal muscle fibres than on erythrocytes. These results support the hypothesis that some cardiotoxins have more affinity for the membranes of excitable cells than for those of other cells such as erythrocytes.

Penetrability of orally toxic protein from cobra venom through the gut of a blowfly

The oral toxicity of a radioiodinated toxic polypeptide isolated from a cobra snake venom as assayed by Sarcophaga falculata blowflies coupled with assays on competitive displacement have indicated that: (a) During 3--4 h 8% of the orally active toxin is able to pass through the digestive system of the fly; (b) the orally active toxin after passing the gut binds to body tissues. The strong affinity of the toxin to tissue membranes explains its absence in the insect's hemolymph following oral applications as well as injection. The removal of traces of phospholipase A, which is extremely toxic, by injection of the orally active toxin has significantly lowered its injection toxicity without affecting its oral toxicity, thus indicating the absence of any interaction with phospholipases in oral toxicity. This conclusion was supported by additional experimentation.

The synergism of cardiotoxin and phospholipase A2 in hemolysis

The synergistic effect of exogenous cobra phospholipase A2 on the hemolysis brate of guinea pig erythrocytes by highly purified snake venom cardiotoxins was investigated. In the presence of phospholipase A2 the reaction was not only faster and had a lower activation energy but followed a sigmoidal instead of a linear time course. Similar results were obtained using porcine pancreatic phospholipase A2. Significantly, addition of even a trace of cobra phospholipase A2 (approx. 0.1%, w/w) was sufficient to bring about the full synergistic effect, emphasizing the stringent purity requirements for any meaningful investigation of cardiotoxin's own action. The possibility that the action of cardiotoxin on its own may involve the stimulation of an endogenous phospholipase is discussed in the light of the results obtained with exogenous cobra enzyme.

The oxidation of methionine and its effect of the properties of cardiotoxin VII1 from Naja melanoleuca venom

Methionine residues 24 and 26 of cardiotoxin VII1 from Naja melanoleuca were oxidised to sulphoxides using N-chlorosuccinimide at pH 8.5. The number of equivalents of oxidant required for complete oxidation suggested that the methionine side-chains existed in a relatively "exposed" conformational state in cardiotoxin. The oxidised cardiotoxin was devoid of lethality. It was also non-haemolytic, both on its own and in the presence of phospholipase A2. However, it was still able to precipitate with anti-cardiotoxin antibody. CD studies indicated that the polypeptide backbone conformation was intact in the oxidised cardiotoxin but some perturbation of tyrosine residues was evident. The possibility of a direct or indirect involvement of the methionine residues in the biological activity of the cardiotoxin is discussed.