Immunological relationships of phospholipase A2 neurotoxins from snake venoms

Polyclonal rabbit antisera were raised against ten snake phospholipase A2 neurotoxins and one snake phospholipase A2 cytotoxin. Immunological cross-reactivities between these toxins, two other snake phospholipase A2 enzymes and pancreatic phospholipase A2 were studied using ELISA technology. All snake phospholipase A2 neurotoxins fell into two main antigenic classes. One antigenic class was composed of all the elapid toxins tested (textilotoxin, taipoxin, notexin, pseudexin and beta-bungarotoxin), the cytotoxic phospholipase A2 from Naja naja atra and pancreatic phospholipase A2. beta-Bungarotoxin seemed to be in an immunological subclass of its own compared to the rest of the elapid toxins. The second antigenic class was comprised of crotalid and viperid phospholipase A2 neurotoxins (crotoxin, concolor toxin, Mojave toxin, vegrandis toxin, ammodytoxin and caudoxin). Our data indicated that the viperid toxins, caudoxin and ammodytoxin, were an immunological subclass apart from the crotalid toxins.

Purification, sequencing and characterization of pseudexin phospholipases A2 from Pseudechis porphyriacus (Australian red-bellied black snake)

Pseudexin is the name given to a mixture of toxic phospholipase A2 isoenzymes isolated from the venom of the Australian red-bellied black snake, Pseudechis porphyriacus. We found that this mixture consists of three components: pseudexins A, B and C, which we individually purified by reverse phase chromatography or by hydrophobic interaction chromatography. Pseudexins A and B had relatively low specific toxicities in mice (i.p. LD50 of 1300 and 750 micrograms/kg, respectively), while C was non-toxic. All three had similar phospholipase A2 activities (43-53 muequiv H+ released/min/mg protein). The complete amino acid sequences of pseudexins A and B were determined. Amino acids were identical at 91 of the 117 residues. The first 28 residues of pseudexin C were determined, sufficient to show that C is structurally similar to A and B, but not identical with either. As judged by reactions with antisera against several other snake phospholipase A2 toxins, pseudexins A, B and C have very similar antigenic structures. We noted extensive homology with other phospholipases.

Monoclonal antibodies against Vero cells that protect against diphtheria toxin

Mice were immunized with a cell line (Vero) that possesses a high number of membrane receptors for diphtheria toxin. Spleen cells from these mice were fused with SP2/0-Ag14 cells and two cell lines (1A2 and 2D2) isolated by screening for the ability of their secreted antibodies to inhibit binding of radiolabeled diphtheria toxin to Vero cells. These antibodies protected Vero cells from the inhibition of protein synthesis mediated by diphtheria toxin. The antibodies were purified, iodinated, and their binding characteristics investigated. At 4 degrees C, the association of 1A2 and 2D2 with Vero cells was saturable (KD approximately 10(-8) M) and indicated about 10(6) binding sites/cell. Diphtheria toxin did not inhibit the binding of either radiolabeled antibody. Monoclonal antibody 1A2 completely inhibited 125I-2D2 binding and vice versa. Trypsin or phospholipase C treatment of Vero cells had no effect on the ability of the monoclonal antibodies to bind to the cells. These findings suggest that: (1) the two monoclonal antibodies recognize the same or closely related epitopes and (2) the antibodies bind a domain distinct from the toxin binding site or to a subcomponent of the diphtheria toxin receptor that is present at many other cell surface sites. These antibodies offer a powerful tool to study the structure, processing and mode of action of diphtheria toxin receptors.

Cellular regulation of diphtheria toxin cell surface receptors

The cellular regulation of diphtheria toxin cell surface receptors was studied. Treatment of Vero cells with cycloheximide reduced their diphtheria toxin binding capacity, while cells treated with actinomycin D did not lose their ability to bind diphtheria toxin. A non-toxic analogue of diphtheria toxin, CRM 197, produced a dose-related depletion of cell surface diphtheria toxin binding capacity that was reversible upon washing the cells. Vero cells depleted of toxin receptors by CRM 197 did not restore their ability to bind diphtheria toxin in the presence of cycloheximide. Phospholipase C treatment of Vero cells reduced their diphtheria toxin binding capacity in a dose-dependent manner. The loss of diphtheria toxin binding capacity was recovered within 2 hr after removal of the enzyme. Protein synthesis inhibition blocked this recovery while actinomycin D partially inhibited it. Receptors prebound with toxin were resistant to phospholipase C treatment, suggesting that the action of the enzyme was directly on the receptor. Inhibition of glycosylation with tunicamycin did not prevent reappearance of toxin receptors after CRM 197 or phospholipase C treatment. These data establish the requirement of a continuous protein synthesis for the maintenance of diphtheria toxin cell surface receptors and also suggest that these receptors do not recycle after binding ligand. A hypothesis is put forward that the diphtheria toxin receptor might be a lipid-linked cell surface protein.

Preparation of a crotoxin neutralizing monoclonal antibody

Crotoxin is a heterodimeric protein composed of an acidic and basic subunit from the venom of Crotalus durissus terrificus and is representative of a number of presynaptically acting neurotoxins found in the venom of rattlesnakes. Four different monoclonal antibodies, typed as IgG1 subclass, were raised against the basic subunit of this toxin. One was a potent neutralizing antibody of intact crotoxin, which could neutralize approximately 1.6 moles of purified crotoxin per mole of antibody. The monoclonal antibody enhanced the neutralizing ability of commercial polyvalent crotalid antivenom against the lethality of crude C. d. terrificus venom four-fold. Paradoxically, this monoclonal antibody by itself was ineffective against the lethality of crude C. d. terrificus venom. Using an enzyme-linked immunosorbent assay, we tested various proteins for competitive inhibition of binding of biotinylated-crotoxin to plates coated with the four individual monoclonal antibodies. Concolor toxin, vegrandis toxin, intact crotoxin, Mojave toxin, and the basic subunit of crotoxin showed increasing effectiveness as displacers of crotoxin from the neutralizing monoclonal antibody. None of the monoclonal antibodies reacted with purified phospholipase A2 enzymes from Crotalus atrox or Crotalus adamanteus, nor any of the components present in the crude venoms from four different elapids known to contain presynaptically acting neurotoxins, which show some sequence identity to crotoxin.

Mosquito inoculation: an alternative bioassay for toxins

Mosquitoes were evaluated as a bioassay host for several classes of biological toxins. Mosquitoes were sensitive to snake toxic or neurotoxic phospholipase A2 enzymes (but not to nontoxic phospholipase A2 enzymes), cobrotoxin, saxitoxin, microcystin and the scorpion insect sodium channel toxin. Mosquitoes were not sensitive to ricin, diphtheria toxin, anthrax toxin, botulinum toxin, tetanus toxin, conotoxin G or a scorpion sodium channel toxin toxic to mammals. Specific antisera neutralization tests with mosquitoes gave comparable results to those of a mouse assay. The mosquito is a suitable bioassay animal for many, but not all biological toxins, and offers a safer, more efficient and economical assay than mice.

Ion-conducting channels produced by botulinum toxin in planar lipid membranes

The interaction of botulinum neurotoxin (Botx) with planar lipid membranes was studied by measuring the ability of the toxin to form ion-conducting channels. Channel formation was pH dependent. At physiological pH, Botx formed no channels, whereas at pH 6.6, the toxin formed channels with a unit conductance of 12 pS in 0.1 M NaCl. The rate of channel formation increased with decreasing pH, reaching a maximum at pH 6.1, and then decreased at lower values of pH. The channels, once formed, were permanent entities in the membrane throughout the course of an experiment and fluctuated between an open and a closed state. The rate of channel formation depended upon the square of the toxin concentration, suggesting an aggregation step is involved in channel formation. The data were consistent with the hypothesis that Botx enters cells through endocytosis, followed by its release into the cytoplasm at low pH.

Effects of beta-bungarotoxin and Naja naja atra snake venom phospholipase A2 on acetylcholine release and choline uptake in synaptosomes

Beta-bungarotoxin is a potent presynaptically acting snake venom toxin that exhibits phospholipase A2 activity. We compared the effects of beta-bungarotoxin and a less toxic snake venom phospholipase A2 on synaptosomal 3H-acetylcholine release and 3H-choline uptake. The purpose of these experiments was to study the mode by which beta-bungarotoxin inhibits 3H-acetylcholine release in this preparation. Under non-depolarizing conditions, both beta-bungarotoxin and Naja naja atra phospholipase A2 stimulated 3H-acetylcholine release from a synaptosomal fraction preloaded with 3H-choline. Beta-bungarotoxin was more potent, but less efficacious, than N. naja atra phospholipase A2. In contrast, both toxins inhibited 3H-acetylcholine release from the synaptosomal fraction incubated with 3H-choline after toxin exposure. In agreement with the results obtained by monitoring acetylcholine release, beta-bungarotoxin and N. naja atra phospholipase A2 appeared to block 3H-choline uptake into the synaptosomal fraction non-competitively. Although the toxins may cause the release of unlabeled choline from synaptosomes, the block of labeled choline uptake could not be explained by decreased specific activity of 3H-choline in the bathing medium. Therefore, beta-bungarotoxin and N. naja atra phospholipase A2 block 3H-acetylcholine release from synaptosomes indirectly by inhibiting the uptake of 3H-choline necessary for 3H-acetylcholine synthesis. In comparing these results using 3H-choline to those in the literature obtained with deuterated choline, there appears to be a difference in apparent toxin action that relates to the type of label (3H or 2H) attached to choline.

Cross-reactivity and neutralization by rabbit antisera raised against crotoxin, its subunits and two related toxins

Antisera were raised against intact crotoxin (Crotalus durissus terrificus), Mojave toxin (Crotalus scutulatus scutulatus) and concolor toxin (Crotalus viridis concolor), as well as the subunits of crotoxin. Double immunodiffusion and enzyme-linked immunosorbent assays (ELISA) demonstrated antigenic similarity between these three purified toxins and their subunits. Additionally, when crotoxin antisera were pre-incubated with each of the three toxins before injection, the lethal activity of all were neutralized equally well. Antiserum was considerably more effective in neutralizing crotoxin in vivo when the toxin was injected i.m. than when injected i.v. Antisera against both intact crotoxin and its basic subunit were an order of magnitude more effective than crotoxin acidic subunit antiserum in crotoxin neutralization. Purified phospholipase A2 from Crotalus adamanteus and Crotalus atrox showed weak cross-reactivity with antisera raised against intact crotoxin and its subunits in the ELISA. Our results suggest that crotalid neurotoxins can be detected and neutralized by polyclonal antibodies raised against any intact toxin or basic subunit in this group of homologous toxins.

Decreasing burned children's pain behavior: impacting the trauma of hydrotherapy

We evaluated the effects of cartoon viewing with the use of a star feedback chart on two burned children's pain behavior during their physical therapy sessions. In addition, the degree to which the observational data corresponded with physical therapists' and mothers' ratings of the children's pain, fear, and cooperativeness was examined. Using a reversal single-subject design, the results showed that the children's pain behavior substantially decreased during experimental treatment sessions compared to their baseline levels. The rating scale data indicated that the physical therapist's and mother's rating of pain, anxiety, and cooperativeness were all correlated significantly with the observational data (p less than .05). The contributions of respondent and operant conditioning to the occurrence and treatment of pain behavior in burned children are discussed.

Effect of energy inhibitors on cell surface diphtheria toxin receptor numbers

The effects of various metabolic inhibitors on cell-surface receptors for diphtheria toxin were studied. Incubation of Vero cells at 37 degrees C with fluoride, azide, salicylate, and, to a lesser degree, with deoxyglucose resulted in an apparent loss of receptors for the toxin. With some drugs the loss was rapid (half-time approximately 8-9 min) while with others considerably longer time periods were required (half-time greater than or equal to 40 min). The loss of toxin receptors could also be induced by the drugs at 4 degrees C. Washing out the drugs and incubating the cells at 37 degrees C led to a complete restoration of receptors to control numbers. Restoration did not occur if cells were incubated at 4 degrees C. The regain of binding capacity was not dependent on RNA or protein synthesis. It is suggested that metabolic inhibitors may block a continuous process required to maintain toxin receptors in a functional state at the cell surface.

Inhibition of diphtheria toxin degradation and cytotoxic action by chloroquine

Chloroquine was found to prevent the cytotoxic action of diphtheria toxin on cultured monkey kidney cells. Analysis of the cellular processing of 125I-labeled diphtheria toxin showed that chloroquine does not affect the rate or extent of toxin uptake but substantially blocks degradation. These studies provide strong evidence that diphtheria toxin enters monkey kidney cells primarily by adsorptive endocytosis and suggest that lysosomal processing is involved in intracellular activation of the proenzyme form of the toxin.

The secondary structure of staphylococcal enterotoxins A, B and C

The circular dichroism (CD) of staphylococcal enterotoxins A, B and C was measured. The CD of enterotoxins B and C were almost identical from 250 to 320 nm, but differed from the CD of enterotoxin A. The spectrum of enterotoxin A in this wavelength region contained the same bands with respect to both location and sign, but with significant differences in intensity. The CD spectra of enterotoxins B and C were also much more alike from 190 to 250 nm. Although all three enterotoxins had a major negative extremum at 215--218 nm, its magnitude was equal in enterotoxins B and C, but was substantially decreased in enterotoxin A. The secondary structure of the enterotoxins contained little alpha-helix as analyzed with CD models. A secondary structure of entertoxin B compured from a scheme based on a joint prediction histogram of five separate methods, placed 29 residues in alpha-helices, 71 in beta-pleated sheets, 88 in beta-turns and 55 in aperiodic conformation.

Receptor-mediated internalization and degradation of diphtheria toxin by monkey kidney cells

The receptor-mediated internalization and degradation of radiolabeled diphtheria toxin by cultured monkey kidney cells was studied. The ability of a number of enzymes and chemicals to remove cell surface-bound toxin was tested; the combination of pronase and inositol hexaphosphate (PIHP) proved most effective. Using PIHP, the kinetics of toxin-cell association at 37 degrees C was resolved into two compounds: surface binding and internalization. The PIHP assay also allowed estimation of the half-time of toxin internalization (about 25 min). An assay involving precipitation of culture supernatants with trichloroacetic acid was developed and used to measure the rate of degradation and excretion of cell-associated toxin. Agents which markedly inhibited toxin internalization similarly prevented degradation, implying an intracellular location for the degradative process. The primary radioactive product excreted by Vero cells was monoiodotyrosine. The extent and rate of toxin degradation indicated lysosomal involvement. Finally, agents which blocked internalization or degradation, or both, (e.g. antibody and concanavalin A), protected cells from the cytotoxin action of diphtheria toxin, suggesting that these processes are necessary for expression of biological effect.

Protection of mammalian cells from diphtheria toxin by exogenous nucleotides

Exogenous nucleotides were found to protect mammalian cells from the lethal effects of diphtheria toxin. Protective potency of a given nucleotide was base specific and phosphate chain length dependent. Full expression of protective potency required an intact nucleotide, but the effect did not appear to be mediated by nucleotide-induced phosphorylation. Nucleotides antagonized the binding of diphtheria toxin to its cell surface receptor in a manner that correlated with the degree of protection. It was concluded that cellular protection from diphtheria toxin by nucleotides results from inhibition of toxin-receptor binding and that nucleotides therefore may serve as valuable research tools for future studies.

Association of diphtheria toxin with Vero cells. Demonstration of a receptor

The interaction of radiolabeled diphtheria toxin with highly sensitive mammalian cell lines was studied. Toxin bound to (or was taken up by) Vero cells at 4 and 37 degrees C in a highly specific manner. At both temperatures, excess unlabeled toxin competed for up to 90% of the cell-associated label. The association at 37 degrees C was biphasic, increasing to a peak at 1 to 2 h and falling thereafter. At 4 degrees C, association increased with time to a steady state. Both fragment B and CRM-197 competed for the association of labeled toxin with cells. The magnitude of association correlated with the cytotoxic sensitivity of several cell lines. Both pH and exogenous nucleotides affected the association in a manner consistent with effects on cytotoxicity. The label associated with cells at 4 degrees C was largely intact toxin, while that at 37 degrees C was degraded. At 4 degrees C, the association was saturable (K = 9 X 10(8) liters/mol), was reversible, and indicated about 1 to 2 X 10(5) binding sites/cell.

Differential chemical protection of mammalian cells from the exotoxins of Corynebacterium diphtheriae and Pseudomonas aeruginosa

Many drugs or chemicals had markedly different effects on the cytotoxicity induced by Pseudomonas aeruginosa exotoxin A (PE) or Corynebacterium diphtheriae exotoxin (DE). The glycolytic inhibitor NaF protected cells from DE but potentiated the cytotoxicity of PE. Another energy inhibitor, salicylic acid, also protected cells from DE but had no effect with PE. Colchicine and colcemid did not affect the cytotoxicity of either toxin. Cytochalasin B exhibited a modest protection from DE but no effect with PE. Ouabain, a specific inhibitor of the Na+, K+-dependent adenosine 5'-triphosphatase (ATPase), did not affect the cytotoxicity of either toxin. Ruthenium red, a specific inhibitor of the Ca2+, Mg2+,-dependent ATPase, conferred marked protection from DE-induced cytotoxicity but did not affect PE-induced cytotoxicity. A number of local anesthetics were tested, and they too presented differential results with PE and DE. Most chemicals that affected toxin-induced cytotoxicity had little or no influence on the in vitro adenosine 5'-diphosphate-ribosylation catalyzed by either toxin. This work presents further evidence that PE and DE have different mechanisms of intoxication and suggests that these differences lie in the attachment or internalization stages of intoxication.

Serum effects on the response of mammalian cells to the exotoxins of Pseudomonas aeruginosa and Corynebacterium diphtheriae

The response of mammalian cells to Pseudomonas and diphtheria exotoxins was studied. A method was developed whereby the sensitivity of cells to these two toxins could be quantitated. The method is versatile and can be used to study the effects of toxins on many cellular metabolic or transport processes. The type of serum used in the culture medium significantly influenced the response of cells to the toxins. Calf, horse, and human sera protected cells while fetal calf serum did not. Precipitation with (NH4)2SO4 demonstrated the probable presence of toxin-specific antibody in the protective calf serum while none was detected in the nonprotective fetal calf serum. The level of antibody in calf serum, as titrated by hemagglutination, was sufficient to account for all the observed protection. It is suggested that fetal calf serum be used for all future cell culture studies of bacterial toxins.

Response of cultured mammalian cells to the exotoxins of Pseudomonas aeruginosa and Corynebacterium diphtheriae: differential cytotoxicity

The sensitivities of 21 mammalian cell lines to the exotoxins of Pseudomonas aeruginosa and Corynebacterium diphtheriae were measured. Each line exhibited 1-4 log differences in sensitivities to the two toxins. No species-specific sensitivities were noted for Pseudomonas exotoxin while diphtheria exotoxin was most potent in cells of monkey origin, followed by human and hamster cells. Rat- and mouse-derived cell lines were very insensitive to diphtheria exotoxin. The rates of cellular intoxication by both toxins exhibited apparent first-order kinetics and were indistinguishable from one another when equipotent doses were used. Our preparation of diphtheria exotoxin appeared to have a slightly higher ADP-ribosylating efficiency than did Pseudomonas toxin. However, neither toxin exhibited cell line-specific differences in ribosylating efficiencies which could have explained the wide range in potencies for intact cells. Our results suggest that there are significant differences in the mechanisms of cellular intoxication by Pseudomonas and diphtheria exotoxins and that these differences probably exist in the attachment or internalization stages of toxin action.

Physicochemical properties of glucocorticoid receptors from mouse fibroblasts

Glucocorticoid-bound receptor macromolecules were prepared in three forms from mouse fibroblast cells: the cytosol receptor, the nuclear extractable receptor, and the nuclear residual form obtained by DNase digestion of chromatin samples. These receptor complexes were studied with respect to gel filtration properties, sedimentation velocities in various salt concentrations, partial specific volumes, isoelectric points, and thermal stability properties. The results indicate that the three forms of the receptor differ in their molecular properties, and nuclear translocation and binding ofthe receptor complex is associated with conformational and physical changes consistent with a reduction in apparent molecular weight.