Action of a thionin isolated from nuts of Pyrularia pubera on human erythrocytes

Stimulation of prothrombinase activity by the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, a segment of a plant thionin

Pyrularia thionin (PT) is a basic 47 amino acid peptide isolated from the nuts of Pyrularia pubera. Its structure and properties have been studied in some detail. Its receptor site is a domain of membrane phosphatidyl serine (PS), where it binds with a relatively high specificity. A segment of its covalent structure, the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, designated serine nonapeptide (SNP), corresponds to amino acids 7-15 of the thionin, except for the position 12 (Ser), which substitutes for Cys, to give stability. This peptide represents what we consider to be the active site of the thionin, and it also binds to PS domains, but less tightly than thionin does. The peptide has an effect on the prothrombinase assay using the chromophore S2238 to measure the thrombin produced by the prothrombinase complex. It is shown that SNP stimulates the prothrombinase complex activity, instead of inhibiting it, as would be expected if it simply covered the PS sites on the membrane of erythrocyte ghosts, used in the prothrombinase assay. SNP appears to substitute for Va in the prothrombinase complex reaction, in a Ca(2+) independent manner, being even more effective in the absence than in the presence of ghosts. In the clotting system, SNP can also substitute for Factor Va.

Modes of membrane interaction of a natural cysteine-rich peptide: viscotoxin A3

Among the very homologous family of alpha- and beta-thionins, known for their antimicrobial activity, the viscotoxin subfamily differs from other members because it is cytotoxic against tumoral cells but weakly hemolytic. We studied the interactions between the most active of these toxins, viscotoxin A3 (VA3), and model membranes made of phosphatidylcholine and phosphatidylserine (PS), the major zwitterionic and acidic phospholipids found in eukaryotic cells. Monolayer studies showed that electrostatic forces are essential for the interaction and are mainly involved in modulating the embedding of the toxin in the PS head group region. This in turn induces membrane stiffening, as shown by fluorescence polarization assays with 1,6-diphenyl-1,3,5-hexatriene and its derivatives. Moreover, vesicle permeabilization analyses showed that there are two modes of interaction, which are directly related to the stiffening effect and depend on the amount of VA3 bound to the surface of the vesicles. We propose an interaction model in which the embedding of VA3 in the membrane induces membrane defects leading to the gradual release of encapsulated dye. When the surfaces of the vesicles are saturated with the viscotoxin, complete vesicle destabilization is induced which leads to bilayer disruption, all-or-none encapsulated dye release and rearrangement of the vesicles.

Accidental cell death and generation of reactive oxygen intermediates in human lymphocytes induced by thionins from Viscum album L

The cytotoxic mechanisms of thionins from Viscum album L., the viscotoxins, were investigated in human granulocytes and lymphocytes. The time course of viscotoxin effects indicate accidental cell death, i.e. membrane permeabilization, degradation of cytoplasm and chromatin, swelling of mitochondria with loss of their cristae, and generation of reactive oxygen intermediates within 1-2 h, followed by secondary apoptosis-associated events. The viscotoxin homologue purothionin from whole-wheat flour and viscotoxin B, however, did not induce cell death in cultured lymphocytes. Cytotoxicity of cationic and amphipathic viscotoxin was prevented only by cleavage of its disulphide bridges.

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.

Interaction of Escherichia coli heat-stable enterotoxin B with cultured human intestinal epithelial cells

Binding of Escherichia coli heat-stable enterotoxin B (STb) to the human intestinal epithelial cell lines T84 and HT29 and to polarized T84 cells was studied to define the initial interaction of this peptide toxin with target cells. Equilibrium and competitive binding isotherms showed that 125I-STb bound specifically to T84 and HT29 cells; however, the toxin-epithelial cell interactions could be characterized by low-affinity binding (< or = 10(5) M(-1)) to a high number of binding sites (> or = 10(6) per cell). STb binding to T84 and HT29 cells as a function of 125I-STb concentration did not approach saturation at levels well above the effective biological concentration of STb for fluid secretion. Treatment of the 125I-STb-bound T84 and HT29 cells with an acidic saline solution to remove surface-bound toxin revealed that only approximately 55% +/- 10% of 125I-STb could be removed by this treatment at 4 degrees C, suggesting that approximately half of the bound STb was stably associated with the plasma membrane and/or internalized into the cytoplasm. Similar results were obtained when binding and internalization experiments were conducted at 22 and 37 degrees C. Immunofluorescence studies demonstrated that the strongest signal for STb appeared in the plasma membrane even after acid treatment. Toxin-treated cells also displayed diffuse cytoplasmic staining, indicating that once cell bound, STb did not appear to preferentially associate with membrane vesicles or cellular organelles. Binding and subsequent internalization of 125I-STb were not affected by treatment of the cells with trypsin, endoglycosidase F/peptide N-glycosidase F, Vibrio cholerae neuraminidase, tunicamycin, or 5 mM sodium chlorate, which blocks sulfation of surface proteoglycans. In addition, the internalization process was not altered by preincubation of the cells with the cytoskeleton inhibitors cytochalasin D and colchicine or cellular perturbants (i.e., 0.45 M sucrose and 5 mM sodium azide), indicating that cell surface proteins or carbohydrates did not function as STb receptors. The binding of 125I-STb to polarized T84 cells was also examined, and the total and nonspecific binding isotherms were found to overlap, indicating that the apical surface of polarized T84 cells did not contain a specific receptor for STb. In comparison to undifferentiated cells, twice the amount of bound STb (approximately 80% +/- 10%) was removable from polarized T84 cells after treatment with acidic solution. The percentage of surface-bound STb to polarized T84 cells did not vary significantly with the transepithelial electrical resistance of the cells or when STb was applied basolaterally. Together, our results indicate that STb binds with relatively low affinity to the plasma membrane of cultured intestinal epithelial cells and polarized T84 cells, probably to membrane lipids, and becomes stably associated with the lipid bilayer. The fact that a significant portion of the bound STb becomes free in the cytoplasm, even at a low temperature, suggests that the bound toxin may directly traverse the membrane bilayer.

Thionins: properties, possible biological roles and mechanisms of action

Thionins are low-molecular-weight proteins (M(r) ca. 5000) occurring in seeds, stems, roots and leaves of a number of plant species. The different members of this family of plant proteins show both sequence and structural homology, and are toxic to bacteria, fungi, yeasts and various naked cells in vitro. Toxicity requires an electrostatic interaction of the positively charged thionin with the negatively charged phospholipids making up the membrane, followed by either pore formation or a specific interaction with a certain lipid domain. This domain might be composed of phosphoinositides, which mediate transduction of environmental signals in eukaryotes. Their in vitro toxicity to plant pathogenic bacteria and fungi could reflect a direct role in plant defence, although, in view of the many divergent activities displayed by thionins both in vitro and in vivo, a biological role other than inhibition of microbial growth is equally plausible.

Enhancement of adenylate cyclase activity in S49 lymphoma cell membranes by the toxin thionin from Pyrularia pubera

We investigated the mode of action of Pyrularia thionin on adenylate cyclase in S49 lymphoma cell membranes. Thionin increased the forskolin-induced cAMP production in both wild-type and cyc- variant cell membranes. Thionin increased the apparent efficacy, but not potency, of the stimulation of adenylate cyclase by forskolin. Our results also indicated that the stimulatory G protein (Gs) was not involved in the action of thionin on adenylate cyclase; however, thionin appeared to reduce the effect of Gi on adenylate cyclase. We examined the effect of thionin on the temperature dependence of adenylate cyclase activity. The effect of thionin was not quantitatively the same at all temperatures. The effect was greatest in the range of about 30 degrees C to 50 degrees C. In addition, cardiotoxin, a structurally distinct peptide that alters the properties of biological membranes similarly to thionin, also enhanced the activity of adenylate cyclase. It seems reasonable to suspect that thionin influences the activity of adenylate cyclase indirectly by acting on the membrane.

Pyrularia thionin binding to and the role of tryptophan-8 in the enhancement of phosphatidylserine domains in erythrocyte membranes

Pyrularia thionin is a small, strongly basic peptide which interacts readily with cellular and synthetic membranes. With cells it induces hemolysis, depolarizes the cellular membrane with an accompanying influx of Ca2+, and activates an endogenous phospholipase A2. Evidence points toward a binding site involving phosphatidylserine (PS). This study shows that addition of the peptide to erythrocyte membranes as well as to vesicles formed from phospholipids isolated from erythrocyte membranes causes an enhancement of phospholipid domains which are made visible by the use of fluorescence digital imaging microscopy with fluorescent derivatives of PS (NBD-PS) and phosphatidylcholine (NBD-PC). Addition of thionin caused a large increase in NBD-PS domains, with an accompanying enrichment of NBD-PC in another separate domain. Double-labeling experiments performed with a Texas Red derivative of thionin show that the peptide binds to the domain enriched in NBD-PS. P thionin inactivated by modification of Trp-8 with N-bromosuccinimide lost the ability to enhance PS domains, although it bound to the membrane with the same affinity as native P thionin. This shows that binding to the membrane is not in itself sufficient to cause the NBD-PS and NBD-PC redistribution into domains.

Effects of different antagonists on depolarization of cultured chick myotubes by cobra venom cardiotoxins and Pyrularia thionin from the plant Pyrularia pubera

Cardiotoxins (3.12 and 3.12.1) purified from cobra venom (Naja naja siamensis) are basic single-chain polypeptides of about 60 residues. Although they depolarize nerve and muscle cells and have cytolytic effects, their mechanism of action is still unknown. Pyrularia thionin (P-thionin) isolated from nuts of the parasitic plant Pyrularia pubera is a strongly basic, single-chain polypeptide containing 47 residues. It is known to be haemolytic and cytotoxic, and to depolarize muscle cells, but its mechanism of action is unclear. The present studies explored the possible similarities between P-thionin and cobra venom cardiotoxins by comparing their effects on depolarization of cultured chick skeletal muscle cells in the presence and absence of possible antagonists. Cardiotoxins and P-thionin depolarized cultured chick skeletal muscle cells, but with P-thionin showing a steeper concentration-dependence. Ca2+ was more effective at reducing cardiotoxin action than P-thionin, while the Ca(2+)-channel blockers Ni2+ (100 microM) and verapamil (100 microM) had no blocking effects on the toxins. Ca2+ may block the binding of both toxins. Indomethacin (100 microM, an inhibitor of cyclooxygenase), quinacrine and dexamethasone (100 microM, inhibitors of phospholipase A2) did not block the effects of the toxins, implying that the actions on cultured chick skeletal muscle cells are not due to activation of endogenous phospholipase A2.

Pyrularia thionin increases arachidonate liberation and prolactin and growth hormone release from anterior pituitary cells

Pyrularia thionin is a 47 amino acid peptide isolated from the nuts of Pyrularia pubera. This peptide does not have intrinsic phospholipase A2 activity, but it increases the liberation of arachidonate from several tissues. Exposure of anterior pituitary cells to this toxin increases the liberation of arachidonate, increases the cellular levels of lysophospholipids, and decreases cellular phospholipids. Thus, phospholipase A2 is involved in the liberation of arachidonate stimulated by this peptide. Because this toxin also increases stearate liberation from the pituitary cells, either diacylglycerol lipase, phospholipase A1 or lysophospholipase may be directly or indirectly activated by this toxin. In addition to increasing fatty acid liberation, Pyrularia thionin increases the release of prolactin and growth hormone from anterior pituitary cells over the identical concentration ranges that this toxin liberates the fatty acids. Pyrularia thionin increased arachidonate liberation and prolactin release from perifused pituitary cells within 2 min, and following withdrawal of the toxin, arachidonate liberation and prolactin release returned to near basal levels within 6 min. Dopamine, a physiological inhibitor of prolactin release that closes calcium channels, decreased prolactin release stimulated by Pyrularia thionin. However, dopamine had no effect on the arachidonate liberation stimulated by this peptide. Similarly, D-600, an organic calcium channel blocker, decreased the prolactin and growth hormone release stimulated by the toxin without affecting the toxin-stimulated arachidonate liberation. Therefore, Pyrularia thionin increases arachidonate liberation through the rapid activation of phospholipase A2 by a mechanism that is not dependent on calcium uptake via D-600-inhibitable calcium channels. In contrast, the prolactin and growth hormone release stimulated by this toxin requires calcium uptake via D-600 inhibitable calcium channels.

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.

Binding properties of Pyrularia thionin and Naja naja kaouthia cardiotoxin to human and animal erythrocytes and to murine P388 cells

Pyrularia thionin and snake venom cardiotoxin are strongly basic peptides which induce hemolysis, depolarization of muscle cells and activation of endogenous phospholipase A2. An earlier study of the hemolysis reaction indicated that the two peptides bind to and compete for the same site on human erythrocytes. A recent study examined the hemolysis induced by both peptides as the phosphate and Ca2+ content of the reaction mixture was varied. The results of the recent study (VERNON, L. P. and ROGERS, A., Toxicon 30, 701-709) agree with this companion study on the binding of 125I-labeled pyrularia thionin and cardiotoxin to erythrocytes under the same conditions. Added phosphate ion at 5 mM and removal of membrane-bound Ca2+ by treatment with 10 mM EGTA make more binding sites of the same affinity available to both peptides, which are shown to bind in a competitive fashion to the same site. Addition of 10 mM Ca2+ to the medium decreases peptide binding due to competitive binding of Ca2+ to the same site on the membrane. For human erythrocytes the number of binding sites/cell for the thionin ranged from 0.7 to 1.7 x 10(5) and for cardiotoxin from 0.82 to 1.6 x 10(5). The calculated dissociation constants (Kd) from the Scatchard plots ranged from 0.43 to 1.1 microM for the thionin and from 0.40 to 0.98 microM for the cardiotoxin. The binding sites for thionin and cardiotoxin with sheep erythrocytes were 1.7 and 2.0 x 10(4) sites/cell, respectively, and both cow and horse erythrocytes demonstrated 2.7 x 10(4) sites/cell for the thionin. Binding studies with murine P388 cells showed 7.0 and 9.5 x 10(6) sites per cell for Pyrularia thionin and cardiotoxin, respectively.

Effect of calcium and phosphate ions on hemolysis induced by Pyrularia thionin and Naja naja kaouthia cardiotoxin

Pyrularia thionin is a strongly basic bioactive peptide of 47 amino acids isolated from nuts of Pyrularia pubera. It is hemolytic, cytotoxic and activates an endogenous phospholipase A2 in 3T3 cells. Earlier studies have shown that the cardiotoxin from Naja naja kaouthia has similar activities and binds to the same site as Pyrularia thionin. Since the peptides appear to bind to the phospholipids of cell membranes to elicit their cellular responses, the effect of modifying the electrostatic environment was studied by separately adding phosphate ion and Ca2+, and by removing Ca2+ from the membrane by treatment with EGTA. Analysis of erythrocyte hemolysis for both Pyrularia thionin and cardiotoxin shows that the reactions follow Michaelis-Menten kinetics, with the peptides serving as the substrate. The basal rate of hemolysis in physiological saline is markedly increased by the addition of phosphate in the 5-10 mM range and also by removing membrane-bound Ca2+ by incubation of the cells with 10 mM EGTA. These treatments do not change the apparent K(m) values, but increase the V(max), indicating that more binding sites are made available by these treatments. On the other hand, added Ca2+ in the 5-10 mM range competitively inhibits the reaction by inhibiting the binding of the peptide to the membrane.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Hemolysis of erythrocytes and fluorescence polarization changes elicited by peptide toxins, aliphatic alcohols, related glycols and benzylidene derivatives

Hemolysis rates of human erythrocytes induced by C2 and C8-C14 straight chain 1-alkanols, 1,2-alkanediols and the corresponding benzylidene derivatives (benzaldehyde acetals) have been studied and compared with hemolysis rates obtained by three peptide toxins. The peak of activity occurs at C12 for the alkanols and glycols and at C10 for the benzylidene derivatives. The most active compound is 1-dodecanol, followed by 1,2-dodecanediol and the C10 benzylidene acetal, which show 50% hemolysis at 15, 99 and 151 microM, respectively, at 37 degrees C. A few lysolecithins and longer chain cis-unsaturated alcohols were studied for comparison purposes, and were found to be more active than 1-dodecanol. The most active were the 16:0 lysolecithin and cis-9-tetradecene-1-ol, which gave 50% hemolysis at concentrations of 2.8 and 5.6 microM respectively. The hemolytic activities of 1-dodecanol, 1,2-dodecanediol and the C10 benzylidene acetal were compared to activities of Pyrularia thionin and melittin with cow, horse, sheep, pig and human erythrocytes. Whereas the peptide toxins showed clear specificity for human erythrocytes, no selectivity was shown by any of the other compounds tested. Addition of the thionin or Naja naja kaouthia cardiotoxin to erythrocyte ghosts caused a slight but reproducible increase in the order of the phospholipid bilayer, as measured with the fluorescent probe NBD-PC. Cardiotoxin gave a greater response than did the P thionin, and extensively iodinated P thionin gave a smaller change than did P thionin. Similar results were obtained with melittin, but this peptide gave a markedly greater response than all other peptides. Addition of dodecanol or the C10 benzylidene acetal caused a marked increase in membrane fluidity. All of these data indicate that the organic compounds interact directly with and are incorporated nonspecifically into the membrane lipid bilayer, but the peptide toxins interact specifically with some component on the surface of the membrane, either a protein or specific phospholipid domain, followed by insertion into the membrane and decreasing phospholipid movement.

Phospholipase activation in the cytotoxic mechanism of thionin purified from nuts of Pyrularia pubera

Treating NIH3T3 fibroblast cells with Pyrularia pubera thionin (100 micrograms/ml) stimulated release of labelled free fatty acids from phospholipids biosynthetically labelled by incorporation of [3H]arachidonic acid. Since Pyrularia thionin exhibited no detectable phospholipase activity, it was concluded that the release response represented activation of endogenous phospholipases in the cells. The phospholipase activated by Pyrularia thionin (100 micrograms/ml) stimulated the release of 61% of the incorporated [3H]arachidonate in the presence of 1.8 mM extracellular calcium with maximum activation at 90 min following an initial lag period of about 20 min. The release response exhibited little dependence on extracellular calcium at this thionin concentration, but at concentrations 20 micrograms/ml, extracellular calcium appeared to be inhibitory to phospholipase activation. Some characteristics of the fatty acid release response are consistent with activation of a lysosomal phospholipase being part of the cellular response to Pyrularia thionin. Activation of a lysosomal enzyme can occur independently or as a result of coordinate activation of the whole lysosome, which would expose other cellular components of degradative lysosomal enzymes. Consistent with coordinate activation of lysosomal enzymes, Pyrularia thionin also stimulates the production of small, trichloroacetic acid-soluble peptides and nucleic acid fragments from biosynthetically-labelled RNA and proteins in treated cells. It is not clear from the results obtained what role, if any, activation of lysosomal enzymes plays in the overall toxic response to Pyrularia thionin in NIH3T3 cells. However, Pyrularia pubera thionin may represent a useful tool for studying the regulation of lysosomal enzymes and their roles in cells.

Cellular responses to Pyrularia thionin are mediated by Ca2+ influx and phospholipase A2 activation and are inhibited by thionin tyrosine iodination

Pyrularia thionin, isolated from nuts of Pyrularia pubera, is a strongly basic peptide of 47 amino acids. The amino acid sequence and configuration of its four disulfide bonds place this plant peptide, known to be hemolytic, cytotoxic, and neurotoxic, among the thionins. We report and compare several cellular responses mediated by Pyrularia thionin: hemolysis of human erythrocytes, activation of an endogenous phospholipase A2 in Swiss 3T3 cells, cytotoxicity toward HeLa and mouse B16 melanoma cells in culture, viability of rat hepatocytes and lymphocytes measured by trypan blue exclusion, and lethality in mice. Cellular responses related to ion movement include a toxin-mediated influx of Ca2+ into mouse P388 cells measured by Fura-2 fluorescence, depolarization of mouse P388 plasma membrane measured by fluorescence of bis(1,3-diethylthiobarbituric acid)trimethine oxonol (bisoxonol), and depolarization of frog (Rana pipiens) sartorius muscle determined by direct measurement of membrane potential. Graded iodination of Pyrularia thionin leads to a related loss of activity for hemolysis, phospholipase A2 activation, cytotoxicity, and lethality in mice. The mediated Ca2+ influx into and depolarization of P388 cells require Ca2+ in the external medium and are inhibited by 100 microM Ni2+. Depolarization of sartorius muscle by Pyrularia thionin also requires a functional Ca2+ channel, as shown by verapamil inhibition. This muscle depolarization also involves phospholipase A2 activation because dexamethasone and quinacrin, but not indomethacin, protect against depolarization. The IC50 values for viability of rat hepatocytes and splenic lymphocytes measured by trypan blue exclusion were 0.17 and 40 microM, respectively. The general response of cells to Pyrularia thionin involves a membrane alteration leading to depolarization and a channel-mediated influx of Ca2+. There is a related activation of phospholipase A2 that results in loss of membrane integrity, hemolysis in the case of erythrocytes, and eventually cell death. Iodination of Pyrularia thionin leads to a corresponding inhibition of all three cellular responses, which indicates an essential role for tyrosine in either maintenance of peptide structure or interaction of the peptide with cellular membranes.

Hemolytic activity of thionin from Pyrularia pubera nuts and snake venom toxins of Naja naja species: Pyrularia thionin and snake venom cardiotoxin compete for the same membrane site

Pyrularia thionin (P. thionin) is a strongly basic peptide of 47 amino acids which is hemolytic, cytotoxic and neurotoxic. It shows the greatest hemolytic activity toward human erythrocytes. Rabbit, guinea pig and pig erythrocytes show decreasing activity in that order, and little or no activity is shown with sheep, horse, cow or mouse erythrocytes. Crotalus venoms are inactive, but the venoms from Naja naja atra, Naja naja ceylonicus and Naja naja melanoleuca and, more specifically, cardiotoxin from Naja naja kaouthia have significant hemolytic activities toward human erythrocytes. The cardiotoxin preparation used had no phospholipase activity, and was less active than P. thionin (23% compared to 35% hemolysis by P. thionin in 60 min at 10 micrograms/ml toxin). Since iodinated P. thionin is inactive, it was used as an inhibitor of hemolysis catalyzed by native P. thionin, N. ceylonicus venom and by cardiotoxin. Examination of the kinetics of the reactions catalyzed by N. ceylonicus venom and cardiotoxin in the absence and presence of iodinated P. thionin shows that both N. ceylonicus venom and cardiotoxin exhibit Michaelis-Menten kinetics, yielding apparent Km values of 7.4 micrograms/ml and 0.69 microM, respectively. These values compare to an apparent Km for P. thionin of 1.6 microM for erythrocyte hemolysis and a binding constant of 2.1 microM (Osorio e Castro, V. R. Van Kuiken, B. A. and Vernon, L. P. (1989) Action of a thionin isolated from nuts of Pyrularia pubera on human erythrocytes. Toxicon 27, 501). The inhibition constants Ki for iodinated P. thionin in the reactions with N. ceylonicus venom and cardiotoxin are 3.8 and 5.3 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)