Amino acid sequence of a presynaptic neurotoxin from the venom of Notechis scutatus scutatus (Australian tiger snake)

Four Decades of Cytochrome P450 2B Research: From Protein Adducts to Protein Structures and Beyond

This article features selected findings from the senior author and colleagues dating back to 1978 and covering approximately three-fourths of the 60 years since the discovery of cytochrome P450. Considering the vast number of P450 enzymes in this amazing superfamily and their importance for so many fields of science and medicine, including drug design and development, drug therapy, environmental health, and biotechnology, a comprehensive review of even a single topic is daunting. To make a meaningful contribution to the 50th anniversary of Drug Metabolism and Disposition, we trace the development of the research in a single P450 laboratory through the eyes of seven individuals with different backgrounds, perspectives, and subsequent career trajectories. All co-authors are united in their fascination for the structural basis of mammalian P450 substrate and inhibitor selectivity and using such information to improve drug design and therapy. An underlying theme is how technological advances enable scientific discoveries that were impossible and even inconceivable to prior generations. The work performed spans the continuum from: 1) purification of P450 enzymes from animal tissues to purification of expressed human P450 enzymes and their site-directed mutants from bacteria; 2) inhibition, metabolism, and spectral studies to isothermal titration calorimetry, deuterium exchange mass spectrometry, and NMR; 3) homology models based on bacterial P450 X-ray crystal structures to rabbit and human P450 structures in complex with a wide variety of ligands. Our hope is that humanizing the scientific endeavor will encourage new generations of scientists to make fundamental new discoveries in the P450 field. SIGNIFICANCE STATEMENT: The manuscript summarizes four decades of work from Dr. James Halpert's laboratory, whose investigations have shaped the cytochrome P450 field, and provides insightful perspectives of the co-authors. This work will also inspire future drug metabolism scientists to make critical new discoveries in the cytochrome P450 field.

A pan-specific antiserum produced by a novel immunization strategy shows a high spectrum of neutralization against neurotoxic snake venoms

Snakebite envenomation is a neglected tropical disease of high mortality and morbidity largely due to insufficient supply of effective and affordable antivenoms. Snake antivenoms are mostly effective against the venoms used in their production. It is thus crucial that effective and affordable antivenom(s) with wide para-specificity, capable of neutralizing the venoms of a large number of snakes, be produced. Here we studied the pan-specific antiserum prepared previously by a novel immunization strategy involving the exposure of horses to a ‘diverse toxin repertoire’ consisting of 12 neurotoxic Asian snake toxin fractions/ venoms from six species. This antiserum was previously shown to exhibit wide para-specificity by neutralizing 11 homologous and 16 heterologous venoms from Asia and Africa. We now show that the antiserum can neutralize 9 out of 10 additional neurotoxic venoms. Altogether, 36 snake venoms belonging to 10 genera from 4 continents were neutralized by the antiserum. Toxin profiles previously generated using proteomic techniques of these 36 venoms identified α-neurotoxins, β-neurotoxins, and cytotoxins as predominant toxins presumably neutralized by the antiserum. The bases for the wide para-specificity of the antiserum are discussed. These findings indicate that it is feasible to generate antivenoms of wide para-specificity against elapid neurotoxic venoms from different regions in the world and raises the possibility of a universal neurotoxic antivenom. This should reduce the mortality resulting from neurotoxic snakebite envenomation.

Toxinology provides multidirectional and multidimensional opportunities: A personal perspective

In nature, toxins have evolved as weapons to capture and subdue the prey or to counter predators or competitors. When they are inadvertently injected into humans, they cause symptoms ranging from mild discomfort to debilitation and death. Toxinology is the science of studying venoms and toxins that are produced by a wide variety of organisms. In the past, the structure, function and mechanisms of most abundant and/or most toxic components were characterized to understand and to develop strategies to neutralize their toxicity. With recent technical advances, we are able to evaluate and determine the toxin profiles using transcriptomes of venom glands and proteomes of tiny amounts of venom. Enormous amounts of data from these studies have opened tremendous opportunities in many directions of basic and applied research. The lower costs for profiling venoms will further fuel the expansion of toxin database, which in turn will provide greater exciting and bright opportunities in toxin research.

Varespladib (LY315920) and Methyl Varespladib (LY333013) Abrogate or Delay Lethality Induced by Presynaptically Acting Neurotoxic Snake Venoms

The phospholipase A₂ (PLA₂) inhibitor Varespladib (LY315920) and its orally bioavailable prodrug, methyl-Varespladib (LY333013) inhibit PLA₂ activity of a wide variety of snake venoms. In this study, the ability of these two forms of Varespladib to halt or delay lethality of potent neurotoxic snake venoms was tested in a mouse model. The venoms of Notechis scutatus, Crotalus durissus terrificus, Bungarus multicinctus, and Oxyuranus scutellatus, all of which have potent presynaptically acting neurotoxic PLA₂s of variable quaternary structure, were used to evaluate simple dosing regimens. A supralethal dose of each venom was injected subcutaneously in mice, followed by the bolus intravenous (LY315920) or oral (LY333013) administration of the inhibitors, immediately and at various time intervals after envenoming. Control mice receiving venom alone died within 3 h of envenoming. Mice injected with O. scutellatus venom and treated with LY315920 or LY333013 survived the 24 h observation period, whereas those receiving C. d. terrificus and B. multicinctus venoms survived at 3 h or 6 h with a single dose of either form of Varespladib, but not at 24 h. In contrast, mice receiving N. scutatus venom and then the inhibitors died within 3 h, similarly to the control animals injected with venom alone. LY315920 was able to reverse the severe paralytic manifestations in mice injected with venoms of O. scutellatus, B. multicinctus, and C. d. terrificus. Overall, results suggest that the two forms of Varespladib are effective in abrogating, or delaying, neurotoxic manifestations induced by some venoms whose neurotoxicity is mainly dependent on presynaptically acting PLA₂s. LY315920 is able to reverse paralytic manifestations in severely envenomed mice, but further work is needed to understand the significance of species-specific differences in animal models as they compare to clinical syndromes in human and for potential use in veterinary medicine.

So many roads traveled: A career in science and administration

I have traveled many roads during my career. After spending my first 19 years in Los Angeles, I became somewhat of an academic nomad, studying and/or working in six universities in the United States and three in Sweden. In chronological order, I have a B.A. in Scandinavian languages and literature from UCLA, a Ph.D. in biochemistry from Uppsala University, and an M.S. in toxicology from the Karolinska Institute. I have been in schools of natural science, pharmacy, and medicine and have worked in multiple basic science departments and one clinical department. I have served as a research-track and tenured faculty member, department chair, associate dean, and dean. My research has spanned toxinology, biochemistry, toxicology, and pharmacology. Through all the moves, I have gained much and lost some. For the past 40 years, my interest has been cytochrome P450 structure-function and structure-activity relationships. My lab has focused on CYP2B enzymes using X-ray crystallography, site-directed mutagenesis, deuterium-exchange MS, isothermal titration calorimetry, and computational methods in conjunction with a variety of functional assays. This comprehensive approach has enabled detailed understanding of the structural basis of the remarkable substrate promiscuity of CYP2B enzymes. We also have investigated the mechanisms of CYP3A4 allostery using biophysical and advanced spectroscopic techniques, and discovered a pivotal role of P450-P450 interactions and of multiple-ligand binding. A major goal of this article is to provide lessons that may be useful to scientists in the early and middle stages of their careers and those more senior scientists contemplating an administrative move.

Exploring the structural and functional aspects of the phospholipase A2 from Naja spp

Naja spp. venom is a natural source of active compounds with therapeutic application potential. Phospholipase A₂ (PLA₂) is abundant in the venom of Naja spp. and can perform neurotoxicity, cytotoxicity, cardiotoxicity, and hematological disorders. The PLA₂s from Naja spp. venoms are Asp 49 isoenzymes with the exception of PLA₂ Cys 49 from Naja sagittifera. When looking at the functional aspects, the neurotoxicity occurs by PLA₂ called β-toxins that have affinity for phosphatidylcholine in nerve endings and synaptosomes membranes, and by α-toxins that block the nicotinic acetylcholine receptors in the neuromuscular junctions. In addition, these neurotoxins may inhibit K⁺ and Ca⁺⁺ channels or even interfere with the Na⁺/K⁺/ATPase enzyme. The disturbance in the membrane fluidity also results in inhibition of the release of acetylcholine. The PLA₂ can act as anticoagulants or procoagulant. The cytotoxicity exerted by PLA₂s result from changes in the cardiomyocyte membranes, triggering cardiac failure and hemolysis. The antibacterial activity, however, is the result of alterations that decrease the stability of the lipid bilayer. Thus, the understanding of the structural and functional aspects of PLA₂s can contribute to studies on the toxic and therapeutic mechanisms involved in the envenomation by Naja spp. and in the treatment of pathologies.

Snake Venom PLA2, a Promising Target for Broad-Spectrum Antivenom Drug Development

Snakebite envenomation is a neglected global health problem, causing substantial mortality, disability, and psychological morbidity, especially in rural tropical and subtropical zones. Antivenin is currently the only specific medicine for envenomation. However, it is restricted by cold storage, snakebite diagnosis, and high price. Snake venom phospholipase A₂s (svPLA₂s) are found in all kinds of venomous snake families (e.g., Viperidae, Elapidae, and Colubridae). Along with their catalytic activity, svPLA₂s elicit a wide variety of pharmacological effects that play a pivotal role in envenomation damage. Hence, neutralization of the svPLA₂s could weaken or inhibit toxic damage. Here we overviewed the latest knowledge on the distribution, pathophysiological effects, and inhibitors of svPLA₂s to elucidate the potential for a novel, wide spectrum antivenom drug targeting svPLA₂s.

Cross-Neutralisation of In Vitro Neurotoxicity of Asian and Australian Snake Neurotoxins and Venoms by Different Antivenoms

There is limited information on the cross-neutralisation of neurotoxic venoms with antivenoms. Cross-neutralisation of the in vitro neurotoxicity of four Asian and four Australian snake venoms, four post-synaptic neurotoxins (α-bungarotoxin, α-elapitoxin-Nk2a, α-elapitoxin-Ppr1 and α-scutoxin; 100 nM) and one pre-synaptic neurotoxin (taipoxin; 100 nM) was studied with five antivenoms: Thai cobra antivenom (TCAV), death adder antivenom (DAAV), Thai neuro polyvalent antivenom (TNPAV), Indian Polyvalent antivenom (IPAV) and Australian polyvalent antivenom (APAV). The chick biventer cervicis nerve-muscle preparation was used for this study. Antivenom was added to the organ bath 20 min prior to venom. Pre- and post-synaptic neurotoxicity of Bungarus caeruleus and Bungarus fasciatus venoms was neutralised by all antivenoms except TCAV, which did not neutralise pre-synaptic activity. Post-synaptic neurotoxicity of Ophiophagus hannah was neutralised by all antivenoms, and Naja kaouthia by all antivenoms except IPAV. Pre- and post-synaptic neurotoxicity of Notechis scutatus was neutralised by all antivenoms, except TCAV, which only partially neutralised pre-synaptic activity. Pre- and post-synaptic neurotoxicity of Oxyuranus scutellatus was neutralised by TNPAV and APAV, but TCAV and IPAV only neutralised post-synaptic neurotoxicity. Post-synaptic neurotoxicity of Acanthophis antarcticus was neutralised by all antivenoms except IPAV. Pseudonaja textillis post-synaptic neurotoxicity was only neutralised by APAV. The α-neurotoxins were neutralised by TNPAV and APAV, and taipoxin by all antivenoms except IPAV. Antivenoms raised against venoms with post-synaptic neurotoxic activity (TCAV) cross-neutralised the post-synaptic activity of multiple snake venoms. Antivenoms raised against pre- and post-synaptic neurotoxic venoms (TNPAV, IPAV, APAV) cross-neutralised both activities of Asian and Australian venoms. While acknowledging the limitations of adding antivenom prior to venom in an in vitro preparation, cross-neutralization of neurotoxicity means that antivenoms from one region may be effective in other regions which do not have effective antivenoms. TCAV only neutralized post-synaptic neurotoxicity and is potentially useful in distinguishing pre-synaptic and post-synaptic effects in the chick biventer cervicis preparation.

A distinct three-helix centipede toxin SSD609 inhibits I(ks) channels by interacting with the KCNE1 auxiliary subunit

KCNE1 is a single-span transmembrane auxiliary protein that modulates the voltage-gated potassium channel KCNQ1. The KCNQ1/KCNE1 complex in cardiomyocytes exhibited slow activated potassium (I(ks)) currents. Recently, a novel 47-residue polypeptide toxin SSD609 was purified from Scolopendra subspinipes dehaani venom and showed I(ks) current inhibition. Here, chemically synthesized SSD609 was shown to exert I(ks) inhibition in extracted guinea pig cardiomyocytes and KCNQ1/KCNE1 current attenuation in CHO cells. The K(+) current attenuation of SSD609 showed decent selectivity among different auxiliary subunits. Solution nuclear magnetic resonance analysis of SSD609 revealed a distinctive three-helix conformation that was stabilized by a new disulfide bonding pattern as well as segregated surface charge distribution. Structure-activity studies demonstrated that negatively charged Glu19 in the amphipathic extracellular helix of KCNE1 was the key residue that interacted with SSD609. The distinctive three-helix centipede toxin SSD609 is known to be the first polypeptide toxin acting on channel auxiliary subunit KCNE1, which suggests a new type of pharmacological regulation for ion channels in cardiomyocytes.

Structural basis for functional diversity of animal toxins

The diversity of biological functions that are exerted by toxins from snake and scorpion venoms is associated with a limited number of structural frameworks. At present, one predominant basic fold has been observed among scorpion toxins whereas six folds have been found among snake toxins. Most toxin folds have the capacity to accept multiple insertions, deletions and mutations and to exert various recognition functions. We suggest that such folds may serve as guides to engineer new protein functions.

The role of multifunctional delivery scaffold in the ability of cultured myoblasts to promote muscle regeneration

Many cell types of therapeutic interest, including myoblasts, exhibit reduced engraftment if cultured prior to transplantation. This study investigated whether polymeric scaffolds that direct cultured myoblasts to migrate outwards and repopulate the host damaged tissue, in concert with release of angiogenic factors designed to enhance revascularizaton of the regenerating tissue, would enhance the efficacy of this cell therapy and lead to functional muscle regeneration. This was investigated in the context of a severe injury to skeletal muscle tissue involving both myotoxin-mediated direct damage and induction of regional ischemia. Local and sustained release of VEGF and IGF-1 from macroporous scaffolds used to transplant and disperse cultured myogenic cells significantly enhanced their engraftment, limited fibrosis, and accelerated the regenerative process. This resulted in increased muscle mass and, improved contractile function. These results demonstrate the importance of finely controlling the microenvironment of transplanted cells in the treatment of severe muscle damage.

Snake venoms and their toxins: An Australian perspective

Australia is home to a vast collection of highly venomous terrestrial and marine snakes. As such, Australia has proven to be an excellent source of investigative material for both local and international toxinologists. Research on snake venoms initially focussed on identifying the most lethal species, and the venom components responsible for the lethality, so that treatment strategies could be implemented. Since then, the focus of research has included the isolation and characterisation of toxins (primarily neurotoxins), examination of the efficacy of commercially available antivenoms and, more recently, the use of liquid chromatography/mass spectrometry (LCMS) to aid in the analysis of whole venoms. Given the vast quantity of research undertaken over the past 70 yr we have tried to provide a short insight into some of this excellent work and identify areas requiring further examination.

Functional characterization of a basic D49 phospholipase A2 (LmTX-I) from the venom of the snake Lachesis muta muta (bushmaster)

The whole venom of Lachesis muta muta is preponderantly neurotoxic but moderately myotoxic on the chick biventer cervicis preparation (BCp). We have now examined these toxic activities of a basic phospholipase A(2), LmTX-I, isolated from the whole venom. LmTX-I caused a significant concentration-dependent neuromuscular blockade in the BCp. The time to produce 50% neuromuscular blockade was 14.7+/-0.75 min (30 microg/ml), 23.6+/-0.9 min (10 microg/ml), 34+/-1.7 min (2.5 microg/ml) and 39.2+/-3.6 min (1 microg/ml), (n=5/concentration; p<0.05). Complete blockade with all tested concentrations was not accompanied by inhibition of the response to ACh. At the highest concentration, LmTX-I (30 microg/ml) significantly reduced contractures elicited by exogenous KCl (20mM), increased the release of creatine kinase (1542.5+/-183.9 IU/L vs 442.7+/-39.8 IU/L for controls after 120 min, p<0.05), and induced the appearance of degenerating muscle fibers ( approximately 15%). Quantification of myonecrosis indicated 14.8+/-0.8 and 2.0+/-0.4%, with 30 and 10 microg/mlvenom concentration, respectively, against 1.07+/-0.4% for control preparations. The findings indicate that the basic PLA(2) present on venom from L. m. muta (LmTX-I) possesses a dominant neurotoxic action on isolated chick nerve-muscle preparations, whereas myotoxicity was mainly observed at the highest concentration used (30 microg/ml). These effects of LmTX-I closely reproduce the effects of the whole venom of L. m. muta in chick neuromuscular preparations.

Chemical modification of arginine residues of Notechis scutatus scutatus notexin

Notexin, a presynaptic phospholipase A2 (PLA2) neurotoxin isolated from Notechis scutatus scutatus venom, was inactivated by arginine-specific reagents, phenylglyoxal and 1,2-cyclohexanedione. Kinetic analyses of the modification reaction revealed that the inactivation of notexin followed pseudo-first order kinetics and the loss of PLA2 activity was correlated with the incorporation of one molecule of modification reagent per toxin molecule. However, the results of amino acid analysis and sequence determination revealed that two arginine residues at positions 43 and 79 of notexin were modified simultaneously. Modification of the arginine residues was accompanied with a decrease in the ability to inhibit the indirectly evoked contraction of chick biventer cervicis muscle and bind with synaptic membranes. The secondary structure of the toxin molecule did not significantly change after modification with phenylglyoxal as revealed by the CD spectra. The modified derivative retained its affinity for Ca2+, indicating that the modified arginine residues did not participate in Ca2+ -binding. Together with the notion that Arg-43 and Arg-79 of notexin are located in the proximity of its catalytic site and toxic site, respectively, our results suggest that modification of Arg-43 and Arg-79 should differently contribute to the observed decrease in the PLA2 activity and neurotoxic effect of notexin.

Comparison of the in vitro neuromuscular activity of venom from three Australian snakes (Hoplocephalus stephensi, Austrelaps superbus and Notechis scutatus): efficacy of tiger snake antivenom

1. Tiger snake antivenom, raised against Notechis scutatus venom, is indicated not only for the treatment of envenomation by this snake, but also that of the copperhead (Austrelaps superbus) and Stephen's banded snake (Hoplocephalus stephensi). The present study compared the neuromuscular pharmacology of venom from these snakes and the in vitro efficacy of tiger snake antivenom. 2. In chick biventer cervicis muscle and mouse phrenic nerve diaphragm preparations, all venoms (3-10 microg/mL) produced inhibition of indirect twitches. In the biventer muscle, venoms (10 micro g/mL) inhibited responses to acetylcholine (1 mmol/L) and carbachol (20 micromol/L), but not KCl (40 mmol/L). The prior (10 min) administration of 1 unit/mL antivenom markedly attenuated the neurotoxic effects of A. superbus and N. scutatus venoms (10 microg/mL), but was less effective against H. stephensi venom (10 microg/mL); 5 units/mL antivenom attenuated the neurotoxic activity of all venoms. 3. Administration of 5 units/mL antivenom at t90 partially reversed, over a period of 3 h, the inhibition of twitches produced by N. scutatus (10 microg/mL; 41% recovery), A. superbus (10 microg/mL; 25% recovery) and H. stephensi (10 microg/mL; 50% recovery) venoms. All venoms (10-100 microg/mL) also displayed signs of in vitro myotoxicity. 4. The results of the present study indicate that all three venoms contain neurotoxic activity that is effectively attenuated by tiger snake antivenom.

Functional characteristics of a phospholipase A(2) inhibitor from Notechis ater serum

A phospholipase A(2) inhibitor has been purified p6om the serum of Notechis ater using DEAE-Sephacel chromatography. The inhibitor was found to be composed of two protein subunits (alpha and beta) that form the intact complex of approximately 110 kDa. The alpha-chain is a 30-kDa glycoprotein and the beta-chain a nonglycosylated, 25-kDa protein. N-terminal sequence analysis reveals a high level of homology to other snake phospholipase A(2) inhibitors. The inhibitor was shown to be extremely pH and temperature stable. The inhibitor was tested against a wide variety of phospholipase A(2) enzymes and inhibited the enzymatic activity of all phospholipase A(2) enzymes tested, binding with micromole to nanomole affinity. Furthermore, the inhibitor was compared with the Eli-Lilly compound LY311727 and found to have a higher affinity for human secretory nonpancreatic phospholipase A(2) than this chemical inhibitor. The role of the carbohydrate moiety was investigated and found not to affect the in vitro function of the inhibitor.

Targeting of venom phospholipases: the strongly anticoagulant phospholipase A(2) from Naja nigricollis venom binds to coagulation factor Xa to inhibit the prothrombinase complex

The strongly anticoagulant basic phospholipase A(2) (CM-IV) from Naja nigricollis venom has previously been shown to inhibit the prothrombinase complex of the coagulation cascade by a novel nonenzymatic mechanism (S. Stefansson, R. M. Kini, and H. J. Evans Biochemistry 29, 7742-7746, 1990). That work indicated that CM-IV is a noncompetitive inhibitor and thus it interacts with either factor Va or factor Xa, or both. We further examined the interaction of CM-IV and the protein components of the prothrombinase complex. Isothermal calorimetry studies indicate that CM-IV does not bind to prothrombin or factor Va, but only to factor Xa. CM-IV has no effect on the cleavage of prothrombin by factor Xa in the absence of factor Va. However, in the presence of factor Va, CM-IV inhibits thrombin formation by factor Xa. With a constant amount of CM-IV, raising the concentration of factor Va relieved the inhibition. The phospholipase A(2) enzyme inhibits by competing with factor Va for binding to factor Xa and thus prevents formation of the normal Xa-Va complex or replaces bound factor Va from the complex. Thus factor Xa is the target protein of this anticoagulant phospholipase A(2), which exerts its anticoagulant effect by protein-protein rather than protein-phospholipid interactions.

Crystal structure of agkistrodotoxin, a phospholipase A2-type presynaptic neurotoxin from agkistrodon halys pallas

The crystal structure of agkistrodotoxin containing eight copies of molecules in the asymmetric unit has been determined at 2.8 A resolution to a crystallographic R factor of 0.207 by the molecular replacement technique. Two spatially adjacent regions of agkistrodotoxin molecule, turn 55-61 and stretch 85-91, are remarkably different from those of non-neurotoxic isoforms in conformation and electrostatic characteristics. These regions are likely to be involved in the recognition of agkistrodotoxin towards the specific receptor at the presynaptic membrane. The structural comparison of the interfacial recognition site with non-neurotoxic isoforms reveals a decreased hydrophobicity and lack of residues with bulky hydrophobic side-chains (i.e. Trp) to serve as membrane anchors. This structural feature of agkistrodotoxin may be related to the reduced non-specific binding of the toxin to non-targeted membrane before it arrives at the presynaptic membrane and recognizes the putative receptor. A unique hydrophobic patch including residues I19, P20, F21, A23, F24, M118 and F119 is found on the surface of the molecule near the entrance of the hydrophobic channel which plays an important role in crystal packing. The interaction mode between the patches might give a clue to the binding of the neurotoxin on the membrane. The agkistrodotoxin molecules in the asymmetric unit form two tetramers and each tetramer exhibits a novel "dimer of dimers"-like structure. A molecule-spanning four-stranded antiparallel beta-sheet is formed by the beta-wings of two molecules within a tetramer.

Purification and properties of phospholipases A2 from the crown-of-thorns starfish (Acanthaster planci) venom

Two phospholipases A < inf2 (named AP-PLA2-I and II) were purified from the crown-of-thorns starfish (Acanthaster planci) venom. Both enzymes were confirmed to be PLA2s, based on the results that they showed hemolytic activity only in the presence of phosphatidylcholine (PC) and also released fluorescent fatty acids from PC with labeled fatty acids at the sn-2 position. The enzyme activity of both PLA2s was enhanced by Ca2+ but reduced by Cu2+ and Zn2+. The molecular mass of AP-PLA2-I was estimated to be 28 kDa by gel filtration and 15 kDa by SDS-PAGE, indicating that AP-PLA2-I is a dimer composed of the same subunit. In contrast, AP-PLA2-II was judged to be a monomer with a molecular mass of 12 kDa (gel filtration) or 15 kDa (SDS-PAGE). The amino acid compositions of the two enzymes were comparable to each other; Asx, Glx and Gly were rich in both molecules, while Met, His and Trp were poor. Analyses by a sequencer determined the first 62 amino acid residues for both PLA2s. In the AP-PLA2-I preparation, minor amino acids were additionally found at 17 positions, suggesting the coexistence of another PLA2-component. As compared to the N-terminal sequences of the known PLA2s, both AP-PLA2-I and II were identified as class I enzymes not only because they have Cys-11 and lack Cys-51 but also because they contain the elapid loop in the region 53-61.

Molecular evolution of snake toxins: is the functional diversity of snake toxins associated with a mechanism of accelerated evolution?

Recent studies revealed that animal toxins with unrelated biological functions often possess a similar architecture. To tentatively understand the evolutionary mechanisms that may govern this principle of functional prodigality associated with a structural economy, two complementary approaches were considered. One of them consisted of investigating the rates of mutations that occur in cDNAs and/or genes that encode a variety of toxins with the same fold. This approach was largely adopted with phospholipases A2 from Viperidae and to a lesser extent with three-fingered toxins from Elapidae and Hydrophiidae. Another approach consisted of investigating how a given fold can accommodate distinct functional topographies. Thus, a number of topologies by which three-fingered toxins exert distinct functions were investigated either by making chemical modifications and/or mutational analyses or by studying the three-dimensional structure of toxin-target complexes. This review shows that, although the two approaches are different, they commonly indicate that most if not all the surface of a snake toxin fold undergoes natural engineering, which may be associated with an accelerated rate of evolution. The biochemical process by which this phenomenon occurs remains unknown.

cDNA sequence analysis and mutagenesis studies on the A chain of beta-bungarotoxin from Taiwan banded krait

The cDNA encoding the A chain of beta-bungarotoxin (beta-Bgt) was constructed from the cellular RNA isolated from the venom glands of Bungarus multicinctus (Taiwan banded krait). The deduced amino acid sequence encoding the A chain revealed that the determined chain was different from the known A chains (A1, A2, A3, A4, and A5). Nevertheless, the amino acid sequence and the cDNA sequence of the novel A chain were highly homologous with those of other A chains. The gene encoding the A chain of beta-Bgt was subjected to mutagenesis, and the Tyr-11, Cys-15, and Leu-72 of the A chain were substituted by Cys-11, Ser-15, and Cys-72, respectively. Instead of the six disulfide bonds observed with the A chain, the resulting mutant contained seven disulfide linkages in its molecular structure which simulated those of presynaptic PLA2 neurotoxins and PLA2 enzymes. However, the mutant did not exhibit a higher phospholipase activity than that noted with the recombinant A chain. These results seem to suggest that, in the absence of the B chain, the six pairs of disulfide bonds in the recombinant A-chain molecule are enough to maintain its active conformation for exerting the phospholipase activity.

Chemical modification of notexin from Notechis scutatus scutatus (Australian tiger snake) venom with pyridoxal-5'-phosphate

Notexin from Notechis scutatus scutatus snake venom was subjected to Lys modification with pyridoxal 5'-phosphate (PLP), and one major modified derivative was purified on a cation-exchanger SP-8HR column. The results of amino acid analysis and sequence determination revealed that only 2 Lys residues at positions 82 and 115 out of 11 Lys residues in notexin were modified. The incorporation of PLP into the protein was accompanied by the loss of 53% lethal toxicity, but the modified notexin showed an about 1.2-fold increase in enzymatic activity. However, the secondary structure of the toxin molecule did not significantly change after modification with PLP as revealed by the CD spectra, and the antigenicity of PLP derivative remained unchanged. The modified derivative retained its affinity for Ca2+, indicating that the modified Lys residues did not participate in Ca2+ binding. These results indicate that modification of Lys residues causes a differential effect on the enzymatic activity and lethal toxicity of notexin, and suggest that notexin might possess two functional sites, one responsible for the catalytic activity and the other associated with its lethal effect.

Anions and the anomalous gel filtration behavior of notexin and scutoxin

Based on their mol. wts, notexin and scutoxin elute later than expected from gel filtration columns in multiple peaks [Francis et al. (1991) Toxicon 29, 85-96]. Notexins present in these peaks have identical amino acid sequences and unmodified amino acid side-chains. Scutoxin is an isoform of notexin which contains arginine at position 16 and glutamate at position 82. Like notexin, it also elutes in different fractions on a gel filtration column, yet the fractions show identical amino acid sequence. This perplexing chromatographic behavior appears to be caused by the association of these proteins with different anions, since dissolving notexin in buffers containing different anions produces up to a 30% change in elution volume. Certain anions promote an apparent reduction in the interaction of notexin with gel filtration matrix, hence earlier elution. These anions include citrate, 3-phosphoglycerate and 2-phosphoglycerate, which also inhibit the PLA2 activity of notexin. However, even under conditions which minimize protein-matrix interaction the toxins elute later than expected based on their mol. wt.

Conodipine-M, a novel phospholipase A2 isolated from the venom of the marine snail Conus magus

We describe the purification and first biochemical characterization of an enzymatic activity in venom from the marine snail Conus magus. This enzyme, named conodipine-M, is a novel phospholipase A2 with a molecular mass of 13.6 kDa and is comprised of two polypeptide chains linked by one or more disulfide bonds. The amino acid sequence of conodipine-M shows little if any homology to other previously sequenced phospholipase A2 enzymes (PLA2s). Conodipine-M thus represents a new group of PLA2s. This is remarkable, since conodipine-M displays a number of properties that are similar to those of previously characterized 14-kDa PLA2s. The enzyme shows little, if any, phospholipase A1, diacyglycerol lipase, triacylglycerol lipase, or lysophospholipase activities. Conodipine-M hydrolyzes the sn-2 ester of various preparations of phospholipid only in the presence of calcium and with specific activities that are comparable to those of well known 14-kDa snake venom and pancreatic PLA2s. The Conus enzyme binds tightly to vesicles of the negatively charged phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphomethanol and catalyzes the hydrolysis of this substrate in a processive fashion. Conodipine-M does not significantly discriminate against phospholipids with unsaturated versus saturated fatty acids at the sn-2 position or with different polar head groups. Linoleoyl amide and a phospholipid analog containing an alkylphosphono group at the sn-2 position are potent inhibitors of conodipine-M. We suggest that the functional resemblance of conodipine-M to other PLA2s might be explained by the utilization of similar catalytic residues.

Purification and characterization of myotoxin IV, a phospholipase A2 variant, from Bothrops asper snake venom

A basic myotoxic protein was purified from Bothrops asper venom. Like other basic Bothrops myotoxins, myotoxin IV induces acute muscle damage after intramuscular injection in mice and disrupts negatively charged liposomes but not positively charged ones. Furthermore, this protein exerts a weak anticoagulant effect only at concentrations of 40 micrograms/ml or higher, and is devoid of phospholipase A2 activity. Rabbit polyclonal antibodies to B. asper myotoxin II, a related lysine-49 isoform, cross-react strongly with myotoxin IV by enzyme immunoassay, indicating a high degree of antigenic similarity between these two proteins. The fact that both toxins have similar amino acid compositions and amino-terminal sequences, including leucine-5 and glutamine-11, 2 amino acid residues conserved in all lysine-49 phospholipase A2 variants purified, strongly suggests that B. asper myotoxin IV is a lysine-49 phospholipase A2.

Broad cytolytic specificity of myotoxin II, a lysine-49 phospholipase A2 of Bothrops asper snake venom

The cytotoxic activity of Bothrops asper myotoxin II, a lysine-49 phospholipase A2 isoform, on different cell types in culture, was investigated. Myotoxin II caused a dose-dependent cytolytic effect on all cell types tested, characterized by rapid release of cytoplasmic lactic dehydrogenase and drastic morphological cell alterations. Quantitative differences in the susceptibility to myotoxin II among cell types fell within a relatively narrow range, and in general, the toxin was cytolytic at concentrations of 50-100 micrograms/ml (3-7 microM), when assays were performed using culture medium as a diluent. Toxin activity was markedly enhanced if phosphate-buffered saline was utilized instead of medium. The cytotoxic activity of myotoxin III, an aspartate-49 isoform from the same venom, on both endothelial cells and skeletal muscle myoblasts was higher than that of myotoxin II, suggesting that, although phospholipase A2 activity is clearly not required for the induction of cell damage, it may have an enhancing role. In contrast to B. asper myotoxins, other basic phospholipases A2 with myotoxic activity in vivo (notexin from Notechis scutatus, and two enzymes isolated from Vipera russelli venom) did not affect endothelial cells and myoblasts. Pretreatment of cells with neuraminidase, tunicamycin, or protamine, did not alter their susceptibility to myotoxin II. At low temperatures (2-4 degrees C) myotoxin II was devoid of cytolytic effect. Washing and neutralization experiments using heparin with low affinity for antithrombin or mouse monoclonal antibody MAb-3 suggest that at low temperatures myotoxin II binds very weakly to the cells, and that its normal interaction with the putative target is probably not only based on charge, but that a membrane penetration event may be required.

Proteins isolated from the venom of the common tiger snake (Notechis scutatus scutatus) promote hypotension and hemorrhage

Notechis scutatus scutatus venom contains several toxic acidic proteins called HTa-i which promote hypotension and hemorrhage in mice. They have apparent mol. wts in the 18,000-21,000 range, i.v. LD50 values between 0.5 and 1.5 micrograms/g, and no detectable phospholipase, arginine esterase, proteolytic or hemolytic activities. A polyclonal antibody raised against HTg binds to other purified proteins, suggesting that they are isoforms of the same protein. Many other elapid crude venoms contain proteins which recognize the polyclonal antibody raised against HTg. Crotalid and viperid crude venoms do not recognize this antibody, although some of their component proteins are known to exhibit hypotensive and hemorrhagic activities. A combination of gel-filtration on Sephacryl S-200, cation-exchange and anion-exchange chromatography allows isolation of the N. s. scutatus proteins in high purity. They are the first hypotension-inducing proteins to be purified from an Australian elapid.

Studies on the subunit structure of textilotoxin, a potent presynaptic neurotoxin from the venom of the Australian common brown snake (Pseudonaja textilis). 3. The complete amino-acid sequences of all the subunits

The complete amino-acid sequences of subunits A, B, C and D of textilotoxin, the presynaptic neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis, were determined. These confirmed that it is structurally the most complex of any of the known snake venom neurotoxins. Textilotoxin consists of 623 amino-acid residues in five subunits (subunit A, 118 residues; subunit B, 121 residues; subunit C, 118 residues; subunit D, two chains of 133 residues each). All subunits A, B, C and D contain the putative phospholipase A2 active site. Only subunit A showed any lethality on its own (4 mg/kg i.v. in mice). Subunit D contained two identical covalently-linked subunits and was weakly glycosylated. All subunits were necessary for maximum lethality at 1 micrograms/kg mice intraperitoneally. Details of the sequences of the subunits A, B and C are reported and interesting homology with other snake venom phospholipase A2 neurotoxins indicated.

Notechis 11'2, a non-toxic phospholipase A2 from the venom of Notechis scutatus scutatus

Previously, we deduced the amino acid sequence of a novel phospholipase-A2-like protein (PLA2) from the nucleotide sequence of a cDNA isolated from a library prepared from the venom gland of the Australian elapid Notechis scutatus scutatus. The corresponding protein has now been identified, purified from the venom and named Notechis 11'2. Its complete amino acid sequence has been determined by automated Edman degradation of both the whole protein and peptides generated by Staphylococcus aureus protease digestion and chemical cleavage at a tryptophan residue. As predicted from its sequence which contains all the residues putatively required for PLA2 activity, Notechis 11'2 exhibits an esterase activity, preferentially against neutral phospholipids. However, despite its sequence homology with other highly toxic PLA2 present in the venom of Notechis scutatus scutatus, notechis 11'2 has no lethal activity. This observation further supports the view that the lethal activity of PLA2 from Notechis scutatus scutatus is not due to the esterasic activity only.

Conservative and variable regions of homologous snake phospholipases A2 sequences: prediction of the taxon-specific peptides structure

Homologous amino acid sequences of phospholipases A2 (PLA2) of snakes belonging to the families Elapidae, Viperidae, and Colubridae were considered in order to study the conservative and variable regions location. The PLA2 sequences were divided into two groups (taxons) according to the phylogenetic tree reconstructed from the pair similarity matrix. Results of the intergroup comparison were plotted to facilitate the identification of significant conservative and variable regions. It was shown that the results of the comparison between two phylogenetic groups of snake PLA2 did not much depend on the number of each group representatives and did not markedly change if one of the groups was represented by the single sequence. The knowledge of the number and location of conservative and variable regions and their dependence on the phylogenetic relations between compared taxa may be used to predict a synthetic peptide structure to obtain specific antibodies against PLA2 of one of these taxons. Such prediction is possible if there is a specific region conservative for one taxon but variable for two of them.

Venom constituents of Notechis scutatus scutatus (Australian tiger snake) from differing geographic regions

Column chromatography and polyacrylamide gel electrophoresis of Notechis scutatus scutatus venom showed that the venoms from different geographical locations had variations in their constituents. The venom collected from South Australia region contained both notexin and notechis II-5. The relative quantity of notechis II-5 was about three times that of notexin. On the other hand, the venom from Victoria region contained large amounts of notexin, but lacked notechis II-5. Instead, an unknown nontoxic protein, designated as notechis II-5b, exhibiting weak phospholipase A2 activity appeared in the position of notechis II-5 elution. This protein had an N-terminal sequence of N-L-I-Q-L-S-N-M-I-K-C-A-I-P-G-S-Q-P-L-F, sharing 45% homology with notexin and notechis II-5 and 60% homology with notechis II-1. The antibodies raised against Trp-modified notexin inhibited the enzymatic activities of notexin and notechis II-5 by 88 and 68%, respectively. However, the affinity of notexin for the antibodies was nine-fold greater than that of notechis II-5. This result is contrary to the previous finding (Mollier et al., FEBS Lett. 250, 479-482, 1989) in which notexin and notechis II-5 had similar binding affinities for antibodies raised against native notexin. This observation suggests that the antibodies prepared in this study could differentiate between isoforms of notexin.

New toxins from the venom of the common tiger snake (Notechis scutatus scutatus)

Scutoxin A and B represent two isoforms of a new toxic protein from the venom of the Australian tiger snake, Notechis scutatus scutatus. Both isoforms, of apparent mol. wt 13,000, are less basic than either notexin or notechis II-5. They both have similar i.v. LD50-values in mice of ca 0.006 micrograms/g, and phospholipase activities of about 136 mumoles of fatty acid released/min/mg at 37 degrees C when acting on phosphatidylcholine in the presence of Triton X-100. Toxicities of the scutoxins are the same as notexin and about seven times more potent than notechis II-5. ELISAs and western blot analyses indicate that the new toxins are immunologically similar to notexin and notechis II-5, with phospholipase activities falling between these latter two proteins. When crude venom is initially passed over a gel filtration column, each scutoxin isoform co-elutes in a different fraction with notexin. Gel filtration experiments using purified samples of notexin and scutoxin have failed to demonstrate any evidence for the formation of higher mol. wt protein complexes. Peptide mapping suggests the presence of five glutamate residues in one of the protein isoforms. These findings, together with the high toxicity and active phospholipase levels, demonstrate that the new proteins are not the previously reported non-toxic and enzymatically inactive notechis II-1. The combination of gel filtration on Sephacryl S-200 and cation-exchange chromatography used to isolate the scutoxins also permits recovery of notexin and notechis II-5 in high purity.

Crystallization and preliminary diffraction analysis of caudoxin and notexin; two monomeric phospholipase A2 neurotoxins

Two monomeric neurotoxic phospholipases A2 have been crystallized and their diffraction properties characterized. Crystals of caudoxin (from the venom of Bitis caudalis) and notexin (from the venom of Notechis scutatus scutatus) were grown at neutral pH, in the absence of calcium ion, and diffract to a resolution of 2.3 and 1.6 A, respectively.

Studies on the venom components of the long-glanded coral snake, Maticora bivirgata

The venom of the Asian long-glanded coral snake, Maticora bivirgata, was fractionated into five fractions, S1-S5, by passing through a Sephadex G-50 column. Fraction S2 contains two phospholipases A2, PLA2 I and PLA2 II, fraction S3 contains four cytotoxin homologues, maticotoxins A, C, D1 and D2, and fractions S4 and S5 contain a large amount (about 1 mg/specimen) of adenosine accompanied with smaller amounts of inosine and guanosine. The amino-terminal amino acid sequences of PLA2, I, PLA2 II and maticotoxin A suggest that Maticora bivirgata is closely related to Bungarinae, especially to genera Hemachatus and Naja.

Preparation and characterization of monoclonal antibodies against pseudexin

Fifteen hybridoma cell lines secreting monoclonal antibodies against pseudexin were developed. The cell lines were grown as ascites tumors and the resulting antibodies were purified by Protein A affinity-chromatography. Several of the antibodies exhibited extensive ELISA cross-reactions with different phospholipase A2 toxins from various snake venoms, while other of the antibodies reacted only with the pseudexins. Three of the antibodies neutralized pseudexin A and B, but none of the 10 other phospholipase A2 toxins tested. These same three antibodies inhibited the enzymatic activity of pseudexin A and B and also that of notexin. After each antibody was labeled with biotin, competition experiments were carried out to determine the binding relationships among the antibodies and the pseudexins. Competitions were frequently observed, with a low of zero to a high of eight out of the 14 possibilities. Competition experiments were also carried out with biotin-labeled rabbit IgG against the pseudexins. Some of the monoclonal antibodies had no effect on rabbit IgG binding to pseudexin, while others blocked up to 50% of the binding.

The N-terminal amino group essential for the biological activity of notexin from Notechis scutatus scutatus venom

Notexin from Notechis scutatus scutatus snake venom was modified with trinitrobenzenesulfonic acid, and the major trinitrophenylated (TNP) derivative was separated by high-performance liquid chromatography. Modification resulted in the incorporation of only one TNP group on the N-terminal alpha-amino group. The TNP derivative showed a precipitous decrease in enzymatic activity and lethal toxicity, whereas the antigenicity remained unchanged. However, trinitrophenylation did not significantly affect the secondary structure of the toxin molecule as revealed by the CD spectra. The results, that the modification reaction was accelerated by the Ca2+ and that the TNP derivative retains its affinity for Ca2+, indicate that the N-terminal alpha-amino group did not participate in the Ca2(+)-binding. The TNP derivative could be regenerated with hydrazine hydrochloride. The biological activities of the regenerated notexin are almost the same as those of native notexin. These results suggest that the N-terminal alpha-amino group is essential for the phospholipase A2 activity and lethal toxicity of notexin, and that incorporation of the TNP group on the N-terminal alpha-amino group might give rise to a distortion of the active conformation of notexin.

Leukotriene and prostaglandin production in rat brain synaptosomes treated with phospholipase A2 neurotoxins and enzymes

beta-Bungarotoxin (beta-BuTX) and notexin cause an irreversible blockade of neurotransmitter release through specific and potent effects at the presynaptic nerve terminal, however, the mechanism of action is uncertain. We examined the effects of beta-BuTX and notexin on LT and PG production in rat cerebrocortical synaptosomes in order to determine if eicosanoid production might mediate or regulate the pharmacological actions of these phospholipase A2 (PLA2) neurotoxins. The effects of the PLA2 enzymes isolated from Naja naja atra and Naja nigricollis snake venoms (which are not presynaptic selective) on LT and PG production were compared with the effects of beta-BuTX and notexin. N. n. atra PLA2, beta-BuTX, and notexin (all 50 nM) produced a time dependent rise in free fatty acids as measured in synaptic plasma membranes isolated from treated synaptosomes. Both the PLA2 neurotoxins and enzymes stimulated LTC4, LTB4, and PGE2 production, as measured by radioimmunoassay. In all cases, the PLA2 enzymes were more potent than the PLA2 neurotoxins. This observation correlates with their relative enzymatic potencies, as measured by free fatty acid generation. EDTA and BSA antagonized PLA2 induced LTB4 production and BSA also antagonized PLA2 induced PGE2 production. These results suggest that stimulation of eicosanoid production does not mediate the potent and specific presynaptic actions of beta-BuTX and notexin.

On the purification of notexin. Isolation of a single amino acid variant from the venom of Notechis scutatus scutatus

Venom of the Australian tiger snake, Notechis scutatus scutatus was fractionated by conventional ion-exchange chromatography. The fraction containing notexin, a well-known single-chain toxic phospholipase A2, was further purified by reverse-phase high-performance liquid chromatography. Two main components were isolated and the major one corresponded to notexin. The other component, designated as notechis Ns, was an isoform of notexin. Notechis Ns and notexin possessed similar in vitro esterase activity, in vitro neuromuscular activity and in vivo lethality. Amino acid composition and sequence of the Staphylococcus aureus V8-protease peptides demonstrated that primary structures of notechis Ns and notexin differed from each other by a single substitution amongst 119 amino acids: Lys----Arg at position 16.

Myotoxic components of snake venoms: their biochemical and biological activities

Necrosis of skeletal muscle is produced by two types of snake venom components: single chain peptides consisting of 42-44 amino acid residues and phospholipases A2 representing either single chain proteins or existing as complexes of several enzyme subunits or combined with other nonenzymatic proteins. Vacuolation, lysis and necrosis of skeletal muscle cells are the major pathological effects of these myotoxins. Although the exact mode of action of these toxins is not clear, interactions with the plasma membrane leading to permeability changes for ions and to their complete destruction is evident. The high specificities of some venom phospholipases A2 for skeletal muscle cells suggest a specific binding to certain membrane receptors; however, an enzymatic action on membranes may also be involved.

Amino acid sequences of eight phospholipases A2 from the venom of Australian king brown snake, Pseudechis australis

The amino acid sequences of eight phospholipases A2 (Pa-1G, Pa-3, Pa-5, Pa-9C, Pa-10A, Pa-12A, Pa-12C and Pa-15) which had been isolated from the venom of Australian king brown snake (Pseudechis australis) were elucidated. Pa-1G, Pa-3 and Pa-15 showed micro-heterogeneity at the 103rd position and Pa-5 was separated into two components, Pa-5a ([Pro-18 and Tyr-61]Pa-5) and Pa-5b ([ Ser-18 and Phe-61]Pa-5). All the phospholipase A2 molecules except Pa-1Ga and Pa-1Gb which lack the 118th residue, consisted of a single chain of 118 amino acid residues including 14 half-cystine residues and all the common residues among phospholipases A2 from other sources. From comparison studies, Asp-50, Lys-58 and Asp-90 seem to be important for the toxicity, and we propose that the domain for the presynaptic toxicity consists of seven hydrophilic residues, i.e. Arg-43, Lys-46, Asp-50, Glu-54, Lys-58, Asp-90 and Glu-94.