Chromatographic separation of the venom of Bungarus multicinctus and characterization of its components

Bungarus multicinctus multicinctus Snakebite in Taiwan

AbstractAlthough specific antivenom is available in Taiwan, respiratory failure and general pain frequently accompany Bungarus multicinctus envenomation and there have been few reports on the management of B. multicinctus envenomation. We retrospectively analyzed 44 cases of B. multicinctus bite admitted to Taichung Veterans General Hospital (VGH) or to Taipei VGH. Demographic data, treatment, and outcome of patients with and without respiratory failure were compared. In this study, 20.5% patients had bites without noticeable signs or symptoms of significant envenoming, 27.3% developed respiratory failure, and 27.3% experienced general pain. Bivalent specific antivenom for B. multicinctus and N. atra was administered in all envenomed cases. Respiratory failure occurred 1.5-6.5 hours post-bite and general pain occurred 1-12 hours post-bite. Specific antivenom for B. multicinctus and N. atra at the recommended dose (i.e., 2-4 vials) might not effectively prevent respiratory failure and pain. Respiratory failure, general pain, and autonomic effects after B. multicinctus bite were probably caused, at least partly, by β-bungarotoxin. Although general weakness, ptosis, dysarthria, and dilated pupils were significantly associated with respiratory failure, their predictive value could not be accurately determined in such a retrospective study. Due to the rapid onset of respiratory failure, every suspected envenomed case thus should be closely monitored in the first few hours. We recommend the initial administration of four vials of antivenom in all envenomation cases, and a subsequent four vials be considered if the patient's condition is deteriorating. Prospective evaluation of the antivenom dosing regimen is urgently needed to improve B. multicinctus envenomation treatment.

Protease inhibitors from marine venomous animals and their counterparts in terrestrial venomous animals

The Kunitz-type protease inhibitors are the best-characterized family of serine protease inhibitors, probably due to their abundance in several organisms. These inhibitors consist of a chain of ~60 amino acid residues stabilized by three disulfide bridges, and was first observed in the bovine pancreatic trypsin inhibitor (BPTI)-like protease inhibitors, which strongly inhibit trypsin and chymotrypsin. In this review we present the protease inhibitors (PIs) described to date from marine venomous animals, such as from sea anemone extracts and Conus venom, as well as their counterparts in terrestrial venomous animals, such as snakes, scorpions, spiders, Anurans, and Hymenopterans. More emphasis was given to the Kunitz-type inhibitors, once they are found in all these organisms. Their biological sources, specificity against different proteases, and other molecular blanks (being also K⁺ channel blockers) are presented, followed by their molecular diversity. Whereas sea anemone, snakes and other venomous animals present mainly Kunitz-type inhibitors, PIs from Anurans present the major variety in structure length and number of Cys residues, with at least six distinguishable classes. A representative alignment of PIs from these venomous animals shows that, despite eventual differences in Cys assignment, the key-residues for the protease inhibitory activity in all of them occupy similar positions in primary sequence. The key-residues for the K⁺ channel blocking activity was also compared.

Alpha neurotoxins

α-Neurotoxins have been isolated from hydrophid, elapid and, more recently, colubrid snake venoms. Also referred to as postsynaptic neurotoxins or 'curare mimetic' neurotoxins, they play an important role in the capture and/or killing of prey by binding to the nicotinic acetylcholine receptor on the skeletal muscle disrupting neurotransmission. They are also thought to cause respiratory paralysis in envenomed humans. This review will discuss the historical background into the discovery, isolation, structure and mechanism of action of the α-neurotoxins, including targets and cellular outcomes, and then will examine the potential uses of α-neurotoxins as pharmacological tools and/or as drug leads.

Magnetic resonance imaging of bone marrow in oncology, Part 1

Magnetic resonance imaging plays an integral role in the detection and characterization of marrow lesions, planning for biopsy or surgery, and post-treatment follow-up. To evaluate findings in bone marrow on MR imaging, it is essential to understand the normal composition and distribution of bone marrow and the changes in marrow that occur with age, as well as the basis for the MR signals from marrow and the factors that affect those signals. The normal distribution of red and yellow marrow in the skeleton changes with age in a predictable sequence. Important factors that affect MR signals and allow detection of marrow lesions include alterations in fat-water distribution, destruction of bony trabeculae, and contrast enhancement. This two-part article reviews and illustrates these issues, with an emphasis on the practical application of MR imaging to facilitate differentiation of normal marrow, tumor, and treatment-related marrow changes in oncology patients.

Taxonomic distribution and quantitative analysis of free purine and pyrimidine nucleosides in snake venoms

The nucleoside content of 32 elapid and viperid venoms was examined. Free purines, principally adenosine (ADO), inosine (INO), and guanosine (GUA), comprised as much as 8.7% of the solid components of some venoms. Thus, purines are far more abundant in some venoms than many proteinaceous toxins. Hypoxanthine (HYP) was found in about half of elapid and viperine venoms, in which it is a relatively minor constituent (<60 microg/g). Adenosine monophosphate (AMP) was tentatively identified in only three elapid and two viperid venoms. The pyrimidines, uridine (URI) and cytidine (CYT), were also found in most elapid and viperine venoms. In most of these, the amount of uridine was substantially greater than that of cytidine. Thymidine (THY) was not found in any venom, indicating that DNA from disintegration of glandular cells is not the source of venom nucleosides. In contrast to elapid and viperine venoms, most crotaline venoms are devoid of free nucleosides. Elapid and viperine venoms also contained other minor, low molecular weight constituents that could not be positively identified. Some had spectra identical to those of adenosine, nicotinamide adenine dinucleotide (NAD), inosine, xanthosine (XAN), and guanosine, while others had unique spectra. There is no apparent correlation between quantities of venom nucleosides and literature values for the three dominant venom enzymes that release endogenous nucleosides, 5'-nucleotidase (5NUC), phosphodiesterase (PDE), and alkaline phosphomonoesterase (PME).

Suramin inhibits beta-bungarotoxin-induced activation of N-methyl-D-aspartate receptors and cytotoxicity in primary neurons

We demonstrated that beta-bungarotoxin (beta-BuTX), a snake presynaptic neurotoxin, exhibited a potent cytotoxic effect on cultured cerebellar granule neurons. The mechanism of action of beta-BuTX and the cytoprotective agents against beta-BuTX were studied. The neuronal death of cerebellar granule neurons induced by beta-BuTX was manifested with apoptosis and necrosis processes as revealed by neurite fragmentation, morphological alterations, and staining apoptotic bodies with the fluorescent dye Hoechst 33258. By means of microspectrofluorimetry and fura-2, we measured intracellular Ca2+ concentration, [Ca2+]i and found that [Ca2+]i was increased markedly prior to the morphological changes and cytotoxicity. The downstream pathway of the increased [Ca2+]i was investigated: there was increased production of free radicals, decreased mitochondrial membrane potential, and depleted cellular ATP content. MK801 and suramin effectively suppressed these detrimental effects of beta-BuTX. Furthermore, the [3H]MK801 binding was reduced by unlabeled MK801, beta-BuTX, and suramin. Thus, activation of N-methyl-D-aspartate (NMDA) receptors appeared to play a crucial role in the cytotoxic effects following betaBuTX exposure. In conclusion, the novel finding of this study was that a polypeptide beta-BuTX exerted a potent cytotoxic effect through sequential events, including activating NMDA receptors followed by increasing [Ca2+]i, ROS production, and impaired mitochondrial energy metabolism. Suramin, clinically used as a trypanocidal agent, was an effective antagonist against beta-BuTX. Data suggest that suramin might have value to detect the possible pathway of certain neuropathological disorders.

Activation of NMDA receptor partly involved in beta-bungarotoxin-induced neurotoxicity in cultured primary neurons

In this study, we demonstrated that a snake presynaptic toxin, beta-bungarotoxin (beta-BuTX), was capable of binding to NMDA receptors of the cultured primary neurons (cerebellar granule neurons, CGNs). We labeled beta-BuTX with fluorescent FITC (FITC-beta-BuTX) and showed that the binding of FITC-beta-BuTX was inhibited by unlabeled beta-BuTX and MK801 (an NMDA receptor antagonist). Meanwhile, the binding of [3H]-MK801 was also reduced by unlabeled MK801 and beta-BuTX. In addition, beta-BuTX produced a very potent neurotoxic effect on mature CGNs with the EC(50) of 3ng/ml (equivalent to 144pM), but was less effective in immature CGNs. We explored the signaling pathway of neuronal death and found that it was apparently due to the excessive production of reactive oxygen species (ROS) induced by beta-BuTX. MK801 and antioxidants (Vitamin C, N-acetylcysteine (NAC), melatonin, epigallocatechin gallate (EGCG), superoxide dismutase (SOD) and catalase) attenuated not only ROS production but also beta-BuTX-neurotoxicity. The downstream signaling of ROS was identified as the activation of caspase-3. Caspase inhibitor (z-DEVD-fmk) and antioxidants depressed both caspase-3 activation and neurotoxicity. Based on these findings and our previous reports, we conclude that the binding and activation of NMDA receptors by beta-BuTX was crucial step to produce the potent neurotoxic effect. The binding of NMDA receptors resulted in excessive Ca(2+) influx, followed by ROS production and activation of caspase-3. This snake toxin is considered not only to be a useful tool for exploring the death-signaling pathway of neurotoxicity, but also provides a model for searching neuroprotective agents.

Non-conventional toxins from Elapid venoms

Non-conventional toxins constitute a poorly characterized class of three-finger toxins isolated exclusively from Elapidae venoms. These toxins are monomers of 62-68 amino acid residues and contain five disulfide bridges. However, unlike alpha/kappa-neurotoxins and kappa-neurotoxins which have the fifth disulfide bridge in their middle loop (loop II), the fifth disulfide bridge in non-conventional toxins is located in loop I (N-terminus loop). Overall, non-conventional toxins share approximately 28-42% identity with other three-finger toxins including alpha-neurotoxins, alpha/kappa-neurotoxins and kappa-neurotoxins. Recent structural studies have revealed that non-conventional toxins also display the typical three-finger motif. Non-conventional toxins are typically characterized by a lower order of toxicity (LD(50) approximately 5-80 mg/kg) in contrast to prototype alpha-neurotoxins (LD(50) approximately 0.04-0.3 mg/kg) and hence they are also referred to as 'weak toxins'. Further, it is generally assumed that non-conventional toxins target muscle (alpha(2)beta gamma delta) receptors with low affinities several orders of magnitude lower than alpha-neurotoxins and alpha/kappa-neurotoxins. However, it is now known that some non-conventional toxins also antagonize neuronal alpha 7 nicotinic acetylcholine receptors. Hence, non-conventional toxins are not a functionally homogeneous group and other, yet unknown, molecular targets for this class of snake venom toxins may exist. Non-conventional toxins may therefore be a useful source of ligands with novel biological activity targeting the plethora of neuronal nicotinic receptors as well as other physiological processes.

Neuronal death signaling by beta-bungarotoxin through the activation of the N-methyl-D-aspartate (NMDA) receptor and L-type calcium channel

The aim of this study was to elucidate the mechanism of the neurotoxic effect of beta-bungarotoxin (beta-BuTX, a snake presynaptic neurotoxin isolated from the venom of Bungarus multicinctus) on cultured cerebellar granule neurons. beta-BuTX exerted a potent, time-dependent, neurotoxic effect on mature granule neurons. Mature neurons, with an abundance of neurite outgrowths, were obtained after 7-8 days in culture. By means of microspectrofluorimetry and fura-2, we measured the intracellular Ca(2+) concentration ([Ca(2+)](i)) and found it to be increased markedly. BAPTA-AM [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tertrakis(acetoxymethyl ester)], EGTA, MK801 (dizocilpine maleate), and diltiazem prevented not only the elevation of [Ca(2+)](i), but also the beta-BuTX-induced neurotoxic effect. The signaling pathway involved in the elevation of [Ca(2+)](i) in beta-BuTX-induced neurotoxicity was studied. The results obtained indicated that beta-BuTX initially increased the production of reactive oxygen species and subsequently reduced mitochondrial membrane potential and depleted ATP. All of these events in the signaling pathway were blocked by MK801, diltiazem, EGTA, and BAPTA-AM. These findings suggest that the neurotoxic effect of beta-BuTX is mediated, at least in part, by a cascade of events that include the direct or indirect activation of N-methyl-D-aspartate (NMDA) receptors and L-type calcium channels that, in turn, lead to Ca(2+) influx, oxidative stress, mitochondrial dysfunction, and ATP depletion. Therefore, we suggest that this polypeptide neurotoxin, as a result of its high potency and irreversible properties, is a useful tool to elucidate the mechanisms of neurodegenerative diseases.

Long-term lithium treatment prevents neurotoxic effects of beta-bungarotoxin in primary cultured neurons

Lithium is the most commonly used drug for the treatment of manic-depressive illness. The precise mechanisms underlying its clinical efficacy remain unknown. In this study, we found that long-term exposure to lithium chloride protected cultured cerebellar granule neurons (CGNs) against beta-bungarotoxin (beta-BuTX)-induced neurotoxicity. This neuroprotection was exhibited at the therapeutically relevant concentration of 1.2 mM lithium. Pretreatments for 3-5 days (long-term) were required for protection to occur; but a 3 hr treatment (short-term) was ineffective. In contrast, a longer treatment for 6-7 days or a higher concentration of 3 mM lithium led not only to loss of the neuroprotective effect but also to a neurotoxic effect. These findings suggest that lithium protection is limited to its narrow window of concentration and apparently relevant to its narrow therapeutic index in clinical application. Measurement of intracellular calcium [Ca(2+)](i) revealed that neurotoxic concentrations of beta-BuTX markedly increased [Ca(2+)](i), which could be attenuated by long-term, but not short-term, lithium treatment. Thus, the protection induced by lithium in CGNs was attributed to its inhibition of calcium overload. In addition, the Ca(2+) signaling pathway, including reactive oxygen species production and mitochondrial membrane potential reduction, along with the neurotoxic effect of beta-BuTX was blocked by long-term, but not short-term, lithium treatment. All of these results indicate that a crucial step for lithium protection is modulation of [Ca(2+)](i) homeostasis and that lithium neurotoxicity possibly, at least in part, is due to calcium overload. In conclusion, our results suggest that lithium, in addition to its use in treatment of bipolar depressive illness, may have an expanded use in intervention for neurotoxicity.

Professor Chen-Yuan Lee, MD (1915-2001), pharmacologist: snake venom research at the Institute of Pharmacology, National Taiwan University

Professor Chen-Yuan Lee was born in Tainan, Taiwan. In 1940, he joined the staff of the Institute of Pharmacology of the university, now named National Taiwan University. Dr Lee began a study of Daboia russelli formosensis venom under the direction of Professor Tsungming Tu who, in the 1930s, initiated the pharmacological studies of Formosan snake venoms carried out at the Institute. Under Professor Lee's direction, the Institute became known internationally for its work on the isolation, composition and characterization of the pharmacological properties of neurotoxins isolated from Formosan elapid venoms. Sophisticated use of the latest techniques revealed the mode of action of postsynaptic -bungarotoxin and presynaptic -bungarotoxin from Bungarus multicinctus venom, postsynaptic cobrotoxin, cytotoxic cardiotoxin and phospholipase A2 from Naja naja atra venom. Through work undertaken with colleagues at the Institute and in foreign countries, Professor Lee made an important contribution to our understanding of the mode of action of snake neurotoxins, and to their use in the elucidation of neuromuscular transmission. In the past decade, C.-Y. Lee was a prominent campaigner for social and political justice in Taiwan.

Determination of Anti-Acetylcholine Receptor Antibodies in Myasthenic Patients by Use of Time-resolved Fluorescence

Background: Autoantibodies against nicotinic acetylcholine receptor (nAChR) in myasthenia gravis (MG) patients are usually detected by radioimmunoprecipitation assays using extracted acetylcholine receptors labeled irreversibly with 125I-α-bungarotoxin (α-BuTx). To provide a nonradioactive immunoassay, we established an assay using nAChRs labeled with Eu3+-α-cobratoxin (α-CTx).

Methods: We derivatized α-CTx with a diethylenetriaminepentaacetate moiety and formed a complex with Eu3+. The complex was purified by HPLC, and the fractions were tested for binding to Torpedo and human nAChRs. The most active fractions were used to label nAChRs for the immunoprecipitation assay, and the bound Eu3+ was quantified by time-resolved fluorescence.

Results: Eu3+-labeled α-CTx competed with 125I-α-BuTx for binding to Torpedo nAChRs and saturated the binding sites of human nAChRs, with a Kd of 7.2 × 10−9 mol/L. Results of the immunoassay performed with Eu3+-labeled α-CTx were similar to those obtained with 125I-α-BuTx, with a slightly higher limit of detection [0.3 nmol/L (n = 6) vs ∼0.1 nmol/L for isotopic assay]. None of 34 negative sera tested (16 healthy controls, 10 patients with nonmyasthenia-related disease, 8 patients seronegative for MG) gave a value &gt;0.3 nmol/L. Of the 35 positive myasthenic sera (with antibody values, previously determined by isotopic assay, of 0.4–1290 nmol/L) compared in the two assays, 32 tested positive with the Eu3+ assay. Linear regression analysis yielded the equation: y = 1.035x − 0.013 nmol/L; Sy|x = 0.172 nmol/L; r2 = 0.977.

Conclusions: The new time-resolved fluorescence method for quantification of antibodies to nAChRs in MG patients provides a performance similar to that of the widely used isotopic assay and could be used in laboratories with restricted use of isotopes.

Immunochemical study on beta1-bungarotoxin using polyclonal and monoclonal antibodies

The number of antigenic determinants of beta1-bungarotoxin (beta1-Bgt), A chain and B chain were determined to be seven, five and two, respectively, by quantitative precipitin reactions and analysis of the soluble complex formed from beta1-Bgt and Fab fragments of the antibody. The gel filtration patterns on a size exclusion column revealed that the soluble complexes formed from non-precipitating antibody and beta1-Bgt at a different molar ratio all emerged in the void volume, indicating that the molecular weight of the soluble complex is around 6000 kDa or more. The gel filtration pattern from Fab fragments of non-precipitating antibody also revealed that only 1 or 2 molecules of Fab fragments are bound to beta1-Bgt, A chain and B chain instead of seven, five and two molecules as in the case of precipitating antibody. Besides, twenty-three stable monoclonal antibodies (mAbs) were prepared against beta1-Bgt by the hybridoma technique. Most of these mAbs cross-reacted with isotoxins of the beta-Bgt family, as determined by an enzyme-linked immunosorbent assay, but none of them reacted with notexin, notechis II-5 and three PLA2 homologues, PLA2, DE-I and CMS-9.

Neutralizing epitope mapping of six beta1-bungarotoxin monoclonal antibodies and its application in beta1-bungarotoxin peptide vaccine design

Twenty three stable monoclonal antibodies (mAbs) against beta1-bungarotoxin (beta1-bgt) were prepared by the hybridoma technique. Seven of the 23 mAbs (mAbs 2, 6, 8, 11, 17, 21 and 22) could inhibit more than 70% of phospholipase A2 activity of beta1-bgt and neutralize the toxin. Six of these neutralizing mAbs (mAbs 2, 6, 8, 17, 21 and 22) recognized continuous epitopes on the A chain of beta1-bgt and the other one (mAb 11) recognized a conformational epitope on the toxin. The continuous epitopes of these six mAbs were mapped using synthetic peptide and proteolytic enzymes. Experimental results indicate that mAb 17 recognized the A-chain residues 31-37; mAbs 2 and 8 recognized residues 46-51; mAbs 21 and 22 recognized residues 91-98; and mAb 6 recognized residue 100-106. The competitive-antibody-binding inhibition experiments showed that the affinity of these neutralizing mAbs to the native beta1-bgt is compatible with synthetic peptides. Furthermore, mice immunized with BSA-conjugated A-chain-peptide sequences A(31-37), A(46-51), A(91-98) or A(100-106) were protected from a high-dose beta1-bgt challenge. Subsequently, the peptide-immunized sera were passively injected into Balb/c mice and a significantly protective effect was also observed. To our knowledge, this study is the first systematic demonstration of multiple neutralizing B-cell epitopes of beta1-bgt, and this study is also the first report of the protective synthetic-peptide vaccine against beta1-bgt challenge.

Spontaneous recovery from severe neurotoxic envenoming by a Malayan krait Bungarus candidus (Linnaeus) in Thailand

A patient bitten by Bungarus candidus (Malayan krait) developed nausea, vomiting, weakness, and myalgia 30 minutes after being bitten. One hour later, ptosis and occulomotor palsies as well as tightness of his chest were noted. Respiratory failure requiring mechanical respiration appeared 8 hours after the bite and lasted for nearly 96 hours. The two bite sites were virtually painless and resulted in slight transient erythema and edema. No specific antivenin was available, and treatment consisted of respiratory support and management of aspiration pneumonitis. Recovery was complete.

Enzymatic activities affecting exogenous nicotinamide adenine dinucleotide in human skin fibroblasts

The fate of nicotinamide adenine dinucleotide (NAD), AMP, and ADP-ribose supplied to intact human skin fibroblasts was monitored, and the concentrations of intra- and extracellular pyridine and purine compounds were determined by HPLC analysis. Two enzymatic activities affecting extracellular NAD were detected on the plasma membrane, one hydrolyzing the pyrophosphoric bond and yielding nicotinamide mononucleotide (nucleotide pyrophosphatase) and the other cleaving the glycoside link and releasing nicotinamide (NAD-glycohydrolase). No AMP or ADP-ribose was found in the extracellular medium of cells incubated with NAD, the former being completely catabolized to hypoxanthine and the latter degraded to adenine and hypoxanthine.

Resolution of isotoxins in the beta-bungarotoxin family

Although only five isotoxins of the beta-bungarotoxin family had been claimed, this work indicated the existence of more than sixteen isotoxins. Crude snake (Bungarus multicinctus) venom was divided into four main fractions by gel filtration on a Sephadex G-50 column. beta-Bungarotoxin appeared in a major fraction that contained mainly the M(r) 20,000 protein components. The fraction could be further resolved into eighteen peaks designated P1-P18 by HPLC on a Protein Pak SP 5PW column that was eluted with a linear gradient of 0.1-0.6 M CH3COONa in 20 mM NaH2PO4-Na2HPO4 at pH 7.4 P3-P18 were demonstrated to be isotoxins of the beta-bungarotoxin family. Results of protein sequencing for P8, P9 and P11, the three main isotoxins, confirmed that they shared a common phospholipase A2 subunit, which was very similar to although not completely identical with the A1 chain reported previously.

Enhancement of a slow potassium current component by uranyl nitrate and its relation to the antagonism on beta-bungarotoxin in the mouse motor nerve terminal

Uranyl nitrate (UO2(NO3)2) has been shown to be capable of increasing transmitter release from the motor nerve accompanied by the potentiation of nerve evoked muscle contraction. In this paper, we have demonstrated that UO2(2+) induced an initial twitch depression followed by a later twitch potentiation in low (0.35 mM) Ca2+ medium. Although UO2(2+) has been identified as a K(+)-channel blocker, we have found it only partially blocked the fast K(+)-current (IK(f) as recorded in the perineurial sheath of the mouse triangularis sterni preparation. Increasing the concentration of UO2(2+) to a high concentration of 0.4 mM did not further inhibit IK(f) but markedly prolonged the duration of the outward current of the nerve terminals. From the time course of its appearance together with the specific inhibition by 4-aminopyridine, dendrotoxin and beta-bungarotoxin, which has been shown to be capable of blocking the K(+)-current of the motor nerve terminal, it was proposed that UO2(2+) prolonged the duration of the nerve terminal spikes by an enhancement of an IK(s)-like current, which was further characterized by its susceptibility to be enhanced by low K+, low Ca2+ and Cd2+ but attenuated by high K+ and high Ca2+. These cation effects not only supported UO2(2+)-induced IK(s) current but also excluded the possibility of an enhancement of Ca(2+)-activated K(+)-current induced by UO2(2+) plus TEA. The significance of this enhancement of IK(s) induced by UO2(2+) has been elucidated by the finding that dendrotoxin inhibited but tetraethylammonium potentiated not only UO2(2+)-induced IK(s) but also UO2(2+)-induced twitch depression.(ABSTRACT TRUNCATED AT 250 WORDS)

The non-phospholipase A2 subunit of beta-bungarotoxin plays an important role in the phospholipase A2-independent neurotoxic effect: characterization of three isotoxins with a common phospholipase A2 subunit

Three isotoxins (SP I-III) of the beta-bungarotoxin family were purified to homogeneity via a series of isolation procedures including a final step of h.p.l.c. on an SP column washed with a linear gradient of 0.2-0.6 M sodium acetate at pH 7.4. Their proportions varied greatly with the batch of venom. Each isotoxin was demonstrated by SDS/PAGE to contain a phospholipase A2 subunit and a non-phospholipase A2 subunit. The three proteins were reductively alkylated with 4-vinylpyridine and the alkylated derivatives of the two subunits of each isotoxin were separated. N-Terminal sequence analysis of the alkylated derivatives revealed that the three isotoxins probably share a common phospholipase A2 subunit but differ in their non-phospholipase A2 subunits. The non-phospholipase A2 subunits of SP II and SP III were identical with those of beta 2- and beta 1-toxin respectively, except that there was an additional valine inserted between Thr-18 and Val-19 in beta 2-toxin and Pro-18 and Val-19 in beta 1-toxin. The non-phospholipase A2 subunit of SP I differed greatly from that of SP III but was almost identical with that of SP II, except that Lys-14 and Ala-29 in SP II were replaced by Arg-14 and Glu-29 in SP I. Analysis of the effect of CaCl2 on protein fluorescence showed the existence of a low- and a high-affinity site on the different domains of each isotoxin for Ca2+ binding. The three isotoxins showed no great difference in their ability to bind Ca2+ on both the high- and low-affinity site. They had slightly different phospholipase A2 activities but differed to a great extent with respect to their neurotoxic effects. LD50 values increased in the order SP I > SP II > SP III. In contrast, the ability to inhibit the indirectly evoked contraction of chick biventer cervicis muscle was in the order SP III > SP II > SP I.

Ethanol enhances agonist-induced fast desensitization in nicotinic acetylcholine receptors

The reversible decline of the nicotinic acetylcholine receptor's response to acetylcholine during prolonged exposure to acetylcholine is known as desensitization. Here, we studied ethanol's modulation of fast agonist-induced desensitization of the nicotinic acetylcholine receptor in postsynaptic membrane vesicles from Torpedo using a fast kinetic technique: pulsed quenched flow. Preincubation of the vesicles with various concentrations of acetylcholine at 4 degrees C for times ranging from 80 ms to 1.5 s caused fast desensitization, which was revealed as a decreased 86Rb+ influx when the vesicles were subsequently briefly exposed to a saturating concentration of acetylcholine in 86RbCl. Acetylcholine-induced fast desensitization had a maximum observed rate, kdmax, of 6.8 s-1, a half-effect concentration, KD, of 157 microM, and a Hill coefficient of 1.4. Increasing the ethanol concentration up to 1.0 M causes a linear increase in kdmax, such that 1.0 M ethanol doubles the rate. Ethanol (1 M) also decreased KD 10-fold without changing the Hill coefficient. We consider a modified sequential model to interpret our data. Two acetylcholine molecules bind sequentially to the receptor's resting state to form a pre-open (closed) state, which then opens and, at very high acetylcholine concentrations, is inhibited. A priori fast desensitization might occur from any of these acetylcholine-occupied states. If we assume fast desensitization to occur solely from the pre-open state, our data predict an excessively large action of ethanol on the fast desensitization rate constant (> 200-fold increase in the desensitization rate constant at 1 M ethanol). When we assume fast desensitization to occur from all states, ethanol is seen to have two actions.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of MPTP by 4-phenylpyridine on the neuromuscular blockade in mouse phrenic nerve-diaphragm

Potentiation by 4-phenylpyridine (a MAO-B inhibitor) on the neuromuscular blocking action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was studied in mouse phrenic nerve-diaphragm. MPTP blocked nerve-evoked twitches in a concentration (1-200 microM)-dependent manner. 4-Phenylpyridine, but not pargyline or tranylcypromine potentiated this inhibitory effect of MPTP. Pretreatment with 50 microM 4-phenylpyridine, reduced IC50 (concentration for 50% inhibition of twitch amplitude) values of MPTP from 53 to 18 microM and d-tubocurarine from 0.7 to 0.3 microM, respectively. 4-Phenylpyridine also enhanced the inhibitory action of MPTP and d-tubocurarine on acetylcholine (0.1 mM)-induced contracture of the denervated mouse diaphragm. The twitch inhibition induced by alpha-bungarotoxin and the specific binding of [125I]alpha-bungarotoxin to the mouse diaphragm were potentiated by 4-phenylpyridine but the inhibitory action of MPTP and d-tubocurarine on [125I]alpha-bungarotoxin binding were not significantly changed by pretreatment with 4-phenylpyridine. Electrophysiological studies revealed that the inhibitory actions of MPTP and d-tubocurarine on the amplitudes of m.e.p.ps and e.p.ps were augmented by 4-phenylpyridine. These indicate that 4-phenylpyridine enhanced the neuromuscular blocking action of the MPTP and d-tubocurarine at the postsynaptic nicotinic acetylcholine receptors. The implication of this finding is that the possible application of 4-phenylpyridine in the MPTP-induced Parkinson's disease is limited by its potentiation on the neuromuscular blocking action of MPTP.

Nine residues influence the binding of alpha-bungarotoxin in alpha-subunit region 185-200 of human muscle acetylcholine receptor

Identification of residues in the skeletal muscle nicotinic acetylcholine receptor (AChR) that bind snake venom alpha-neurotoxin antagonists of acetylcholine [e.g., alpha-bungarotoxin (alpha-BTx)] provides structural information about the neurotransmitter binding region of the receptor. Using synthetic peptides of the human AChR alpha-subunit region 177-208, we previously localized a pharmacologically specific binding site for alpha-BTx in segment 185-199. To define in more detail the residues that influence the binding of alpha-BTx to this region, we prepared 16 peptide analogues of the alpha-subunit segment 185-200, with the amino acid L-alanine sequentially replacing each native amino acid. Circular dichroism spectroscopy did not reveal changes in the secondary structure of the peptides except for the analogue in which Pro194 was substituted with alanine. This implies that any change in alpha-BTx binding could be attributed to replacement of the native residue's side chain by alanine's methyl group, rather than to a change in the structure of the peptide. The influence of each substitution with alanine was determined by comparing the analogue to the parental sequence alpha 185-200 in solution-phase competition with native human AChR for binding of 125I-labeled alpha-BTx. The binding of alpha-BTx by analogue peptides with alanine substituted for Tyr190, Cys192, or Cys193 was greatly diminished. Binding of alpha-BTx to peptides containing alanine replacements at Val188, Thr189, Pro194, Asp195, or Tyr198 was also reduced significantly (p < 0.003). An unanticipated finding was that substitution of alanine for Ser191 significantly increased alpha-BTx binding (p < 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)

Stabilization of acetylcholine receptors at the neuromuscular synapse: the role of the nerve

The majority of acetylcholine receptors (AChRs) at innervated neuromuscular junctions (NMJs) are stable, with half-lives averaging about 11 days in rodent muscles. In addition to the stable AChRs, approximately 18% of AChRs at these innervated junctions are rapidly turned over (RTOs), with half lives of less than 24 h. We have postulated that RTOs may be precursors of stable AChRs, and that the motor nerve may influence their stabilization. This hypothesis was tested by: (i) labeling AChRs in mouse sternomastoid (SM) muscles with 125I-alpha-BuTx; (ii) denervating one SM muscle in each mouse, and (iii) following the fate of the labeled AChRs through a 5-day period when RTOs were either stabilized or degraded. The hypothesis predicts that denervation should preclude stabilization of RTOs, resulting in a deficit of stable AChRs in denervated muscles. The results showed a highly significant (P less than 0.002) deficit of stable AChRs in denervated as compared with innervated muscles. Control experiments excluded the possibility that this deficit could be attributed to independent accelerated degradation of either RTOs or pre-existing stable AChRs. The observed deficit was quantitatively consistent with the deficit predicted by a mathematical model based on interruption of stabilization following denervation. We conclude that: (i) the observed deficit after denervation of NMJs is due to failure of stabilization of pre-existing RTOs; (ii) RTOs at normally innervated NMJs are precursors of stable AChRs; (iii) stabilization occurs after the insertion of AChRs at NMJs, and (iv) motor nerves play a key role in stabilization of RTOs. The concept of receptor stabilization has important implications for understanding the biology of the neuromuscular junction and post-synaptic plasticity.

Studies on curare-like action of 2,2',2''-tripyridine in the mouse phrenic nerve-diaphragm

1. The curare-like action of 2,2',2''-tripyridine (a synthetic by-product of the herbicide, paraquat) was studied in mouse phrenic nerve-diaphragm preparation. The inhibition by 2,2',2''-tripyridine of nerve-evoked twitches was dependent on the concentration, ranging from 1 to 100 microM, which had no significant effect on the twitch amplitudes evoked by direct muscle stimulation. 2. The twitch inhibition by 2,2',2''-tripyridine was reversible and could be antagonized by anticholinesterase agents such as neostigmine, physostigmine as well as ecothiophate. 3. Pretreatment with either 0.7 microM (+)-tubocurarine or 2.2 microM succinylcholine shifted the concentration-inhibition curve of 2,2',2''-tripyridine to the left. 4. 2,2'2''-Tripyridine inhibited not only acetylcholine-induced contracture of the denervated mouse diaphragm but also that of the chick biventer cervicis muscle. Like (+)-tubocurarine, 2,2',2''-tripyridine protected the twitches from the inhibition by alpha-bungarotoxin and also specifically inhibited the binding of [125I]-alpha-bungarotoxin to the mouse diaphragm. All of these findings indicate that 2,2',2''-tripyridine possesses curare-like action and inhibits the muscle contractions through binding to postsynaptic acetylcholine receptors. 5. The postsynaptic inhibition exhibited by 2,2',2''-tripyridine was also implicated in the tetanic fade, a decrease in the amplitude of miniature endplate potential (m.e.p.p.) and endplate potential (e.p.p.). 6. The clinical implication of these findings is that 2,2',2''-tripyridine may be involved in the cause of respiratory failure in paraquat-intoxicated workers since 2,2',2''-tripyridine is a by-product of paraquat synthesis.

The role of an invariant tryptophan residue in alpha-bungarotoxin and cobrotoxin. Investigation of active derivatives with the invariant tryptophan replaced by kynurenine

Ozone oxidation converted the single, invariant, tryptophan residue to N2-formylkynurenine in alpha-bungarotoxin and cobrotoxin. Upon this modification, the lethal toxicity was significantly reduced in cobrotoxin but mostly retained in alpha-bungarotoxin. Each neurotoxin containing kynurenine instead of tryptophan retained the same antigenicity as the native toxin. Fluorescence and CD spectroscopy revealed that, although the environment and state of the kynurenine residue were similar, [Kyn29]cobrotoxin was much more sensitive to pH change than alpha-[Kyn28]bungarotoxin. In terms of lethal toxicity and conformational stability, the invariant tryptophan residue appears to play a more important role in cobrotoxin, imparting a higher lethal toxicity than that in alpha-bungarotoxin, which has a disulfide bond at Cys29-Cys33.

Alpha-bungarotoxin binds to human acetylcholine receptor alpha-subunit peptide 185-199 in solution and solid phase but not to peptides 125-147 and 389-409

The nicotinic acetylcholine receptor (AChR) of human skeletal muscle has a reducible disulfide bond near the neurotransmitter binding site in each of its alpha-subunits. By testing a panel of overlapping synthetic peptides encompassing the alpha-subunit segment 177-208 (containing cysteines 192 and 193) we found that specific binding of 125I-labelled alpha-bungarotoxin (alpha-BTx) was maximal in the region 185-199. Binding was inhibited by unlabelled alpha-BTx greater than d-tubocurarine greater than atropine greater than carbamylcholine. Peptide 193-208 did not bind alpha-BTx, whereas 177-192 retained 40% binding activity. Peptides corresponding to regions 125-147 (containing cysteines 128 and 142) and 389-409, or peptides unrelated to sequences of the AChR failed to bind alpha-BTx. No peptide bound 125I-alpha-labelled parathyroid hormone. The apparent affinity (KD) of alpha-BTx binding to immobilized peptides 181-199 and 185-199 was approximately 25 microM and 80 microM, respectively, in comparison with alpha-BTx binding to native Torpedo ACh receptor (apparent KD approximately 0.5 nM). In solution phase, both peptides effectively competed with solubilized native human AChR for binding of alpha-BTx, and peptide 185-199 showed little evidence of dissociation after 24 h. Peptides that bound alpha-BTx did so when sulfhydryls were reduced. Cysteine modification, by N-ethylmaleimide or acetamidomethylation, abolished alpha-BTx-binding activity. The data implicate the region of cysteines 192 and 193 in the binding of neurotransmitter to the human receptor.

Kappa 2-bungarotoxin and kappa 3-bungarotoxin: two new neuronal nicotinic receptor antagonists isolated from the venom of Bungarus multicinctus

Neuronal nicotinic acetylcholine receptors are recognized with high affinity by two snake venom kappa-neurotoxins, kappa-bungarotoxin and kappa-flavitoxin. Native and radiolabeled kappa-neurotoxins have been used to localize and quantitate neuronal nicotinic receptors in a variety of species. We now report the identification of two new kappa-neurotoxins. kappa 2-Bungarotoxin and kappa 3-bungarotoxin were purified from the venom of Bungarus multicinctus collected in the province of Guangdong, China. kappa-Bungarotoxin has as yet not been found in this venom, although it is the only kappa-neurotoxin to be isolated thus far from Taiwanese Bungarus multicinctus. The geographical separation of Guangdong and Taiwan might account for this evolutionary divergence within the species. Both of the new kappa-neurotoxins are potent antagonists of nicotinic transmission in the chick ciliary ganglion. kappa 3-Bungarotoxin, the least potent of the kappa-neurotoxins, produces a complete blockage of nicotinic transmission in 60 min at 250 nM. Protection experiments using the short-acting nicotinic antagonists dihydro-beta-erythroidine and (+)-tubocurarine demonstrate that kappa 2-bungarotoxin blocks transmission by binding to the acetylcholine recognition sites of neuronal nicotinic receptors. The isoelectric point of kappa 2-bungarotoxin (pI = 8.9) is similar to that of kappa-bungarotoxin and kappa-flavitoxin, but kappa 3-bungarotoxin is considerably more basic, with pI greater than 11. Partial amino acid sequences are reported for both kappa 2-bungarotoxin and kappa 3-bungarotoxin. These sequences show a high degree of homology (approximately 80%) with other kappa-neurotoxins, and allow the determination of the critical differences between the kappa-neurotoxins and the structurally related alpha-neurotoxins. For example, all 4 kappa-neurotoxins lack a tryptophanyl residue which is invariant and important for function in the alpha-neurotoxins. The kappa-neurotoxins also differ from the alpha-neurotoxins by having an invariant prolinyl residue at a critical sequence position. Heterodimers were detected consisting of one subunit each of kappa 2-bungarotoxin and kappa 3-bungarotoxin. These heterodimers, which form between any combination of two kappa-neurotoxins, appear to be physiologically active and confirm that a further distinction between kappa-neurotoxins and alpha-neurotoxins is the strong tendency of the former to self-associate in solution. The present results help to establish the definition of 'kappa-neurotoxin'. These snake toxins are now being used by a number of laboratories in physiological and biochemical experiments on neuronal nicotinic receptors from a variety of species.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparative study of the biological properties of krait (genus Bungarus) venoms

1. The intravenous median lethal doses (LD50), protease, phosphodiesterase, alkaline phosphomonoesterase, L-amino acid oxidase, acetylcholinesterase, phospholipase A, 5'-nucleotidase, hyauronidase and anticoagulant activities of fourteen samples of venoms from the four common species of krait (Bungarus caeruleus, Bungarus candidus, Bungarus multicinctus and Bungarus fasciatus) were examined. 2. The results indicate that even though there are individual variations in the biological properties of the krait venoms, interspecific differences in the properties can be used for differentiation of the venoms from the four species of Bungarus. Particularly useful for this purpose are the LD50's and the contents of 5'-nucleotidase and hyaluronidase of the venoms.

Selective antagonism to succinylcholine-induced depolarization by alpha-bungarotoxin with respect to the mode of action of depolarizing agents

1. The interactions of alpha-bungarotoxin or tubocurarine with the neuromuscular block and endplate depolarization induced by succinylcholine (SCh) in the phrenic nerve-diaphragm preparation of mice were studied in order to elucidate the role of depolarization by SCh in the neuromuscular blockade. 2. The SCh concentrations required to depress the indirect twitch response by 20% and the evoked endplate potential in cut muscle preparations by 80% were 10 microm and 6 microM, respectively, while only 2 microM SCh was needed to induce maximal endplate depolarization from -80 mV to about -60 mV. 3. SCh blocked the neuromuscular transmission synergistically with either alpha-bungarotoxin or tubocurarine. There was an initial partial reversal of the neuromuscular inhibition caused by tubocurarine, but not that by alpha-bungarotoxin. 4. alpha-Bungarotoxin (0.025 microM) antagonized SCh (10 microM)-induced depolarization more effectively than it depressed miniature endplate potentials and the antagonism was insurmountable by increasing SCh concentration. By contrast, tubocurarine preferentially depressed miniature endplate potentials and antagonized SCh-depolarization competitively. 5. The above difference was attributed to the irreversible nature of alpha-bungarotoxin binding to acetylcholine receptors, to the slow diffusion of the toxin molecule into the synaptic cleft and thus to the more rapid binding with perijunctional receptors compared with junctional ones. 6. It is concluded that the sustained depolarization of the endplate by SCh results largely from an action on the perijunctional receptor in mice and, unlike cats, the neuromuscular block by SCh is not due to the depolarization per se but rather to a direct attenuation of endplate potential.

The lethal and biochemical properties of Bungarus candidus (Malayan krait) venom and venom fractions

Bungarus candidus venom exhibited high hyaluronidase, acetylcholinesterase and phospholipase A activities; low proteinase, 5'-nucleotidase, alkaline phosphomonoesterase and phosphodiesterase activities and moderately high L-amino acid oxidase activity. SP-Sephadex C-50 ion exchange chromatographic fractionation of the venom and Sephadex G-50 chromatography of the major lethal venom fractions indicate that the venom contains at least two highly lethal, basic phospholipases A with LD50 (i.v.) values of 0.02 micrograms/g (F6A) and 0.18 micrograms/g (F4A), respectively; as well as two polypeptide toxins with LD50 (i.v.) values of 0.17 micrograms/g and 0.83 micrograms/g, respectively. The major lethal toxin is the basic lethal phospholipase A, F6A, which accounts for approximately 13% of the venom protein and has a mol. wt of 21,000.

Role of the N-terminal region of the A chain in beta 1-bungarotoxin from the venom of Bungarus multicinctus (Taiwan-banded krait)

The N-terminal alpha-amino groups of beta 1-bungarotoxin (beta 1-Bgt) from Bungarus multicinctus venom were modified with trinitrobenzene sulfonic acid and the modified derivative was separated by high performance liquid chromatography. The trinitrophenylated (TNP) derivative contained two TNP groups at the alpha-amino groups of A chain and B chain and showed a marked decrease in enzymatic activity. Methionine residues at positions 6 and 8 of the A chain were oxidized with chloramine T or cleaved with cyanogen bromide to remove the N-terminal octapeptide. Oxidation of methionine residues and removal of the N-terminal octapeptide caused a precipitous decrease in enzymatic activity, whereas antigenicity remained unchanged. The presence of dihexanoyllecithin influenced the interaction between beta 1-Bgt and 8-anilinonaphthalene sulfonate (ANS) and revealed that beta 1-Bgt consists of two types of ANS-binding sites, one at the substrate binding site of the A chain and the other might be at the B chain. The modified derivatives still retained their affinity for Ca2+ and ANS, indicating that the N-terminal region is not involved in Ca2+ and substrate binding. A fluorescence study revealed that the alpha-amino group of the A chain was in the vicinity of substrate binding site and that the TNP alpha-amino groups were in proximity to Trp-19 of the A chain. In addition, the study showed that the N-terminal region is important for stabilizing the architectural environment of Trp-19. The results, together with the proposal that Trp-19 of the A chain is involved in substrate binding, suggest that the N-terminal region of the A chain plays a crucial role in maintaining a functional active site for beta 1-Bgt.

Rapid synthesis of acetylcholine receptors at neuromuscular junctions

The rate of acetylcholine receptor (AChR) degradation in mature, innervated mammalian neuromuscular junctions has recently been shown to be biphasic; up to 20% are rapidly turned over (RTOs; half life less than 1 day) whereas the remainder are lost more slowly ('stable' AChRs; half life 10-12 days). In order to maintain normal junctional receptor density, synthesis and insertion of AChRs should presumably be sufficiently rapid to replace both the RTOs and the stable receptors. We have tested this prediction by blocking pre-existing AChRs in the mouse sternomastoid muscle with alpha-bungarotoxin (alpha-BuTx), and monitoring the subsequent appearance of 'new' junctional AChRs at intervals of 3 h to 20 days by labeling them with 125I-alpha-BuTx. The results show that new receptors were initially inserted rapidly (16% at 24 h and 28% at 48 h). The rate of increase of 'new' 125I-alpha-BuTx binding sites gradually slowed down during the remainder of the time period studied. Control observations excluded possible artifacts of the experimental procedure including incomplete blockade of AChRs, dissociation of toxin-receptor complexes, or experimentally induced alteration of receptor synthesis. The present demonstration of rapid synthesis and incorporation of AChRs at innervated neuromuscular junctions provides support for the concept of a subpopulation of rapidly turned over AChRs. The RTOs may serve as precursors for the larger population of stable receptors and have an important role in the metabolism of the neuromuscular synapse.

Neural nicotinic acetylcholine responses in solitary mammalian retinal ganglion cells

Using the patch-clamp technique, whole-cell recordings from solitary rat retinal ganglion cells in culture have established the nicotinic nature of the acetylcholine responses in these central neurons. Currents produced by acetylcholine (5-20 mumol/l) or nicotine (5-20 mumol/l) reversed in polarity near -5 mV and were unaffected by atropine (10 mumol/l). Agonist-induced currents were blocked by low doses (2-10 mumol/l) of the classical 'ganglionic' antagonists hexamethonium and mecamylamine, as well as by d-tubocurarine and dihydro-beta-erythroidine (the latter two do not discriminate clearly between ganglionic and neuromuscular junction receptors). Treatment with the potent neuromuscular blocking agent alpha-bungarotoxin (10 mumol/l) did not affect the cholinergic responses of these cells, while toxin F (0.2 mumol/l), a neural nicotinic receptor antagonist, readily abolished acetylcholine-induced currents. Thus, the experiments performed to date show that the nicotinic responses of retinal ganglion cells in the central nervous system share the pharmacology of autonomic ganglion cells in the peripheral nervous system. The ionic current carried by the nicotinic channels was selective for cations, similar to that described for nicotinic channels in other tissues. In addition, single-channel currents elicited by acetylcholine were observed in whole-cell recordings with seals greater than 5 G omega as well as in occasional outside-out patches of membrane. These acetylcholine-activated events, which had a unitary conductance of 48 pS and a reversal potential of 0 mV, represent the ion channels that mediate the neural nicotinic responses observed in these experiments on retinal ganglion cells.

Degradation rates of acetylcholine receptors can be modified in the postjunctional plasma membrane of the vertebrate neuromuscular junction

Denervation of vertebrate muscle causes an acceleration of acetylcholine receptor turnover at the neuromuscular junction. This acceleration reflects the composite behavior of two populations of receptors: "original receptors" present at the junction at the time of denervation, and "new receptors" inserted into the denervated junction to replace the original receptors as they are degraded (Levitt, T. A., and M. M. Salpeter, 1981, Nature (Lond.), 291:239-241). The present study examined the degradation rate of original receptors to determine whether reinnervation could reverse the effect of denervation. Sternomastoid muscles in adult mice were denervated by either cutting or crushing the nerve, and the nerves either allowed to regenerate or ligated to prevent regeneration. The original receptors were labeled with 125I-alpha-bungarotoxin at the time of denervation, and their degradation rate followed by gamma counting. We found that when the nerve was not allowed to regenerate, the degradation decreased from a t1/2 of approximately 8-10 d to one of approximately 3 d (as reported earlier for denervated original receptors) and remained at that half-life throughout the experiment (approximately 36 d). If the axons were allowed to regenerate (which occurred asynchronously between day 14 and day 30 after nerve cut and between day 7 and 13 after nerve crush), the accelerated degradation rate of the original receptors reverted to a t1/2 of approximately 8 d. Our data lead us to conclude that the effect of denervation on the degradation rate of original receptors can be reversed by reinnervating. The nerve can thus slow the degradation rate of receptors previously inserted into the postsynaptic membrane.

Evidence for structural dissimilarity in the neurotransmitter binding region of purified acetylcholine receptors from human muscle and Torpedo electric organ

Acetylcholine receptor (AChR) purified from human skeletal muscle affinity-alkylated with bromoacetyl[methyl-3H]choline bromide ([3H]BAC) in mildly reducing conditions to yield a specifically radiolabeled polypeptide, Mr 44,000, the alpha-subunit. The binding of [125I]alpha-bungarotoxin to AChR was completely inhibited by affinity-alkylation, indicating that the human AChR's binding site for alpha-bungarotoxin is closely associated with the alpha-subunit's acetylcholine binding site. Structures in the vicinity of the alpha-bungarotoxin binding sites of AChRs from human muscle and Torpedo electric organ were compared by varying the conditions of alkylation. Under optimal conditions of reduction and alkylation, both human and Torpedo AChR incorporated BAC in equivalence to the number of alpha-bungarotoxin binding sites. However, with limited conditions of reduction but sufficient BAC to alkylate 100% of the alpha-bungarotoxin binding sites of human AChR, only 71% of the Torpedo AChR's binding sites were alkylated. In optimal conditions of reduction but with the minimal concentration of BAC that permitted 100% alkylation of the human AChR's alpha-bungarotoxin sites, only 74% of the Torpedo AChR's binding sites were alkylated. These data suggest that the neurotransmitter binding region of human muscle AChR is structurally dissimilar from that of Torpedo electric organ, having a higher binding affinity for BAC and an adjacent disulfide bond that is more readily accessible to reducing agents.

Antagonistic action of uranyl nitrate on presynaptic neurotoxins from snake venoms

Uranyl ion (UO2+2) antagonized the neuromuscular blocking action and phospholipase A2 activity of neurotoxins which act presynaptically [beta-bungarotoxin (beta-BuTX) and crotoxin] but did not affect the action of alpha-bungarotoxin and tetrodotoxin. On the basis of the kinetic analysis of the UO2+2 and strontium ion (Sr2+) antagonism of muscle paralysis induced by beta-bungarotoxin, it was found that they inhibited both the binding of the toxin and the steps following binding that brought about the neuromuscular blocking action of beta-bungarotoxin. Uranyl ion was about 50 times more potent than Sr2+ in antagonizing beta-bungarotoxin. High Ca2+ (10 mM) abolished but low Ca2+ (0.25-1.25 mM) medium enhanced the antagonizing action of UO2+2 and Sr2+. In low Ca2+ medium, UO2+2 markedly potentiated the amplitude of the twitch, subsequent addition of beta-bungarotoxin produced three phases of effects on the twitches, e.g. an initial depression, followed by the second facilitation and finally a rapid depression of twitches; however, approx. 70 min after beta-bungarotoxin the small twitches reached a steady state which persisted for more than 350 min. Therefore, it is evident that UO2+2 is the most potent antagonist of beta-bungarotoxin so far tested.

Acetylcholine receptors of human skeletal muscle: a species difference detected by snake neurotoxins

The binding abilities of the nicotinic acetylcholine receptors (AChRs) of the skeletal muscles of man and other vertebrates to two typical curaremimetic toxins, erabutoxin b (Eb) and alpha-bungarotoxin (alpha-BT), were investigated. Fluorescent microscopy using rhodamine-labeled erabutoxin b (TMR-Eb) and FITC-labeled alpha-bungarotoxin (FITC-alpha-BT) revealed that AChRs of human and chimpanzee muscles were stained with FITC-alpha-BT, but not with TMR-Eb. In contrast, the AChRs of mouse muscle were stained with both fluorescent toxins. The stainings of human and chimpanzee AChRs with FITC-alpha-BT were inhibited by preincubation with unmodified alpha-BT, but not with either unmodified Eb or other short-chain neurotoxins. Binding experiments using 125I-labeled Eb ([125I]Eb) and 125I-labeled alpha-BT ([125I]alpha-BT) showed that the affinity of human AChRs for [125I]Eb was unusually low. Electrophysiological experiments showed that both acetylcholine potential and end-plate potential of human muscle were blocked by addition of alpha-BT, but not by Eb. On the contrary, acetylcholine potential of rat muscle was blocked by addition of Eb. All these results indicate that AChRs of human and chimpanzee muscles are different from those of other animals in having an exceptionally low affinity for Eb and other short-chain neurotoxins. The results suggest a heterogeneity among vertebrate AChRs concerning their reactivities to curaremimetic toxins.

Mode of stimulatory actions of cadmium ion on the mouse diaphragm

Effects of Cd2+ on the phrenic nerve-diaphragm preparation of the mouse varied markedly in media containing various Ca2+ concentrations. In normal 2.5 mM Ca2+ medium, Cd2+ inhibited acetylcholine release from nerve endings without appreciable effect on the muscle membrane. However, Cd2+ elicited stimulatory effects on the muscle membrane in low Ca2+ medium (10(-3)-1 mM). These stimulatory effects included the induction of spontaneous contractions, augmentation of twitch responses to direct electrical stimulation and potentiation of the muscle contracture induced by acetylcholine, carbachol and high K+. By contrast, caffeine contracture was not affected by Cd2+. Tetrodotoxin, procaine, cysteine and glycerol pretreatment abolished these stimulatory effects of Cd2+. Moreover, changing the ionic composition of the bathing medium to one containing low Na+, high K+, high Mg2+ or high Ca2+ also antagonized these effects of Cd2+. In contrast, low Mg2+ markedly potentiated the frequency of spontaneous contractions induced by Cd2+. (+)-Tubocurarine and beta-bungarotoxin had no effect on Cd2+-induced spontaneous contractions indicating that they may be myogenic rather than neurogenic in origin. By use of conventional microelectrodes, it was found that Cd2+ not only depolarized the muscle membrane but also induced spontaneous action potentials at a high frequency (173 +/- 17 Hz). It is concluded that increased Na+ permeability of the muscle membrane is the essential step bringing about spontaneous contractions. The binding of Cd2+ to -SH groups of the membrane is closely related to the induction of these effects.

Binding characteristics of the bungarotoxin fraction II-S1 to rat brain membranes

Bungarotoxin fraction II-S1 (designated BGT II-S1), isolated from the venom of Bungarus multicinctus, appears to affect nicotinic transmission in rat sympathetic ganglia through its phospholipase activity. On the other hand, the present investigation suggests that other modes of interaction of this toxin with nervous tissue may also exist as, in a rat brain membrane preparation, binding of this toxin to a specific binding site can be demonstrated. In a buffer containing calcium, binding of [125I]BGT II-S1 saturated with a Bmax of approximately 16 fmol/mg protein and a Kd of 5 nM. This site did not appear to be directly linked to the nicotinic acetylcholine recognition site as the binding was not displaced by nicotinic agents; however, alpha-bungarotoxin, which interacts with a nicotinic-like site in neural tissue, did affect the binding and, conversely, BGT II-S1 inhibited the binding of [125I]alpha-bungarotoxin.

Localization of alpha-bungarotoxin binding sites in the ciliary ganglion of the embryonic chick: an autoradiographic study at the light and electron microscopic level

Binding sites for [125I]alpha-bungarotoxin were localized in the chick ciliary ganglion by light and electron microscopic autoradiography. Groups of four ganglia were incubated for 4 h with 20 nM [125I]alpha-bungarotoxin alone or with radioactive alpha-bungarotoxin plus either 1 microM unlabeled alpha-bungarotoxin or 100 microM d-tubocurarine. Specific binding to various morphological regions was determined by subtracting the densities of autoradiographic grains in the presence of competing nonradioactive ligands from the densities in the absence of those ligands. Most of the specific binding in the ganglion (91%) was associated with neurons. Seventy-four per cent was found within 1.2 micron of neuronal plasma membranes and 17% was found overlying neuronal cell bodies. Analysis of the specific binding associated with neurons, but not with the neuronal plasma membranes, revealed that lysosomes and multivesicular bodies were 9- and 32-fold more heavily labeled than other cellular organelles. The grain density over choroid cell bodies was significantly higher than that over ciliary neurons. Most (greater than 75%) of the autoradiographic grains within 0.25 micron of neuronal plasma membranes were found in "complex" contact regions of the membranes, which are characterized by extensive membrane evaginations. However, after correcting for the amount of plasma membrane present in the various regions of the membrane studied, alpha-bungarotoxin binding was found to be uniform. Few (less than 10%) of the specialized membranes between pre- and postsynaptic neurons were found in these "complex" contact regions suggesting that the bulk of alpha-bungarotoxin binding to neuronal membranes is located at some distance from the sites of transmitter release. The density of alpha-bungarotoxin binding sites in the neuronal plasma membrane was low (approximately 100-200 sites/micron 2 of neuronal membrane) compared to published values of the density of binding sites at the neuromuscular junction. Since alpha-bungarotoxin does not bind preferentially to specialized synaptic membranes, it seems unlikely that the binding sites for this toxin are the neuronal nicotinic receptors.