Mechanism of Neurontoxins in Snake Venom



|Mechanism of the Neurontoxins in Snake Venom |

For

BIO 490 01

Proffessor: Dr. Baskin J. N.

By Lifan Shih

March 19, 1999

Abstract: Mechanism of the Neurontoxins in Snake Venom

The neurotoxins in snake venom disrupt synaptic transmission. They can either inhibit the release of neurotransmitter from exocytosis of synaptic vesicle at the presynaptic site or bind to the neurotransmitter receptor at postsynaptic site. Here two kinds of neurotoxins, bungarotoxin and crotoxin, are illustrated. There are two types of bungarotoxin isolated from Bungarus multicinctus(Taiwan banded krait), alpha-bungarotoxin and beta-bungarotoxin. Alpha-bungarotoxin acts on the postsynatic site. It binds to nicotinic acetylcholine receptor on the postsynaptic membrane irreversibly, and blocks the depolarizition in the postsynaptic site. Belta-bungarotoxin act on presynaptic site. It disrupts the release of synaptic vesicle that acetylcholine is stored by its phospholipase A2 activity, and thus the synaptic transmission is blocked. Crotoxin, isolated from Crotalus durissus terrificus, like beta-bungarotoxin, also has phospholipase A2 activity and thus it can disrupt the release of synaptic vesicle at presynaptic site, but its mechanism is quite different from beta-bungarotoxin. The characteristics of neurotoxin can be applied to neuroscience research. alpha-bungarotoxin has been used for labeling acetylcholine receptor because it bind to nicotinic acetylcholine receptor specifically.

Table of content:

1. Introduction ……………………………………………………4

2. Literature Review………………………………….………….. 5

2.1 Bungarotoxin …………………………………………… 5

2.1.1 Isolation of alpha-bungarotoxin …………………… 6

2.1.2 Isolation of beta-bungarotoxin …………………….. 7

2.2 Crotoxin ……………………………………………….. 7

2.3 Application of alpha-bungarotoxin ……………………. 8

3. Discussion……………………………………………………….. 9

3.1 Possible Mechanisms for beta-bungarotoxin …………. 9

3.2 Application of alpha-bungarotoxin ……………………12

3.3 The advantage in snake venom neurotoxin research …..12

4. Conclusion ……………………………………………………….14

5. Literature Cited ………………………………………………….15

1. Introduction:

In order to realize the mechanism of neurotoxin in snake venom, first we have to know what happens in the normal neuromascular junction. The neuromuscular junction contains the membrane of the axon terminal of the motor neuron, also called presynaptic membrane, or presynaptic site; the motor end-plate of muscle cell, also called postsynaptic membrane, or postsynaptic site; and the space between them called synaptic cleft. Here the events of presynaptic membrane, synaptic cleft, and postsynaptic membrane is delineated:

(1). Presynaptic membrane

The neurotransmitter, acetylcholine is synthesized in the cytosol of the axon terminal, and must be taken up by the synaptic vesicle. Acetylcholine release is triggered by the arrival of an action potential in the axon terminal. The depolarization of the presynaptic membrane causes voltage-gated calcium channels in the active zones to open. The extracellular concentration of calcium is far higher than the intracellular concentration of calcium, so calcium ions will flood the axon terminal as long as the calcium channels are open. The resulting elevation in the internal calcium ion concentration is the signal that causes neurotransmitter to be released from synaptic vesicle.

The vesicle release their contents by a process called exocytosis. The membrane of the synaptic vesicle fuses to the presynaptic membrane at the active zone, allowing the contents of the vesicle to spill out into the synaptic cleft.

(2) Postsynaptic membrane

Acetylcholine released in the synaptic cleft affect the muscle cell by binding to thousands of specific receptor proteins that are embedded in the motor end-plate. The binding of neurotransmitter to the receptor is like inserting a key in a lock; this causes conformation changes in the protein.

Receptor proteins also called Acetylcholine-gated sodium ion channels. They are membrane-spanning proteins consisting of five subunits that come together to form a pore between them. In the absence of neurotransmitter, the pore is closed. When neurotransmitter bind to a specific site on the extracellular region of the channel, the pore is open due to the conformation changes. The resulting elevation in the internal sodium ion concentration depolarizes the muscle cell from resting membrane potential, and eventually causes muscle to contract.

In summary, when the nerve impulse arrives at the presynapic site of the neuromuscular junction, calcium moves into the membranes of axon terminal and triggers the release of acetylcholine. Acetylcholine passes across the synaptic and combines with acetylcholine receptor at the postsynaptic site of neuromascular junction. This causes depolarization of the muscle fiber and eventually muscle contraction. Thus any substances that interfere with normal processes of synaptic transmissions can be neurotoxic. (Tu, venoms: chemistry and molecular biology, p258)

Neurotoxins in snake venom can either disrupt the function of presynaptic site, like beta-bungarotoxin and crotoxin, or disrupt the function of postsynaptic sited, like alpha-bungarotoxin. The detail of how those neurotoxins work would fully described in following reviews.

2. Review of Literature

2.1. Bungarotoxin

The curare-like properties of Bungarus multicinctus (Taiwan banded krait) (Figure 1) venom were recognized by To and Tin (1944a, b). Lee and Peng (1961) concluded that the respiratory failure caused by the venom was primarily peripheral in origin. The site of action was further studied by Chang (1960a), who found that the blocking was confined to the neuromuscular junction, since the conductivity of the phrenic nerve and the twitch response to direct stimulation were unaffected.

For further study, Chang (1960b) used frog rectus abdominus muscle. The venom inhibited the acetylcholine response of the muscle, but the potassium chloride contracture was not affected appreciably. The effect was progressive and could not be reversed by washing out the venom. Chang concluded that the curare like action of the venom resulted from progressive and irreversible occupation of the specific receptorfor acetylcholine in the motor end plate. Further progress was made in studying the mode of action by using isolated alpha-, beta-, and gamma-bungarotoxins. (Chang and Lee, 1963) Chang and Lee reached the important and definitive conclusion that alpha-bungarotoxin block neuromuscular transmission by an irreversible combination with the motor end-plate acetylcholine receptor, whereas beta- and gamma-bungarotoxins exhibit their actions at presynaptic sites. Beta-bungarotoxin destroys synaptic vesicles and thus inhibits the release of acetylcholine from presynaptic sites. (Chen and Lee, 1970)

[pic]

Figure 1. Bungarus multicinctus (Taiwan banded krait)

2.1.1 Isolation of alpha-bungarotoxin:

alpha-bungarotoxin was isolated from the venom of Bungarus multicinctus by Lee and Chang using zone electrophoresis in 1963. They showed that the toxin acted as an irreversible antagonist of cholinergic receptors (Figure 2) the site of the vertebrate nerve-muscle junction. Their finding that this effect culd be prevented by d-tubocurarine, a specific but reversible antagonist of neuromuscular cholinergic receptors, indicated that alpha-bungarotoxin interacted with the cholinergic receptor (Chen and Lee, 1970).

[pic]

Figure 2. Nicotinic-acetylcholine receptor and the binding site of alpha-bungarotoxin

2.1.2 Isolation of beta-bungarotoxin:

Beta-bungarotoxin was isolated from the venom of Bungarus multicinctus by Lee and Chang zone electrophoresis in 1963. It is a nondepolarizing neurotoxin that acts on the presynaptic site (Chang and Lee, 1963). Ultrastructural investigation indicates that the vesicles are completely eliminated in the axonal terminal (Chen and Lee, 1970).

Unlike alpha-bungarotoxin, beta-bungarotoxin is an inhibitor at the presynaptic site and has no postsynaptic action on the membrane potential, action potential, or the sensitivity to acetylcholine at the motor end-plate. (Chang et al., 1973) The presynaptic action of beta-bungarotoxin was compared with that of botulinum toxin (Chang and Huang, 1974). The paralytic actions of beta-bungarotoxin and botulinum toxin appear to take place in two processes. First, they bind with the respective target sites, and, second, they inhibit changes in the target macromolecule of the nerve terminals, leading to failure of transmitter release. The two toxins show a mutual antagonism, especially when beta-bungarotoxin is added before or simultaneously with botulinum toxin (Chang et al., 1973a, b; Chang and Huang, 1974). The mechanism of beta-bungarotoxin acting on postsynaptic site is still unknown. The failure of crotoxin to interfere with the binding of beta-bungarotoxin suggested that the postsynaptic inhibition of beta-bungarotoxin is different from that of crotoxin (Hanley, 1978)

2.2. Crotoxin

The first isolation of a neurotoxin from snake venom was achieved by Slotta and his co-workers from the venom of Crotalus durissus terrificus (Figure 3), or South American rattlesnake (Slotta, 1938; Slotta and Fraenkel-Conrat, 1938a, b, 1939). The crystalline toxin was called crotoxin. From studies by ultracentrifugation and by Tiselius electrophoresis (Gralen and Svedberg, 1938), crotoxin appeared to be homogeneous (Li and Fraenkel-Conrat, 1942). The molecular weight of crotoxin is 30,000, and the pI is 4.7. A molecular weight value of 30,900 was obtained by Paradies and breithaupt (1975)

[pic]

Figure 3. Crotalus durissus terrificus, or South American rattlesnake

Crotoxin is a potent neurotoxin consisting of a basic and weakly toxic phospholipase A2 subunit (component B) and an acidic nonenzymatic subunit (component A). The nontoxic component A enhances the toxicity of the phospholipase subunit by preventing its nonspecific adsorption (Hendon and Fraenkel-Conrat, 1977). Crotoxin produces irreversible blockade of transmission at the vertebrate neurotransmitter in response to the nerve impulse as a result of interference with depolarization-secretion coupling (Brazil and Excell, 1971; Brazil et al., 1973; Chang and Lee, 1977; Hawgood and Smith, 1977b)

The binding of crotoxin and of its subunits to small unilamellar phospholipid vesicles was examined under experimental conditions that prevented any phospholipid hydrolysis. Isolated component B rapidly bound with a low affinity (Kapp in the millimolar range) to zwitterionic phospholipid vesicles and with a high affinity (Kapp of less than 1 microM) to negatively charged phospholipid vesicles. On the other hand, the crotoxin complex did not interact with zwitterionic phospholipid vesicles but dissociated in the presence of negatively charged phospholipid vesicles; the noncatalytic component A was released into solution, whereas component B remained tightly bound to lipid vesicles, with apparent affinity constants from 100 to less than 1 microM, according to the chemical composition of the phospholipids. On binding, crotoxin or its component B caused the leakage of a dye entrapped in vesicles of negatively charged but not of zwitterionic phospholipids. The selective binding of crotoxin suggests that negatively charged phospholipids may constitute a component of the acceptor site of crotoxin on the presynaptic plasma membrane. ( Radvanyi F, Saliou B, Lembezat MP, Bon C, 1989)

Component A might be the chaperone for component B. The proposed function of component A would be to decrease the nonspecific binding of component B by charge neutralization or masking until the target site is reached. This is somewhat supported by the finding of low toxicity for the basic subunits. Thus, without component A the basic component B becomes, essentially, deleted before reaching the target site. (Hendon, Bieber, Tu)

2.3. Application of alpha-bungarotoxin as nicotinic acetylcholine receptor labeling probe:

Recently, some scientists try to use DNA cloning technique to produce neurotoxins in snake venom. From amino acid sequence of neurotoxins, we can deduct the DNA sequence of them, and synthesize the artificial DNA of neurotoxin. Furthermore, we can insert the DNA into the cloning vector like bacteria to let the baceteria make neurotoxins for us. Use this way, the large quantity of neurotoxins are available for research, so the venom research will be more efficient than ever. Also, using the artificial DNA as a probe, we can find the gene of neurotoxins in snake DNA by hybridization technique, and clone the gene in the vector. (Yang, Y. L. & Lo, S. J. 1996. The story of Snake Venom Research in Taiwan, p311) At that time, the unclear functions of beta-bungarotoxin and crotoxin would be clarified.

Many people have doubt toward the importance of snake venom research. The snake venom research can discover the evolution pathway of snakes by comparing the proteins or genes of neurotoxins, but also accelerate the research in the other disciplines. For instance, alpha-bungarotoxin has been widely used in neuroscience research. Alpha-bungarotoxin has been used for labeling and isolating acetylcholine receptor because it binds to nicotinic acetylcholine receptor specifically. For example, the effect of maternal smoking is tested by alpha-bungarotoxin:

It is well established that maternal smoking during pregnancy is a leading preventable cause of low birth weight and prematurity. Less appreciated is that maternal smoking during pregnancy is also associated with alterations in pulmonary function at birth and greater incidence of respiratory illnesses after birth. To determine if this is the direct result of nicotine interacting with nicotinic cholinergic receptors (nAChRs) during lung development, rhesus monkeys were treated with 1 mg/kg/day of nicotine from days 26 to 134 of pregnancy. Nicotine administration caused lung hypoplasia and reduced surface complexity of developing alveoli. Immunohistochemistry and in situ alpha-bungarotoxin (BGT) binding showed that 7 nAChRs are present in the developing lung in airway epithelial cells, cells surrounding large airways and blood vessels, alveolar type II cells, free alveolar macrophages, and pulmonary neuroendocrine cells (PNEC). As detected both by immunohistochemistry and by BGT binding, nicotine administration markedly increased 7 receptor subunit expression and binding in the fetal lung. Correlating with areas of increased 7 expression, collagen expression surrounding large airways and vessels was significantly increased. Nicotine also significantly increased numbers of type II cells and neuroendocrine cells in neuroepithelial bodies. These findings demonstrate that nicotine can alter fetal monkey lung development by crossing the placenta to interact directly with nicotinic receptors on non-neuronal cells in the developing lung, and that similar effects likely occur in human infants whose mothers smoke during pregnancy. (Sekhon HS, Jia Y, Raab R, Kuryatov A, Pankow JF, Whitsett JA, Lindstrom J, Spindel ER, 1999)

The advantage of using alpha-bungarotoxin to label nicotinic-acetylcholine receptor is that alpha-bungarotoxin can conjugate with many kinds of fluorescent compounds. BODIPY FL alpha-bungarotoxin, Oregon Green 514 alpha-bungarotoxin, and fluorescein alpha-bungarotoxin are suitable for use with standard fluoriscein optical filter sets. Oregon Green 514 conjugates exhibit bright green fluorescence and superior photostability, making them especially well suited to fluorescence and confocal laser scanning microscopy. Tetramethylrhodamine alpha-bungarotoxin is currently the dominant red-orange fluorescent probe for staining the nicotinic AchR. Texas Red-X conjugate of alpha-bungarotoxin has a longer-wavelength emission maximum and therefore offers better spectral separation from green fluorescent dyes in multicolor experiments. Eosin alpha-bungarotoxin was designed as a probe for visualizing nicotinic AChRs either by fluorescence microscopy or by electron microscopy using the diaminobenzidine photooxidation technique. Rhodamine-labeled alpha-bungarotoxin was used in conjunction with a sophisticated time-lapse video technique to document nicotinic AChR cluster formation after myoblast fusion. Rhodamine-labeled alpha-bungarotoxin was used to quantitate nicotinic AChRs in a study that showed that several isotypes of agrin, a component of synaptic basal lamina, help trigger the nicotinic AChR aggregation that occurs during neuromuscular junction formation. Studies of reinnervation of adult muscle after nerve damage have employed fluorescein- and rhodamine-labeled a-bungarotoxins to identify and visualize endplates. Nicotinic AChRs can also be labeled with biotinylated alpha-bungarotoxin, which is then localized using enzyme- or fluorophore-labeledconjugates of avidin or streptavidin In addition, the biotinylated toxin can be employed for affinity isolation of the nicotinic AChR using an avidin or streptavidin agarose column and for enzyme-linked immunosorbent assays (ELISAs) designed to detect anti–nicotinic AChR antibodies. (Molecular Probes, Inc., )

3. Discussion:

3.1 Possible mechanisms for beta-bungarotoxin

From the review of literature, we can find that the mechanism of neurotoxins in snake venom has not full discovered yet except alpha-bungarotoxin. We know that beta-bungarotoxin eliminates synaptic vesicles before they release acetylcholine by exocytosis, but we don’t know how beta-bungarotoxin eliminates synaptic vesicles. We don’t know if beta-bungarotoxin acts as the basic component of crotoxin having phospholipase A2 activity to disrupt the phospholipid membrane of synaptic vesicle. Some literatures say beta-bungarotoxin is a kind of phospholipase A2, but the other literatures does not agree with that. The function of subunit A in crotoxin is also not very clear. Maybe the more advanced techniques are needed for the further discovery of the function of neurotoxins in snake venom.

The mechanism of beta-bungarotoxin still has a lot of room for future research. Here the direction of future research is addressed. From the literature review, we know beta-bungarotoxin does pretsynaptic inhibition. There are several ways for beta-bungarotoxin to disrupt postsynaptic function:

(1) Block voltage-gated Na+ K+ channel

The simplest way is to block the voltage-gated sodium and potassium channels in the axon terminal. If beta-bungarotoxin block voltage-gated sodium and potassium channel, the action potential cannot reach the axon terminal, so the synaptic vesicle can not be released.

(2) Block Na+ K+ ATPase

There is another way to block action potential conducting through the axon terminal. The sodium potassium ATPase (also called sodium potassium pump) is essential for maintaining the resting potential of cell membranes. If beta-bungarotoxin disrupt the function of sodium, potassium ATPase, it can disrupt the resting potential, and thus disrupt the conductivity of membrane for action potential.

(3) Block voltage-gated Ca++ channel

Beta-bungarotoxin can also block voltage-gated calcium channel in the axon terminal. The action potential conducted by voltage-gated sodium and potassium channel induces the voltage calcium channel to open, and calcium ions enter the cytosol of axon terminal. If voltage-gated calcium channel is blocked, even the action potential exists, the calcium still cannot enter the cytosol.

(4) Interact with Ca++ pump of mitochondria

We know mitochondria can uptake the excess calcium ions in the cytosol. If beta-bungarotoxin facilitate the activity Ca++ pump of mitochondria, mitochondria will uptake too much calcium ions from cytosol, so the effect of calcium ions is disrupted.

(5) Inhibit binding of Ca++ with calmodulin

After calcium ions enter the cell, four calcium ions will bind to one calmodulin protein. If beta-bungarotoxin disrupts the binding of calcium ions to calmodulin, the cascade reactions induced by calmodulin would also be disrupted.

(6) Inhibit protein kinase C

Calcium-calmodulin complex is an enzyme that activates CAM protein kinase C to phosphorylate synapsin 2. Synapsin 2 anchors synaptic vesicle on microtubule, and it release synaptic vesicle when it is phosphorylated. Beta-bungarotoxin thus can inhibit protein kinase C so that synaptic vesicle can never detach from microtubule.

(7) Inhibit binding of Ca++ with synaptophysin (Docking of synaptic vesicle)

Calcium ions also activates docking of synaptic vesicle. There are two membrane protein, synaptophysin and synaptobrevin, regulate the docking of synaptic vesicle. Synaptobrevin anchors the synaptic vesicle to the presynaptic membrane. In the absence of calcium ion, the binding site of synaptobrevin is shadowed by synaptophysin. Synaptophysin moves from it bind to calcium ion, and the binding site of synaptobrevin is exposed. Beta-bungarotoxin can inhibit calcium to bind synaptophysin, so synaptic vesicle can never dock to presynatic membrane, and the exocytosis of synaptic membrane can never occur.

(8) Inhibit bindng of synaptobrevin, syntaxin, and SNAP 25 (Docking of synaptic vesicle)

In order to anchor synaptic vesicle to presynaptic membrane, Synaptobrevin binds to two membrane proteins on the presynaptic membrane, syntaxin and SNAP 25. Beta-bungarotoxin can disrupt the binding process, either inhibits the activity of synaptobrevin on synaptic vesicle membrane or the activity of syntaxin and SNAP 25 on presynaptic membrane, and makes synaptic vesicle unable to be anchored on presynaptic membrane.

(9) Inhibit the function of synaptotagmin (exocytosis)

In order to release acetylcholine to neuromuscular junction after the synaptic vesicle has already docked to the presynaptic membrane, the membrane of synaptic vesicle has to fuse with presynaptic membrane of the neuron, and that is the process called exocytosis. Exocytosis is also mediated by calcium ions. Calcium ion binds to a membrane protein on the membrane of synaptic vesicle, synaptotagmin. The calcium-synaptotagmin complex then activate vesicle fusion and exocytosis. Beta-bungarotoxin can inhibit the function of synaptotagmin, so the synaptic vesicle can never fuse to presynaptic membrane and the neurotransmitter can never be released to neuromuscular junction.

Except (5) and (6), all the possible ways of beta-bungarotoxin can involve with phospholipase activity. Voltage-gated Na+ K+ channel, Na+ K+ ATPase, voltage-gated Ca++ channel, Ca++ pump, synaptophysin, synaptobrevin, syntaxin, SNAP 25, and synaptotagmin are all membrane proteins. If beta-bungarotoxin is a phospholipase, it can degrade the membrane of synaptic vesicle and presynaptic membrane, and thus all the membrane proteins mentioned above will be impacted. That is why most scientists think beta-bungarotoxin is a phospholipase. We could conduct research to test the nine hypothesis above. The activity of beta-bungarotoxin should not exceed from those hypotheses too far.

3.2 Application of alpha-bungarotoxin

From the literature review, we know the most common application of neurontoxins from snake venom is using alpha-bungarotoxin to label and thus isolate nicotinic acetylcholine receptor. The activity of alpha-bungarotoxin is so simple and straightforward. It bind to nicotinic acetylcholine receptor (nAchR) specifically, so we can use alpha-bungarotoxin to label nAchR in the organs, isolate nAchR by affinity chromatography, and quantitate nAchR by ELISA method. Alpha-bungarotoxin is also relatively inexpensive than other chemical reagent.

In literature review, the effect of maternal smoking by increasing nAchR in the epithelial cells of lungs of the infant has been proved by nAchR. Recently some researches proposed that nicotine could increase the number of nAchR in dopaminergic neurons in the brain. We can also use alpha-bungarotoxin to test the proposal. Not only nicotine-caused diseases but also the other neural and muscular diseases might relate to nAchR. For instance, myasthenia gravis is an auto immune disease that nAchR becomes the target of our antibody. Therefore,we can use alpha-bungarotoxin to investigate the pathology of those disease in the medical field.

Because alpha-bungarotoxin is inexpensive, we can save much money by using alpha-bungarotoxin to do research. Besides, the effect of alpha-bungarotoxin is very straightforward, so we can save many lab procedures, and thus save more research budget. This factor is especially critical for third world countries, which don’t have too many budgets for biomedical research, but wants to have good quality of research. Now insufficient research budget is also a severe problem in the US. Since 80s the federal government and state governments have cut down the educational budget, so the biomedical researches, which needs more money than the other researches, have faced the predicament. Also, the research budget is distributed unevenly. Most of the research budgets only go to a few famous public universities, like UC system in California. The majorities of the public universities, like CSU system in California, even have no money to buy the basic equipment for biomedical education. Therefore, to save money in research becomes more and more important in the US. Alpha-bungarotoxin is very practical in this aspect.

3.3 The advantage in snake venom neurotoxin research

We know the neurotoxins in snake venom provide us information in the study of neuroscience, evolution, and medicine. Just like understanding the cellular physiology from diseases, knowing the development of cell from oncogenic virus, we can understand the neuron system from neurotoxins. Neurotoxin tells us what happens if some part of elements not function well. From this aspect, neurotoxins are powerful mean for us to investigate neuron system. Also, from the specific interaction of neurotoxin to neuron system, like alpha-bungarotoxin to nicotinic acetylcholine receptor, we are able to label the specific molecules or reactions in neuron system. The powerful labeling technique accelerates the progress of research.

The most advantageous aspect in the research is that we can obtain this powerful tool, neurotoxin in relatively little expense. Snakes are very common in the world, so the accessibility of snake venom is better than many other chemicals. To extract neurotoxin from snake venom is very easy. Since neurotoxins are big protein molecule, we can easily separate them from snake venom by very basic separation procedures like centrifugation, acid precipitation, ammonium sulfate precipitation, dialysis, ion exchange chromatography, SDS-PAGE gel electrophoresis, and optical density measurement at 280 nm. Also, it is easy to know whether the protein we isolated from snake venom is a neurotoxin. Neurotoxin normally has behavioral effect in animals. The behavioral change is easier to observe than the other pharmacological effects.

The neurotoxins in snake venom are very different in different snakes, and there are so many species of snakes in the world. Therefore, there are endless neurotoxins waiting us to discover. Every new discovery of neurotoxin and the further research is a brand new research field. The innovations in snake venom research are greater than repeating the researches done by the other people.

Moreover, the distribution of snakes in the world is high specific. Taiwan banded krait is only found in Taiwan, South American rattlesnake is only distributed in part of Brazil, and timber rattlesnake is only found in the eastern region of the US. This phenomenon is advantageous for the researches in local small research institute, for example, CSU system in California. Local research institutes can use its available resource. They don’t have to compete with the famous national research institutes for the resources and never win.

In addition, just as what mentioned above, applying neurotoxins like alpha-bungarotoxin to do research also saves much more budget than ordinary neuroscience research. Above all, research of neurotoxin in snake venom is very advantageous. It provides a new way for biologists in the US.

4. Conclusion:

The neurotoxins in snake venom disrupt synaptic transmission either by presynapic inhibition, like beta-bungarotoxin and crotoxin or postsynaptic inhibition like alpha-bungarotoxin. Alpha-bungarotoxin which inhibit nicotinic-acetylcholine receptor is the most simple mechanism of the three and has been well applied to various research field for labeling nicotinic-acetylcholine receptor. The exact mechanism of beta-bungarotoxin is still not very clear, and leaves us a lot of room for further research. Crotoxin is composed of one basic subunit and one acid subunit. The basic subunit is a phospholipase that break the membrane of synaptic vesicle and the acid subunit is a chaperone protein for the basic subunit. From the literature review in research of neurotoxins in snake venom, we can find that those researches are very helpful in realize our neuron system, and provide us powerful research techniques like alpha-bungarotoxin labeling. The research also spends fewer budgets than most of biomedical researches. Therefore, the mechanism of neurotoxins in snake venom possesses tremendous room for us to research.

5. Literature Cited

Bear. M., Connors. B., Paradiso. M. (1996) Neuroscience. Exploring the Brain, 1st ed. p109

Brazil, O. V., Excell, B. J. (1971). Action of crotoxin and crotactin from the venom of Crotalus durissus terrificus on the frog neuromuscular funtion. . Physiol. 212:34pp.

Chang, C. C. (1960a). Studies on the mechanism of curare-like action of Bungarus multicinctus venom, J. Formosan Med. Assoc., 59, 315.

Chang, C. C. (1960a). Studies on the mechanism of curare-like action of Bungarus multicinctus venom, J. Formosan Med. Assoc., 59, 416.

Chang, C. C. and Huang, M. C. (1974). Comparison of the presynaptic actions of botulinum toxin and beta-bungarotoxin on neuromuscular blocking action, Naunyn-Schmiedeberg Arch. Exp. Pathol. Pharmakol., 282, 129

Chang. C. C. and Lee, C. Y. (1963). Isolation of neurotoxins from the venom of Bungarus multicinctus and their modes of neuromuscular blocking action, Arch. Int. Pharmacodyn., 144, 144.

Chang. C. C., Chen, T. F., and Lee, C. Y. (1973a). Studies of the presynaptic effect of beta-bungarotoxin on neuromuscular transmission, J. Pharacol. Exp. Ther., 184, 339

Chang. C. C., Huang, M. C., and Lee, C. Y. (1973b). Mutual antagonism between botulinum toxin and beta-bungarotoxin, Nature, 243, 166

Chen, I. L. and Lee, C. Y. (1970). Ultrastructural changes in the motor nerve terminals caused by beta-bungarotoxin, Virchows Arch. Pathol. Anat., Physiol., 6, 318

Gralen, N., and Svedberg, T. (1938). The molecular weight of crotoxin. Biochem. J. 32: 1375~1377

Hanley, M. R. (1978). Crotoxin effects in Torpedo california cholinergic excitable vesicles and the role of the phospholipase A activity. Biochem. Biophys. Res. Commun. 82(1): 392-401

Hendon, R. A., and Fraenkel-Conrat, H. (1976). The role of complex formation in the neurotoxicity of crotoxin components A and B. Toxicon 14: 283-289.

Lee, C. Y. (1970). Elapid neurotoxins and their mode of action, Clin. Toxicol., 3, 457.

Lee, C. Y. and Peng, M. T. (1961) An analysis of the respiratory failure produced by the Formosan elapid venom, Arch. Int. Pharmacodyn., 133, 180

Lee, C. Y. and Chang, C. C. (1966) Modes of actions of purified toxins from elapid venoms on neuromuscular transmission, Mem. Inst. Butantan, 33,555

Li, C. H. and Fraenkel-Conrat, H. (1942). Electrophoresis of crotoxin. J. Am. Chem. Soc. 64: 1586-1588

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Radvanyi F, Saliou B, Lembezat MP, Bon C (1989) Binding of crotoxin, a presynaptic phospholipase A2 neurotoxin, to negatively charged phospholipid vesicles. J Neurochem 1989 Oct;53(4):1252-60

Sekhon HS, Jia Y, Raab R, Kuryatov A, Pankow JF, Whitsett JA, Lindstrom J, Spindel ER. (1999) Prenatal nicotine increases pulmonary alpha7 nicotinic receptor expression and alters fetal lung development in monkeys. J Clin Invest 1999 Mar;103(5):637-47

Slotta, K. H. and Fraenkel-Conrat, H. (1939). Crotoxin Nature 144: 290-291

Slotta, K. H. and Fraenkel-Conrat, H. (1938a). 2. Mitt. Uber die Bindungsart des Schnefels. Ber. Dtsch. Chem. Ges. 71, 264.

Slotta, K. H. and Fraenkel-Conrat, H. (1938b). Schlangengifte. 3. Mitt. Reiningung und Kristallisation des Klapperschlangengiftes. Ber. Dtsch. Chem. Ges. 71: 1076~1081

Tu, A. T. 1977. Venoms: Chemistry and Molecular Biology

Yang, Y. L. & Lo, S. J. 1996. The story of Snake Venom Research in Taiwan

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