Toxins, snake

However, several snake toxins have been identified that display an unprecedented degree of mAChR... 

Harris, J.B. (1990). Phospholipases in snake venoms and their effects on nerve and muscle. In Snake Toxins (Harvey, A.L., ed.), pp. 91-129, Pergamon Press, Oxford. 

All sea snakes are poisonous and their venoms are extremely toxic. The LD q for crude sea snake venom can be as low as 0.10 fig/g mouse body weight (i). For purified toxin the LD q is even lower, suggesting the high toxicity of sea snake toxins and venoms. This toxicity is derived from the presence of potent neurotoxins. Compared to snake venoms of terrestrial origin, sea snake venoms have been studied less. Different enzymes reported to be present or absent are summarized in Table I. 

The similarity of the primary structure of different sea snake venoms has already been discussed. Postsynaptic neurotoxins from Elapidae venom have been extensively studied. Elapidae include well-known snakes such as cobra, krait, mambas, coral snakes, and all Australian snakes. Like sea snake toxins, Elapidae toxins can also be grouped into short-chain (Type I) and long-chain (Type II) toxins. Moreover, two types of neurotoxins are also similar to cardiotoxins, especially in the positions of disulfide bonds. However, amino acid sequences between cardiotoxins and sea snake and Elapidae neurotoxins are quite different. In comparing the sequence of sea snake and Elapidae neurotoxins, there is a considerable conservation in amino acid sequence, but the difference is greater than among the various sea snake toxins. 

When a nerve-muscle preparation is stimulated in the presence of a sea snake neurotoxin, there is no twitch. However, when the muscle itself is stimulated directly in the presence of a neurotoxin, the muscle contracts. This means that neurotoxin does not inhibit the muscle itself. Moreover, postsynaptic neurotoxin does not inhibit the release of acetylcholine from the nerve ending. Therefore, the site of snake toxin inhibition must be in the postsynaptic site 20). Later it was shown that a neurotoxin strongly binds to the acetylcholine receptor (AChR). 

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Polyclonal antibody preparations have been used for several decades to induce passive immunization against infectious diseases and other harmful agents, particularly toxins. The antibody preparations are usually administered by direct i.v. injection. While this affords immediate immunological protection, its effect is transitory, usually persisting for only 2-3 weeks (i.e. until the antibodies are excreted). Passive immunization can be used prophylactically (i.e. to prevent a future medical episode) or therapeutically (i.e. to treat a medical condition that is already established). An example of the former would be prior administration of a specific anti-snake toxin antibody preparation to an individual before they travel to a world region in which these snakes are commonly found. An example of the latter would be administration of the anti-venom antibody immediately after the individual has experienced a snake bite. 

Table 10.2. Some polyclonal antibody preparations raised in horses against specific snake toxins...

Snake toxins are potent (irreversible) muscarinic ligands... 

Karlsson, E., Jolkkonen, M., Mulugeta, E., Onali, P., Adem, A. Snake toxins with high selectivity for subtypes of muscarinin acetylcholine receptors, Biochimie 2000, 82, 793-806. 

Potter, L.T. Snake toxins that bind specifically to individual subtypes of muscarinic receptors, Life Sci. 2001, 68, 2541-2547. 

Harvey, A.L., Anderson, A.J. (1985). Dendrotoxins snake toxins that block potassium channels and facilitate neurotransmitter release. Pharmacol. Ther. 31 33-55. 

Harvey AL (ed.) (1991) Snake Toxins International Encyclopedia of Pharmacology and Therapeutics, section 134. New York Pergamon. 

At ETb receptors the order of potency of the natural ligands is ET-1 = ET-2 = ET-3. Synthetic selective agonists include IRL 1620. [Ala ]-ET-1, also the snake toxin sarafotoxin is active. 

Certain snake toxins have been found to bind irreversibly to the acetylcholine receptor, thus blocking cholinergic transmissions. These include toxins such as alpha-bungarotoxin from the Indian cobra. The toxin is a polypeptide containing 70 amino acids which cross-links the alpha and beta subunits of the cholinergic receptor (see Section 11.14.). 

One way of investigating the effect of chemicals at the nicotinic acetylcholine receptor is to isolate the receptor and measure the ability of chemicals to bind to it. In the experiments described here, housefly heads were used as a source of nervous tissue, and a crude membrane preparation was obtained using a method based on that described by Jones et al. (2). The snake toxin, crbungarotoxin (a BGTx), is known to bind to this receptor with a very high affinity (KD"1.1 +/- 0.1nM.(3))... 

Most previous studies are based on the assumption that the toxicity of snake toxins is induced by a narrowly defined active region. As a consequence, single amino acid residues vere chemically modified and the effects of the modification assayed by in vivo toxicity tests (11,14,18). However, it was found that the derivatives modified at all those positions believed to be important still have significant activity. Instead, as has been previously shown by Karlsson (1 1), there is a gradual decrease of, but never a complete reduction in toxicity ( affinity of binding). 

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