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Chapter: Modern Medical Toxicology: Organic Poisons (Toxins): Venomous Bites and Stings

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Snake Venom

Snake venom is nothing but the toxic saliva secreted by modi-fied parotid glands, and is a clear, amber-coloured fluid when fresh.

Snake Venom

·              Snake venom is nothing but the toxic saliva secreted by modi-fied parotid glands, and is a clear, amber-coloured fluid when fresh. It is the most complex of all poisons, containing more than 20 components. The concentration of venom shows diurnal and seasonal variation. Bites inflicted at night and immediately after hibernation are the most severe. Most of the dry weight of venom is constituted by protein, comprising a variety of enzymes, non-enzymatic polypeptide toxins, and non-toxic proteins. Non-protein ingredients of venom include carbohydrates and metals (often in the form of glycoprotein metalloprotein enzymes), lipids, free amino acids, nucleotides, and biogenic amines. The lethal and more deleterious fractions of snake venoms are certain peptides and proteins of relatively low molecular weight (6,000 to 30,000). The peptides appear to have very specific receptor sites, both chemically and physi-ologically.

·              The polypeptide toxins (often called neurotoxins) are found most abundantly in elapid and hydrophid venoms. Postsynaptic alpha neurotoxins such as alpha bungarotoxin and cobrotoxin contain about 60 to 70 amino acid residues, and bind to acetylcholine receptors on the motor end-plate. Presynaptic beta neurotoxins such as beta-bungarotoxin, cobrotoxin, and taipoxin contain about 120–140 amino acid residues, and a phospholipase A subunit, and prevent release of acetylcholine at the neuromuscular junction. Cobra’s alpha bungarotoxin, binds to the acetylcholine receptors and inhibits neural transmis-sion at the neuromuscular junction. Krait’s beta bungarotoxin causes an initial release of acetylcholine, but then damages the nerve terminal and prevents any further release. It is for this reason that krait victims often take longer to recover than cobra victims. The acetylcholinesterase found in most elapid venoms is no longer thought to contribute to their neurotoxicity. Enzyme function and patho-physiological disturbances are most clearly related in the case of viper venom pro-coagulants. For instance, Russell’s viper venom contains at least two proteases, which activate the blood-clotting cascade. RVV-X, a glycoprotein, activates factor X by a calcium-dependant reac-tion, and also acts on factor IX and protein C. RVV-V, an argi-nine ester hydrolase, activates factor V. Echis venom contains a zinc metalloprotein “ecarin” which activates prothrombin. Russell’s viper can induce neurotoxic symptoms in addition to haematological abnormalities. Many species of Russell’s viper have this ability, and it is particularly evident in Southern India and Sri Lanka.

·              Hyaluronidase may serve to promote the spread of venom through tissues. Proteolytic enzymes (hydrolases) may be responsible for local changes in vascular permeability leading to oedema, blistering, and bruising, and to necrosis. Biological amines such as histamine and 5-hydroxytryptamine may contribute to local pain and permeability changes at the site of a snakebite.

·              Sea snake venom contains hyaluronidase, acetylcholinest-erase, leucine aminopeptidase, 5- nucleotidase, phosphomo-noesterase, phosphodiesterase, and phospholipase A. Sea snake venoms are highly toxic. Taking the minimal lethal dose of E.schistosa venom as 0.05 mg/kg for warm-blooded animals, itis estimated that the minimal lethal dosage for a 70 kg man would be 3.5 mg, or about one-third of the venom injected by a fresh adult sea snake.

 

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