Snake Venom
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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.
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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.
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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.
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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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