Snakes have been endowed with a lethal secretion called venom which is used to subdue prey and as a defensive and survival tool. This poisonous mixture contains carbohydrates, nucleic acids, amino acids, lipids and proteins.
The three-finger peptides from Viperidae are among the most potent of these toxins. They are well-characterized based on their structure and function.
Neurotoxins
Neurotoxic snake venoms contain peptide toxins that are highly potent and specific inhibitors of nicotinic acetylcholine receptors. They bind to the orthosteric site of the nAChR and cause the neurotransmitter to break down and block normal nerve transmission. This leads to paralysis, including respiratory failure and death. Snakebite causes about 138,000 deaths each year, most in the tropics1.
Recent advances in transcriptomics and proteomics have enabled us to characterize the composition of hundreds of snake species, uncovering the chemical richness of their venoms. A significant number of these venom proteins exhibit high affinity for vital biomolecules and have been identified as the toxins that are responsible for the clinical symptoms of snakebite2.
In the last two decades, long-chain a-neurotoxins from elapids have been isolated, identified and characterized. The pharmacological profile and structure-function relationship of these long-chain a-neurotoxins, especially abungarotoxin (a-Bgtx) from Taiwan banded krait Bungarus multicinctus8, fulditoxin from coral snake Micrurus fulvius9, and a-cobratoxin from spitting cobra Naja siamensis10, have been studied extensively.
The results of a recent study of a-neurotoxins that target the nAChR orthosteric site showed that resistance to these neurotoxic venoms seems to have convergently evolved on three occasions in Afro-Asian primates. This includes an amplification of the resistance in the Homininae (chimpanzee, gorilla and human clade) toward a-Bgtx and fulditoxin. Cercopithecidae and Ponginae were also found to be resistant, coinciding with their evolutionary biogeographies in Africa and Asia. Conversely, Lemuriformes and Platyrrhini were found to be the most susceptible to binding by these venoms.
Hemotoxins
Traditionally, snake venoms have been classified as haemotoxic or neurotoxic (World Health Organization, 2010a). But the reality is that different snake species can have venoms of both types. Moreover, the same snake can produce venoms with both toxicities simultaneously. Variability in venom composition exists at all taxonomic levels including families, genera and species of snakes, and also over the lifetime of the same individual (Fig 2).
Hemotoxic snake venoms typically induce blood-clotting signs. For example, the venom of Mojave rattlesnake (Crotalus terrificus) causes multiple hemorrhages and respiratory paralysis in bite victims. In contrast, venoms of elapids such as cobras (Naja) and king cobras (Ophiophagus) produce neurotoxic signs.
The hemotoxic effects of viperid venoms are usually due to secretory phospholipases A2 (sPLA2s). These are enzymes that generate free radicals which disrupt membrane homeostasis leading to swelling, severe blistering and oedema. The cytotoxic activity of sPLA2 toxins may also result in cell death or necrosis.
The presynaptic neurotoxicity of venom sPLA2 crotoxin is mainly achieved by a series of events that include binding to a receptor at the presynaptic nerve terminal, disrupting the release of the mediator and causing apoptosis. The mechanism involves binding to proteins 14-3-3 and the N-type sPLA2 receptor. Other mechanisms have also been suggested such as a general membrane-destabilizing effect, phospholipolysis and blocking nAChR at the postsynaptic membrane.
Myotoxins
In venomous snakes, the myotoxic effects of the venom are produced by a variety of low molecular weight proteins including secreted phospholipases A2 (PLA2), three-finger peptides (3FTXs), and cysteine-rich secretory proteins such as L-amino acid oxidases and Kunitz peptides. [1] In the bitten limb, increased vascular permeability and extravasation of plasma and blood lead to swelling and bruising that is mediated by the myotoxins. The inflammatory response is further exacerbated by the action of metalloproteinases, endogenous autacoids and the secondary effects of first-aid measures such as tight tourniquets.
In an indirect test, PLA2s from Bothrops atrox venom were found to induce moderate indirect hemolytic activity in human red cells when exposed to serum, but not Tris-Sucrose buffer alone. These PLA2s displayed significant antigenic variation with the commercial B. atrox antivenom (SAB) and an antiserum against BthTX-I, a Lys-49 PLA2 homolog isolated from the B. atrox venom (anti-PLA2) raised in rabbits, showing distinct recognition patterns.
Three-finger toxins are one of the major protein families of snake venom components and include the a-bungarotoxin isolated more than 20 years ago from African black mamba Dendroaspis polylepis venom and the recently discovered Tx7335 from eastern green mamba D. angusticeps venom. The latter TFT exhibits a potent analgesic effect with much less tolerance than morphine and no respiratory distress, and interacts strongly and selectively with the -aminobutryic acid-sensitive (ASIC) voltage-gated calcium channel.
Disintegrins
The venoms of some vipers contain disintegrins, which are proteins that bind to and inhibit the function of the integrin receptors. Disintegrins are derived from the proteolytic cleavage of soluble vascular cell membrane protein precursors (SVMP). The cysteine-rich domain of SVMPs is cleaved by disintegrins to produce the disintegrin-like domain, which can then bind to integrin receptors and block their function. Disintegrins are classified as monomeric or dimeric according to their length and number of disulfide bonds.
Dimeric disintegrins can be further classified as homo- or heterodimers according to whether their two subunits have identical or different sequences. In general, monomeric disintegrins are smaller than dimeric disintegrins and have shorter residue lengths. Generally, they have four to six disulfide bonds. They can be further subdivided into short, medium, and long disintegrins based on their amino acid sequence.
The venoms of some snakes contain disintegrins that act as anti-platelet agents. For example, saxatilin from Gloydius saxatilis and crotatroxin 2 from Crotalus atrox both reduce platelet aggregation in vitro. Other peptides, such as eritostatin from Eristicophis macmahoni and leucurogin from Bothrops leucurus, show antiangiogenic activity by inhibiting the binding of fibronectin to integrin receptors. Heterodimeric disintegrins, such as MLDG-2 and EMF-10 from Bothrops atrox, also display anti-angiogenic properties by blocking the activation of the integrin-matrix metalloproteinase pathway. These peptides have potential therapeutic applications in the treatment of cancer and other diseases that may involve excessive platelet aggregation.