Saturday, February 18, 2012

Colubridae Toxin and Venom

A general trend in the evolution of the modern (Caenophidian) snakes is the production of toxic oral secretions. Vidal, this volume). The family ‘‘Colubridae’’ is the largest family of modern snakes and contains roughly two-thirds of the extant described species. It is generally agreed that this family is polyphyletic, but characters which would differentiate ‘‘hidden’’ families are scant; Vidal, this volume). It is therefore not surprising that this wide variety of unrelated species worldwide have a homolog of the venom gland of front-fanged snakes (the Duvernoy’s gland; often have enlarged posterior maxillary teeth and produce venom. Estimates of the number of venomous colubrids approach 700 species

Venomous colubrids include such divergent genera as Tantilla (often < 20 cm in length as adults) and Boiga (up to 300 cm as adults). Most may not produce a venom capable of causing serious damage to humans, but at least five species (Dispholidus typus, Thelotornis capensis, Rhabdophis tigrinus, Philodryas olfersii and Tachymenis peruviana) have caused human fatalities. However, many species of colubrid snakes are moderate to large in size (1–2 meters), and lack of serious effects of bites may largely be due to rare encounters and the natural inclination of a bitten individual to disengage a biting snake rapidly (e.g., Kuch and Mebs, this volume). Many species specialize on noxious invertebrates (spiders, scorpions, centipedes), while others include the various classes of vertebrates as prey. 

Venom composition is related to prey type/form, and natural history data on snake feeding patterns are important to toxinologists because the types of toxins necessary to facilitate handling of vertebrate prey are likely to be much different than those necessary to subdue scorpions and centipedes. Novel snake venom toxins are likely to occur among these specialists, and such toxins may be specific blockers of analogs in invertebrates of vertebrate ion channels/ligand receptors. In addition, dietary specialists (such as bird-feeding species) may produce taxa-specific toxins.

The term ‘‘venom’’ has been somewhat misunderstood by both the lay public and by many scientists, and it seems useful here to define what is meant by a venom, as there is also disagreement as to its application to the Duvernoy’s secretion of colubrid snakes. I find the definition of Russell to be reasonably inclusive and most useful: ‘‘the toxic substance produced by a plant or animal in a highly developed secretory organ or group of cells. . .which is delivered during the act of biting or stinging’’. An important distinction of this definition is that it defines a route of administration (roughly, injection) and that it allows for a venom to be composed of one to many toxins. For much of the lay public, venom is synonymous with ‘‘toxin’’; this incorrect usage of terminology has served to cloud issues concerning venomous animals and on occasion has lead to incorrect application of snakebite management techniques. 

A toxin is defined here as a specific molecular entity derived from an organism which has a deleterious effect on another organism, and it is not a term interchangeable with venom or poison (both of which could contain one to many toxins). Therefore, in this paper, the Duvernoy’s secretion of colubrid snakes will be considered as a venom, consisting of enzymes, several toxins and other compounds, and it is homologous with the venoms of the front-fanged snakes (families Elapidae and Viperidae), both in many compositional features and in general biological role (as a trophic adaptation). Note, however, that the specific biological roles of colubrid venoms and their components, as for many front-fanged snake venom components, are at best poorly defined.


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