Venom Composition

Reptile venoms are typically complex mixtures of primarily peptides and proteins, and the myriad biological effects these molecules produce in envenomated prey and humans are similarly complex and potent. In this book, the many authors discuss the venom apparatus of reptiles, consider the current status of phylogenetic relations of venomous reptiles, explore specific families of venom components, and provide current approaches to the treatment of human envenomations worldwide. In this introduction to the book, variation in venom composition and the factors leading to this variation are discussed. Major patterns of venom compositional trends are identified for the main clades of venomous reptiles, and the identification of novel toxins and interesting structural variants,
as well as elucidation of their biological activities and significance, will remain fertile areas of research for many years to come.

The polyphyletic family Colubridae contains approximately
two-thirds of the described species of advanced snakes, and
nearly half of these (approx. 700 species) produce a venom in a specialized cephalic gland, the Duvernoy’s gland. Biochemical and pharmacological information is lacking for venoms of most species, and modest detailed information on venom composition is available for only a few species which represent a potential health threat to humans. However, colubrid venoms represent a vast source of novel compounds, and some toxins, such as the 20–26 kD CRISP-related venom proteins (helveprins), have only recently been identified in both colubrid and elapid/viperid venoms. Difficulties associated with extraction have been addressed, and it is now possible to obtain venom sufficient for many analyses from even small species. There appears to be a greater number of venom components shared among the colubrids
and the front-fanged snakes than has been previously
noted, and it is probable that as analytical methods improve,
more similarities will emerge. It is clear that colubrid venoms
are homologous with front-fanged snake venoms, but overall
composition as well as biological role(s) of colubrid venoms
may be quite different. Metallo- and serine proteases have been identified in several colubrid venoms, and phospholipase A2 is a more frequent component than has been previously recognized. Venom phosphodiesterase, acetylcholinesterase and prothrombin activator activities occur in some venoms, and postsynaptic neurotoxins and myotoxins have been partially characterized for venoms from several species. Some venoms show high toxicity toward inbred mice, and others are toxic to birds and/or frogs only. Because many colubrids feed on non-mammalian prey, lethal toxicity toward mice is likely only relevant as a measure of potential risk posed to humans. Development of a non-mammalian vertebrate animal model would greatly facilitate systematic comparisons of the pharmacology of colubrid venoms and their components, and such a model would be more appropriate for evaluation of colubrid venom toxicity. Proteomics has the potential to increase our understanding of these venoms rapidly, but classical approaches to toxinology can also contribute
tremendously to this understudied field. As more colubrid
venoms are analyzed, new compounds unique to colubrid
venoms will be identified, and this work in turn will lead to a
better understanding of the evolution and biological significance of snake venoms and venom components.

A B S T R A C T A paradigm of venom research is adaptive evolution of toxins as part of a predator-prey chemical arms race. This study examined differential co-factor dependence, variations relative to dietary preference, and the impact upon relative neutralisation by antivenom of the procoagulant toxins in the venoms of a clade of Australian snakes. All genera were characterised by venoms rich in factor Xa which act upon endogenous prothrombin. Examination of toxin sequences revealed an extraordinary level of conservation, which indicates that adaptive evolution is not a feature of this toxin type. Consistent with this, the venoms did not display differences on the plasma of different taxa. Examination of the prothrombin target revealed endogenous blood proteins are under extreme negative selection pressure for diversification, this in turn puts a strong negative selection pressure upon the toxins as sequence diversification could result in a drift away from the target. Thus this study reveals that adaptive evolution is not a consistent feature in toxin evolution in cases where the target is under negative selection pressure for diversification. Consistent with this high level of toxin conservation, the antivenom showed extremely high-levels of cross-reactivity. There was however a strong statistical correlation between relative degree of phospholipid-dependence and clotting time, with the least dependent venoms producing faster clotting times than the other venoms even in the presence of phospholipid. The results of this study are not only of interest to evolutionary and ecological disciplines, but also have implications for clinical toxinology.

The molecular origin of waglerin peptides has remained enigmatic despite their industrial application in skin cream products to paralyse facial muscles and thus reduce the incidence of wrinkles. Here we show that these neurotoxic peptides are the result of de novo evolution within the prepro region of the C-type natriuretic peptide gene in Tropidolaemus venoms, at a site distinct from the domain encoding for the natriuretic peptide. It is the same region that yielded the azemiopsin peptides from Azemiops feae, indicative of a close relationship of this toxin gene between these two genera. The precursor region for the molecular evolution is a biodiversity hotspot that has yielded other novel bioactive peptides with novel activities. We detail the diversity of components in this and other species in order to explore what characteristics enable it to be such a biodiscovery treasure trove. The unusual function of Tropidolaemus venoms may have been selected for due to evolutionary pressures brought about by a high likelihood of prey escape.