ABSTRACT
Small metabolites and peptides are present in the venoms of all venomous snakes (Elapidae, Viperinae and Crotalinae). Venoms examined to date contain >900 metabolites and peptides, and with those present in trace quantities, the number probably exceeds 2500. Some small organic compounds are likely present at levels sufficient to contribute significantly to prey envenomation, given that their known pharmacologies are consistent with snake envenomation strategies. Metabolites include purine nucleosides and their bases, neurotransmitters, neuromodulators, guanidino compounds, carboxylic acids, amines, mono- and disaccharides, and amino acids. Peptides of 2–15 amino acids are also present in significant quantities, particularly in crotaline and viperine venoms. Some constituents are specific to individual taxa, while others are more universally distributed. Some metabolites apparently support high anabolic activity in the gland rather than having toxic functions. Overall, the most abundant organic metabolite is citric acid, due to its predominance in viperine and crotaline venoms, where it chelates divalent cations to minimize venom degradation by venom metalloproteases and damage to glandular tissue by phospholipases. However, in terms of their concentrations in individual venoms, adenosine and adenine are most abundant, although hypoxanthine, guanosine, inosine and guanine all number among the 50 most abundant organic constituents. A novel purine, ethyl adenosine carboxylate, occurs in Dendroaspis polylepis venom, in which it probably contributes to the profound hypotension caused by this venom. Venom N6-methyladenine may block adenine deaminase and guanine deaminase, while indole-3-acrylic acid inhibits xanthine oxidase. These compounds may serve the purpose of enhancing 306the levels of adenosine, inosine and guanosine in some venoms. Acetylcholine is apparently present in significant quantities only in the highly excitotoxic mamba venoms, while γ-aminobutyric acid reaches significant levels only in Bothrops moojeni venom. 4-guanidinobutyric acid and 5-guanidino-2-oxopentanoic acid seem to be present in all venoms.
