The tremendous natural pharmacological libraries that are venoms have been shown to be rich source of investigational ligands or even therapeutic agents. This usefulness is due to the potency and specificity of the individual venom molecules. Australian elapid snake venoms contain some of the most potent yet least studied toxins. The comprehensive literature review undertaken for this thesis revealed the myriad of inventive ways that the venoms have accomplished unique activities or specificities.
LC-MS analysis of the low molecular weight fractions of selected venoms from Australian elapid snakes revealed the presence of components having molecular weights not consistent with any known class of elapid venom toxins. Edman degradation sequencing followed by database homology searching revealed these components to be similar in sequence to a hypotensive family of circulating peptidic hormones called natriuretic peptides. Shared was a seventeen amino acid loop formed by a single cysteine bond, with N- and C-terminal tails formed by extension on either side of the loop. Several residues were conserved before, within and after the cysteines that form this loop. However, significant differences existed, particularly within the sequence forming the C-terminal tail. All of the peptides except one (0-4112) had prolines substituted for other residues in the C-terminal tail. These additional prolines may influence receptor specificity through changes in the conformation of the C-terminal tail.
Natriuretic peptides act upon vascular smooth muscle, kidneys, adrenals and other organs. A primary function of these peptides is to both acutely and chronically lower systemic blood pressure through the relaxation of vascular smooth muscle. These circulating peptides exert many of their biological and physiological effects primarily through binding to two receptors: NPR-A (natriuretic peptide receptor A) and NPR-B (natriuretic peptide receptor B). These receptors function to produce vascular relaxation by increasing the intracellular concentration of the secondary messenger molecule cGMP and as such are more properly called GC-A and GC-B, respectively.
The isolated venom peptides were assayed for generalized hypotensive activity, relaxation of precontracted vascular tissue and binding to either GC-A or GC-B. These assays were undertaken to ascertain the similarity in action to the natriuretic peptides that they structurally resemble. Preliminary results indicated that the crude venoms and isolated peptides were potently hypotensive when injected intravenously into the anaesthetized rat. However, the majority of the isolated peptides were not similar in action to the natriuretic peptides on either the isolated tissues or the GC receptors. Only one venom peptide (0-4112) was equipotent to the natriuretic peptide ANP in all assays. These differences in activity were consistent with the differences in sequences between the venom peptides. Isolated venom peptides with multiple prolines differed from circulating forms of natriuretic peptides in specificity and potency of action. The only peptide lacking multiple prolines (0-4112) was equipotent to ANP. These results shed light onto structure-function relationships of natriuretic peptide and may prove to be excellent investigational ligands.