Snake venom is something of an all around remedy for heart disease depending, of course, on the dosage. The reason is the venom's potential to activate components of the coagulation cascade without producing blood clots,thereby deactivating a victim's ability to coagulate its blood when necessary. Thus it comes as no surprise that biologists have focused on the characterization of snake venom in great detail. Geoff Birrell and his colleagues from the University of Queensland in Brisbane have succeeded in extending our understanding of snake venom by investigating the venom's proteome or `venome' in Molecular and Cellular Proteomics, by characterising some of the components of this `healing' mixture.
The team used venom extracted from over 40 Eastern brown snakes(Pseudonaja textiles) and analysed the venom's composition with two-dimensional gel electrophoresis. They found that the venom was comprised of about 200 proteins. Analysing the relative positions of the protein spots on the gel, the team noticed several horizontal chains of protein spots that suggested that some of these proteins are undergoing post-translational modification.
Next, the authors applied mass spectrometry to 49 trypsin-digested venom proteins in order to identify them, and found many of the usual venom suspects. Several prothrombin activator complex proteins were present (Factor Va-like protein and Xa-like heavy chain peptides), which activate the coagulation cascade but fail to initiate blood clotting because they cause the disappearance of fibrinogen, the precursor required for clot formation.
They also identified textilotoxin, a neurotoxin that inhibits the release of acetylcholine at the presynaptic membrane, thus, essentially paralyzing the prey. The four subunits of textilotoxin were identified as members of the phospholipase A2 family, which have also evolved in the Eastern brown snake to prevent blood coagulation and inhibit muscle function in its victims.
Knowing that some of the venom components had been post-translationally modified, the authors tested for two major types of post-translational modification, phosphorylation and glycosylation. Using modification-specific stains the team found that several venom proteins were modified through glycosylation. They further characterized the modifications with proteins that detect glycosylation and found that sialic acids and N-linked sugars were incorporated into some of the venom components. Further characterizing the sugars, the team showed the presence of N-acetyl-galactosamine and N-acetyl-glucosamine as well as sialic acid attached to some venom proteins.
However, the scientists explain that the function of the carbohydrate protein-modifications is unclear. They point out that none of the human homologs that are involved in blood clotting are glycosylated and thus question the role of these sugars in the pseudo-activation of the clotting cascade. Instead, they speculate that these sugars may stabilize the clotting factors in the venom solution. The team also point out that glycosylation may help keep the proteins in solution, as glycoproteins have a tremendously high affinity for water.
As we learn more and more from proteomic studies of biological tissues and materials, the diversity of protein post-translational modification is going to challenge us considerably and leave us with the impression that the genomic diversity that currently overwhelms us could turn out to be relatively simple by comparison.