Snakes and spiders are the classic venomous animals. But producing toxic venom is not the exclusive ability of reptiles and arachnoids.
But a very small number of mammalian species also posses toxic venom, including the male platypus.
However, although the clinical signs of platypus venom are well known (excruciating pain, rapid swelling, hyperalgesia), many of the phenotypic effects have not been matched to chemical players with specific cellular/molecular interactions. Now, research published in the Journal of the American Chemical Society identifies a peptide that is highly concentrated within platypus venom, and matches that peptide to previously reported cellular consequences of venom injection.
Previous research has identified a number of platypus venom constituents, including defensin-like peptides, C-type natriuretic peptides (OvCNPs),NGF, and hyaluronidase. Additionally, previously research from the lab of Daisuke Uemura of the Nagoya University showed that crude platypus venom induces a potent influx of calcium into human neuroblastoma cells. Now, Masaki Kita and colleagues have expanded on that research, identifying Heptapeptide 1 (a primary component of venom fluid) as the main effector of this calcium increase.
They attempted to identify the mechanism of this increase, testing for binding of heptapeptide 1 to L- and N- type voltage gated calcium channels, potassium channels (Ka, Katp, or hERG), or sodium channels. Heptapeptide 1 did not bind to any of these channels, leading Kita et al to conclude that heptapeptide 1 "may specifically bind to other receptor channels or biomacromolecules, such as GTP-binding regulatory proteins, as in the case of the direct activation of Gi proteins by mastoparan or substance P."
Additionally, they conclude that the influx of calcium was not caused by a the high concentration of heptapeptide 1 (which is found in concentrations of 200 ng/uL within the venom), but rather by specific interactions with target molecules. What those molecules are is a mystery to be reveled in another paper.
One evolutionary curiosity: the defensin-like peptides found within the platypus venom are also found within reptile venom. However, genetic analysis in 2008 revealed that the platypus peptides evolved independently from the reptile peptides, although both were derived from the same gene family.
NYTimes article about the research, including a (highly approved of by this author) reference to a literary example of platypus poisoning.
The original article: Kita et al. Duck-Billed Platypus Venom Peptides Induce Ca2+ Influx in Neuroblastoma Cells. J. Am. Chem. Soc. 2009, 131: 18038-18039. DOI: 10.1021/ja908148z
Update 1/25/2010: To clarify a question brought up while discussing this blog post: the venom is released from spurs located on the platypus' hind legs.