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Are snake venoms toxic to snakes?


 
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Do snakes bite other snakes and can they poison each other? Associate Professor Kevin Broady looks at what protective devices snakes have to avoid death by snake venom.     
Kevin Broady was puzzled by the finding that King brown snakes which feeds on other venomous snakes (brown snakes, tiger snakes), are not affected by their venom. Searching the literature he found a few clues about how the venom might be inhibited. It has been shown that venomous snakes in Europe (vipers) and the Americas (rattlesnakes) both had components in their blood that protect them against venoms of their own and sometimes of other species.

The venom only poses a threat if it enters the bloodstream and can travel to the nervous system.
Because venoms are essentially complex proteins, they are readily destroyed by digestive enzymes and so there is no danger of poisoning occurring once the venomous prey has been consumed. This is what makes it possible, for example, for the Taiwanese custom of drinking rice wine that contains snake venom or blood without any ill effects.

Kevin Broady's group studied Australian snakes, looking for similar protective mechanisms.
They found the serum from all the species studied (King brown snakes, tiger snakes, brown snakes and Inland Taipans) all contained similar protective molecules that inhibit the toxicity of venoms. This is perhaps not surprising given that all venomous Australian snakes belong to the Elapid family and their venoms have similar toxic components.

These inhibitors all act to block the toxicity of the same components in the venoms called phospholipases (PLA2s) which are enzymes that also have neurotoxicity. Because the inhibitors block the PLA2 molecules they are called PLA2 inhibitors.
Looking at the venoms from snakes across the world, they all contain varying proportions of these PLA2 compounds and these are the components that contribute to the extreme toxicity of the venom.

Interestingly phospholipases are a whole famly of enzymes that generally are not toxic. PLA2s are, for example, found in all mammals, in bile produced in the liver and help to break down fats, and are obviously not toxic at all. However, it seems that venomous snakes have found ways to modify the enzymes, making them toxic. In Australian snakes, PLA2s are major components of the venom because they are neurotoxic and so help in the capture of the snake's prey. Overseas, the PLA2 may be only a small component of the venoms which often contain compounds that rapidlty start to dissolve the tissues of the prey.

All of the PLA2 inhibitors can be divided into three groups. The alpha group act against the PLA2s in vipers only, strongly inhibiting toxin from other vipers. The beta group are only found in one snake in Japan. The gamma group is found in Europe, the Americas and Australia.
Kevin Broady's group was interested in this group because they were much more universal in their ability to inhibit PLA2s and thought this might be a way to come up with a universal inhibitor or control agent for venoms.
The group worked mainly with the Tasmania triger snake and succeeded in isolating the active component and sequencing the molecule which is a large and complex protein. While it was initially hoped that this could be used to develop a universal antivenene or anti-inflammatory drug, unfortunately the complexity of the molecule does not allow this to occur. The way in which the molecules prevent the PLA2 acting seems to relate to them being large complex proteins which enables them, essentially, to swallow the venom PLA2, acting like a biological blotting paper for the venom.

Text: V.B. February 2009

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