(Simul-blogged with The Lab Lemming - check it out.)
Fluoroacetic acid is plain old acetic acid, plus a fluorine. It is a poison of some potency. Oddly, neither the "fluoro" or "acetic acid" part is associated with any general tox risks - acetic acid, as you'll see, is central to metabolism, and fluorinated compounds aren't inherently bad. It is a "mechanism based" poison.
The overwhelming majority of energy in your food is made useful via the Citric Acid (Krebs) Cycle. Some of it you get directly, and some of it is claimed later, but without Krebs, you're dead.
Fluoroacetic acid will result in the production of fluorocitric acid by citrate synthase. This would be not such a big deal, except for aconitase, which chomps away at regular citric acid, moving it along to the next step in the cycle. Given some fluoroacetate-derived fluorocitrate, aconitase will be inhibited from working on even regular citrate. There goes your energy. Cutting off a step in the citric acid cycle throws a major wrench into the works as far as metabolism goes - it'll kill ya good, just like another mitochondrial poison, dinitrophenol.
As noted in Lab Lemming's post, fluoroacetate is found in certain acacia trees! No doubt this is to discourage would-be munchers. Plant aconitase appears to have a higher tolerance for fluoroacetate than that of mammals (caveat: old article and not my field, comments welcome).
In 1859, a man named Thomas Austin brought twenty-four rabbits to Australia (good hunting, you see). Within a decade, there were millions of their offspring. Australia is a weird place - lots of stuff exists there that doesn't exist anywhere else, and lots of stuff that's benign elsewhere is a blight there. Rabbits eat and reproduce like crazy and are believe to have resulted in mass extinctions of huge numbers of aboriginal Australian animals. The Western Shield program has used scattered oats tainted with fluoroacetate, among other poisons, to blunt the population of avidly grazing rabbits and give native species a slightly better chance.
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Aconitase is the enzyme in the citric acid cycle that isomerizes citrate for further processing. It does this functional group shift via an intermediate called cis-Aconitate, which is the namesake of the enzyme.
See also its sodium salt, known in the U.S. as "Compound 1080" and widely spread by ranchers in attempts to kill off coyotes...
Any ideas where the name aconitase (or aconitate) comes from?
I know there's a plant genus called Aconitum...
1080 is commonly used all over Australia to kill foxes, pigs, cats, and other feral animals. The theory is that because a lot of native plants have fluoroacetate in them, native creatures have developed better resistance to this poison than the introduced species have.