Skin Chemistry: Poison frogs upgrade toxins from prey
Susan Milius, Science News Sept. 6, 2003; Vol. 164, No. 10
the first time, scientists have found a poisonous frog that takes up a toxin
from its prey and then tweaks the chemical to make it a more deadly weapon.
At least three species
of the 4-to-5-centimeter-long Dendrobates frogs of the New World tropics
modify an alkaloid to create one that's about five times as poisonous, according
to a team led by John W. Daly of the National
Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) in Bethesda,
Md. The souped-up poison, one of a class called pumiliotoxins, ends up as
a protective agent in the frogs' skin, the researchers report in an upcoming
Proceedings of the National Academy of Sciences.
"It's an important thing, showing how chemistry connects the life of one organism to another," comments chemical ecologist Jerrold Meinwald of Cornell University. Although scientists have found that some creatures other than frogs customize a basic toxin for various purposes, "I don't know of any other examples of improving a defensive weapon," Meinwald says.
The new work grows out of years of research that started with a puzzle regarding dart-poison frogs, which belong to the family that includes Dendrobates. Frogs in three other families in South America, Australia, and Madagascar also carry poisons in their skin. However, when zoos and aquariums raise these supposedly deadly creatures, frogs from all but one Australian genus grow up harmless.
Daly and his collaborators in the early 1990s proposed that the wild frogs must be picking up the toxins from food and storing them in their skins. Since then, the scientists have found that ants and other arthropods in the frogs' habitat carry most of the poisons that show up in frogs' skin.
The finding that some frogs change the toxins they have eaten came as an unexpected twist of a theoretical study. NIDDK chemists Jingyuan Ma and Herman Ziffer were working with an alkaloid called pumiliotoxin 251D, one of the skin toxins of the frog Dendrobates auratus. The scientists produced both the form of the alkaloid found in nature plus a mirror-image form.
Daly and Valerie Clark dusted these substances onto termites and fruit flies and fed the spiced prey to captive frogs. When Thomas F. Spande and another NIDDK chemist, H. Martin Garraffo, analyzed the skins of these frogs, some 80 percent of the natural form of 251D had been converted to another toxin, allopumiliotoxin 267A. It has an extra hydroxyl group on one of its two rings. The unnatural form of 251D, however, showed up unchanged in frog skin.
The frogs must have a specific enzyme that retrofits just one form, the researchers conclude. Two other Dendrobates species modified the natural form, but two species in related genera didn't.
When the scientists tested allopumiliotoxin 267A on mice, they found it a much more potent poison than its precursor. "It's the first case found where a frog is clearly modifying one of the sequestered alkaloids," says Spande. "We were very surprised."References:
Daly, J.W., H.M. Garraffo, T.F. Spande, V.C. Clark, J. Ma, H. Ziffer, et al. In press. Evidence for an enantioselective pumiliotoxin 7-hydroxylase in dendrobatid poison frogs of the genus Dendrobates. Proceedings of the National Academy of Sciences.
Daly, J.W. . . . H.M. Garraffo, T.F. Spande, et al. 2002. Bioactive alkaloids of frog skin: Combinational bioprospecting reveals that pumiliotoxins have an anthropod source. Proceedings of the National Academy of Sciences 99(Oct. 29): 13996-14001. Available at http://www.pnas.org/cgi/content/full/99/22/13996.
Gorman, J. 2002. Toxic tools: Frogs down under pack their own poison. Science News 161(April 13):229. Available to subscribers at http://www.sciencenews.org/20020413/fob6.asp.
Milius, S. 2001. Poison birds copy 'don't touch' feathers. Science News 160(Oct. 6):214. Available to subscribers at http://www.sciencenews.org/20011006/fob6.asp.
______. 2001. Lifestyles of the bright and toxic overlap. Science News 159(April 14):230. Available at http://www.sciencenews.org/20010414/fob6.asp.
______. 2000. Second bird genus shares dart-frog toxins. Science News 158(Oct. 21):263. References and sources available at http://www.sciencenews.org/20001021/fob7ref.asp.
For more information on the Dendrobates frog, check out the University of Michigan's Animal Diversity site.
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