By Ed Edelson
Certain tropical frogs may want flies to get close enough to eat, but not too close. According to new research, the skin of these amphibians contains a powerful and natural mosquito repellent. But that's only part of the story.
The toxin, dubbed pumiliotoxin 251d, is one of thousands of alkaloid chemicals on the skins of tropical frogs in central and South America, explained researcher John W. Daly, a scientist emeritus at the National Institute of Diabetes, Digestive and Kidney Disease (NIDDK), in Bethesda, Md. "I'm interested in this compound because of suggestions that it might have value in finding a treatment for autoimmune diseases such as rheumatoid arthritis and diabetes," Daly explained.
His team published the findings in this week's issue of the Proceedings of the National Academy of Sciences. Daly is famous in his field. For four decades, he has done field work in nine countries, "looking at frogs and finding more unique substances," he said. He has identified molecules that have the same heart-boosting effect as digitalis and have become the subject of intense research by major pharmaceutical companies.
Unhappily, that research has flagged recently, said Thomas F. Spande, a research chemist as NIDDK, who took part in the mosquito study. "Some compounds were very effective in regulating the function of the heart," Spande said. "They were being tested, but nobody ever went with it. Maybe [the companies] were put off by the name 'toxins.'" It's highly unlikely that the molecule described in the new study will be put to use as a mosquito repellent for human use, Spande added. "We certainly don't want to create the impression that this is the world's best repellent for human use," he said. "It's way too expensive and way too toxic."
The new study is the latest in a series looking at the toxins found on the skin of tropical frogs, Spande said. "Naively, we first thought they were there primarily to deter predetation -- to give the predator such a reaction in the mouth that he would spit [the frog] out," he said. But, "it seemed strange that if the main role was to deter predation, the frog would have so many of these alkaloids -- 20 or 30 different kinds," Spande said. "So we started working on other roles." The study used a test in which mosquitoes were exposed to a pipette coated with a diluted solution of the molecule taken from frogs' skins. The natural version was "many times more effective" than a laboratory-created version, Spande said.
There are still more properties of the toxins to be explored, he said. "Certain of these alkaloids are antibacterial, antifungal and antiviral," Spande said. "Many of these frog skin alkaloids are being investigated in neurochemistry in blocking certain receptors." But it appears that the NIDDK scientists will soon lose their role in such investigations. "Our program is being terminated," Spande said. "I hope we can move to another institute."