A Toxic Tale Of Serendipity
This post is from guest blogger Lila Guterman, senior editor for C&EN's Journal News & Community group, and was written for the "Our Favorite Toxic Chemicals" blog carnival.
I have long had a fondness for weird, complex, toxic natural products. (Witness three recent stories I assigned and edited.) So when ScienceGeist announced the “Our Favorite Toxic Chemicals” blog carnival, I knew I’d have to participate. It would give me the chance to tell the story of homobatrachotoxin.
The story begins in 1963, with a young NIH chemist, John W. Daly, taking a trip to Colombia at the request of his boss to investigate the chemistry of frog secretions. His discoveries would launch his career as a
chemical ecologist and pharmacologist, and would spur an outpouring of research into the toxin he discovered, batrachotoxin.
He published the chemical structure, the biological effects, and a partial synthesis, in 1971. Batrachotoxin and its natural analogs, homobatrachotoxin and batrachotoxinin A, are among the most toxic natural substances known. Just 200 ng kill a mouse in 8 minutes; the lethal dose for people is thought to be around 100 µg.
Daly and colleagues found that batrachotoxin binds to sodium channels, opening them. Researchers now use batrachotoxin to study how these channels interact with anesthetics, anticonvulsants, and antiarrhythmia agents.
These alkaloid toxins are secreted by certain species of Colombian poison-dart frogs – so called because Indians in Western Colombia used their secretions to poison the tips of blow darts. The most poisonous frog, the bright yellow Phyllobates terribilis, secretes batrachotoxin at levels high enough to kill several people.
Daly developed an unorthodox method for deciding whether to collect a frog in the wild – one he was lucky, or prudent, enough not to try on Phyllobates terribilis: “It involved touching the frog, then sampling it on the tongue. If you got a burning sensation, then you knew this was a frog you ought to collect,” he told the NIH Record, in 2002. In his work with South and Central American frogs, Daly and his collaborators ended up isolating more than 500 new natural products.
A series of chance occurrences led Daly to perhaps the most intriguing discovery of his career: that homobatrachotoxin is also secreted by a songbird that lives halfway around the world.
Phyllobates terribilis. Credit: Micha Rieser
In 1989, John P. Dumbacher, then a graduate student in ornithology, was studying birds of paradise in New Guinea. His nets sometimes caught other birds, including a songbird called the hooded pitohui (Pitohui dichrous).
“These are large birds that can cut your hands, and as I struggled to free them, they bit and scratched my hands,” he told COSMOS Magazine this year. “These little scratches really stung, so I just put my fingers in my mouth to clean the cut, and after a minute or so my lips and tongue began to tingle and burn.”
Dumbacher once described the feeling in the pages of C&EN as “a lot like tasting hot chili peppers or touching a 9-volt battery.”
Dumbacher had trouble finding a chemist who would take seriously the idea that a bird could be poisonous. (He’d originally been skeptical, too: It took a second trip to New Guinea, in 1990, to convince himself that the birds were secreting toxins, he told Smithsonian ZooGoer.) Only Daly agreed to look at the secretions, and he immediately recognized the mass spectrum of a batrachotoxin.
In 1992, Dumbacher, Daly, and colleagues published their discovery in Science: homobatrachotoxin in the skin and feathers of three species in the genus Pitohui. The levels of the toxin were far lower than in the frogs, but its presence was still startling: How could these two unrelated animals, separated by oceans and thousands of miles, both secrete a single complex steroidal toxin?
It was also the first paper to report a toxic substance from a bird.
“People were stunned by that first report,” Paul J. Weldon, of the Smithsonian Institution, told C&EN in 2000. “It floored the heck out of me.”
In 2000, the two researchers, with colleague Thomas F. Spande, reported the presence of homobatrachotoxin
in another bird in New Guinea. This time it was the blue-capped ifrita, from a different genus. Their finding raised the question: Do birds engage in chemical defense more commonly than scientists had realized?
Daly and Dumbacher also wanted to know where this outrageous organic compound came from. They realized the frogs and birds must both eat something that carried the toxin.
A hint came in 2004: After more than a decade of studying the pitohui diet, they finally found a group of beetles in the genus Choresine that carry batrachotoxin – and at high concentrations. (They found them thanks to a tingling sensation that local villagers reported from contact with the beetles.) The researchers think that the poison-dart frogs must eat relatives of the Choresine beetles that live in Colombia, though they have yet to discover them. Field research in that country has become far more difficult, due to its long-lasting civil war.
Still more questions remain unanswered: How did the birds and frogs evolve to sequester the highly potent alkaloid poison and to avoid its toxic effects? How do the beetles synthesize the toxin? Or do they eat something else that in turn made the batrachotoxin?
Dumbacher, who is now an ornithologist with the California Academy of Sciences, continues to study these questions.
But Daly’s work is done. He died in 2008. (NIH hosted and videotaped a tribute to his life and prodigious career. The Journal of Natural Products also published a special issue in his and another deceased scientist’s honor.)
ScienceGeist asked carnival participants to “talk about a toxic chemical that plays some nontoxic role in their lives.” This one doesn’t play a nontoxic role for me – apart from being the story that got away.
I first learned of the story behind the batrachotoxins in 2000 – it may even have been from the C&EN story I’ve already linked a few times.
I was fascinated. I immediately called John Daly and interviewed him about the work. But at the time, I couldn’t find a way to wrangle it into a story that would work for the weekly publication where I was a science reporter. I’ve wanted to write about it for years but never found a way. I’m glad that C&EN profiled
Daly in 2006, before I joined the staff.
The name of the toxin perhaps should have rung a bell for me back in 2000. The first total synthesis of the molecule had been published in 1998. It was by my undergraduate adviser, Yoshito
Kishi. One of the coauthors had been a labmate of mine. I wish I could remember whether he’d been working on it during my years in the lab, in ‘93 and ‘94.
The New Guinea songbirds Pitohui dichrous (left) and Ifrita kowaldi contain batrachotoxin alkaloids such as homobatrachotoxin, previously found only in poison-dart frogs. Credit: Photos by John Dumbacher