New alkaloid types encountered in mites

May 16, 2007 07:03 GMT  ·  By

Poison frogs are famous for their toxin which is used for poisoning arrow tips. Their bright vivid colors warn the predators on their toxicity.

The toxins from the frog skin protecting it from predators and infections are chemicals called alkaloids. But frogs cannot synthesize the alkaloids themselves, they have to acquire them from their food items. If poison frogs are fed with fruit flies, which have no alkaloids, they miss the venom.

"But until recently, it's been a mystery what the dietary sources are." said Ralph Saporito of Florida International University (FIU) in Miami.

As ants are known to synthesize alkaloids, they were also believed to be the main source for more than 800 known alkaloids encountered in poison frogs. But Saporito's team discovered that frogs in the lowlands of Panama and Costa Rica get most of their toxins from tiny arthropods called oribatid mites.

The other novelty is that this group was not even previously known to synthesize alkaloids at all. Oribatid mites are decomposers of dead plant materials in soil and leaf litter.

The team found about 80 different alkaloids in mites and over half of them were also present in the skin of the strawberry poison dart frogs of the area.

"The mite-produced compounds included representatives of nearly half of the 24 major chemical classes of alkaloids known from poison frogs," said Saporito.

"It is surprising that mites-rather than ants-were found to be the major source of alkaloids. The source of many alkaloids in poison frogs have [still] not been identified." commented Christopher Raxworthy of the American Museum of Natural History in New York.

Alkaloids, like caffeine and nicotine, are extremely varied and with diverse and specific properties. Many are bitter, warning about their toxicity and a lot of them are used as medicinal drugs, like in the case of the belladonna or henbane.

The new alkaloids could be the source for powerful new drugs. The team also analyzed up 40 previously unknown alkaloids specific to mites.

"Many of these compounds show interesting biological activity and are structurally unprecedented in nature," said co-author John Daly, of the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland.

"The frogs are not harmed by the toxic compounds, presumably because the alkaloids are consumed in very small amounts and become concentrated in the frogs' bodies over time. The frogs don't chemically transform [the alkaloids] at all, they just store them in their skin glands and then secrete them," Daly suggested.

"The next big question to answer is how the poison frogs accomplish this trick. Virtually nothing is known about how the frogs actually take the alkaloids from a dietary source and store them in their glands." Saporito added.