Bioactive alkaloids of frog skin: Combinatorial bioprospecting reveals that pumiliotoxins have an arthropod source

Nearly 500 alkaloids have been detected in skin extracts from frogs of the family Dendrobatidae. All seem to have been sequestered unchanged into skin glands from alkaloid-containing arthropods. Ants, beetles, and millipedes seem to be the source of decahydroquinolines, certain izidines, coccinellines, and spiropyrrolizidine oximes. But the dietary source for a major group of frog-skin alkaloids, namely the pumiliotoxins (PTXs), alloPTXs, and homoPTXs, remained a mystery. In hopes of revealing an arthropod source for the PTX group, small arthropods were collected from eight different sites on a Panamanian island, where the dendrobatid frog (Dendrobates pumilio) was known to contain high levels of two PTXs. The mixed arthropod collections from several sites, each representing up to 20 arthropod taxa, contained PTX 307A and/or alloPTX 323B. In addition, the mixed arthropod collections from several sites contained a 5,8-disubstituted indolizidine (205A or 235B), representing another class of alkaloids previously unknown from an arthropod. An ant alkaloid, decahydroquinoline 195A, was detected in the mixed arthropod collections from several sites. Thus, “combinatorial bioprospecting” demonstrates that further collection and analysis of individual taxa of leaf-litter arthropods should reveal the taxa from which PTXs, alloPTXs, and 5,8-disubstituted indolizidines are derived

Isolation and characterization of γ-L-glutamyl-S-(trans-1-propenyl)-L-cysteine sulfoxide from sandal (Santalum album). Interesting occurrence of sulfoxide diastereoisomers in nature

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A common pumiliotoxin from poison frogs exhibits enantioselective toxicity against mosquitoes

Neotropical poison frogs (Dendrobatidae) contain a variety of lipophilic alkaloids in their diffusely distributed cutaneous glands, including a major class of compounds known as pumiliotoxins. Pumiliotoxins are highly toxic and are believed to protect frogs against predators. Their potential activity against ectoparasites, however, has not been investigated. We tested female yellow fever mosquitoes (Aedes aegypti) for responses to 8-hydroxy-8-methyl-6-(2′-methylhexylidene)-1-azabicyclo[4.3.0]nonane, designated pumiliotoxin 251D [PTX (+)-251D], a skin alkaloid present in all genera of dendrobatids and in other anurans, and to its unnatural enantiomer, PTX (−)-251D. Both enantiomers of PTX 251D presented on silicone feeding membranes reduced landing and feeding by A. aegypti, but PTX (+)-251D did so at lower concentrations. PTX (+)-251D also induced toxicosis, shown when mosquitoes failed to fly off membranes. Similarly, mosquitoes confined with copper wires coated with PTX (+)-251D exhibited greater latencies to fly off the substrate and a higher incidence of leg autotomy than did those confined with the (−)-enantiomer. Our results on the contact toxicities of PTX 251D enantiomers parallel those reported for mice injected with them. The presentation of serial dilutions of PTX (+)-251D to A. aegypti revealed a minimum toxic concentration of 0.1 μg/cm(2). This value is substantially lower than that estimated for the cutaneous abundance of this compound in some frogs, an observation consistent the function of PTX 251D in anuran chemical defense against ectoparasitic arthropods