Dendrobatid Poison Frogs Research Articles

Alkaloids of Anuran Skin: Antimicrobial Function?

A variety of alkaloids, most of which occur or are structurally related to alkaloids that occur in skin glands of dendrobatid poison frogs, were assayed for antimicrobial activity against the Gram-positive bacterium Bacillus subtilis, the Gram-negative bacterium Escherichia coli and the fungus Candida albicans. Certain pyrrolidines, piperidines and decahydroquinolines, perhydro-histrionicotoxin, and a synthetic pumiliotoxin were active against B. subtilis. Only 2-n-nonylpiperidine was active against E. coli. One pyrrolidine, two piperidines, two decahydroquinolines, and the synthetic pumiliotoxin were active against the fungus C. albicans. The results suggest that certain of the skin alkaloids of poison frogs, in addition to being noxious to predators, may also benefit the frog through protection against skin infections.

Convergent evolution of chemical defense in poison frogs and arthropod prey between Madagascar and the Neotropics.

With few exceptions, aposematically colored poison frogs sequester defensive alkaloids, unchanged, from dietary arthropods. In the Neotropics, myrmicine and formicine ants and the siphonotid millipede Rhinotus purpureus are dietary sources for alkaloids in dendrobatid poison frogs, yet the arthropod sources for Mantella poison frogs in Madagascar remained unknown. We report GC-MS analyses of extracts of arthropods and microsympatric Malagasy poison frogs (Mantella) collected from Ranomafana, Madagascar. Arthropod sources for 11 "poison frog" alkaloids were discovered, 7 of which were also detected in microsympatric Mantella. These arthropod sources include three endemic Malagasy ants, Tetramorium electrum, Anochetus grandidieri, and Paratrechina amblyops (subfamilies Myrmicinae, Ponerinae, and Formicinae, respectively), and the pantropical tramp millipede R. purpureus. Two of these ant species, A. grandidieri and T. electrum, were also found in Mantella stomachs, and ants represented the dominant prey type (67.3% of 609 identified stomach arthropods). To our knowledge, detection of 5,8-disubstituted (ds) indolizidine iso-217B in T. electrum represents the first izidine having a branch point in its carbon skeleton to be identified from ants, and detection of 3,5-ds pyrrolizidine 251O in A. grandidieri represents the first ponerine ant proposed as a dietary source of poison frog alkaloids. Endemic Malagasy ants with defensive alkaloids (with the exception of Paratrechina) are not closely related to any Neotropical species sharing similar chemical defenses. Our results suggest convergent evolution for the acquisition of defensive alkaloids in these dietary ants, which may have been the critical prerequisite for subsequent convergence in poison frogs between Madagascar and the Neotropics.

Evidence for an enantioselective pumiliotoxin 7-hydroxylase in dendrobatid poison frogs of the genus Dendrobates.

Dendrobatid poison frogs readily accumulate alkaloids from diet into skin, where such compounds serve as a chemical defense against predators. Arthropods seem to be the source of decahydroquinolines (DHQs), several izidines, coccinellines, spiropyrrolizidines, pumiliotoxins (PTXs), and allopumiliotoxins (aPTXs). A DHQ iso-223F, and PTX (+)-251D were fed to poison frogs of the dendrobatid genera Dendrobates, Epipedobates, and Phyllobates. The two alkaloids were accumulated in skin unchanged except for the three species of Dendrobates, where approximately 80% of accumulated PTX (+)-251D was stereoselectively hydroxylated to aPTX (+)-267A. The unnatural enantiomer PTX (-)-251D was accumulated efficiently when fed to Dendrobates auratus, but was not hydroxylated. The enantiomers of PTX 251D and their desmethyl analogs were synthesized from N-Boc-protected (-)- and (+)-proline methyl esters. Both PTX (+)-251D and aPTX (+)-267A proved to be potent convulsants in mice, with (+)-267A being approximately 5-fold more toxic than (+)-251D. Both alkaloids were hyperalgesic at the site of injection. The unnatural PTX (-)-251D caused no overt effect in mice. Thus, the evolutionary development of a pumiliotoxin 7-hydroxylase would have provided frogs of the genus Dendrobates with a means of enhancing the antipredator potency of ingested PTXs.

Occurrence and significance of decahydroquinolines from dendrobatid poison frogs and a myrmicine ant: use of 1H and 13C NMR in their conformational analysis

Structures for 2,5-disubstituted decahydroquinolines (DHQs) are reported for the two diastereomeric pairs cis-275B (14) and cis-275B' (15) and 5-epi-trans-269AB (18) and trans-269AB (19), all isolated from skin extracts of dendrobatid frogs, and for 5-epi-cis-275B' (16) and 5-epi-trans-275B (17) found in the extracts of virgin queens of a myrmicine ant [Solenopsis (Diplorhoptrum) azteca]. Detection of such DHQs in an ant, their first reported occurrence, strengthens a dietary hypothesis for the origin of the approximately 30 DHQs that have been detected in extracts of frog skin. NMR data on the two conformers of cis-decahydroquinoline permit assignment of ring conformations and stereochemistry to cis-DHQs of the "N-endo" type or the "N-exo" type. These conformations are also assigned on whether H-8a is equatorial or axial as determined with E-COSY or 1D-HOHAHA spectra.

A New Subclass of Alkaloids from a Dendrobatid Poison Frog: A Structure for Deoxypumiliotoxin 251H

Deoxypumiliotoxin 251H [4], representing a new subclass of the pumiliotoxin-A class of alkaloids, has been isolated from a dendrobatid frog, Epipedobates tricolor. The structure, elucidated by nmr, gc-Ftir, and mass spectral analyses, is proposed to be an 8-methyl-6-(5-hydroxy-2-methylhexylidene)-1-azabicyclo[4.3.0]nonan e. The absolute stereochemistry and the relative configuration of the hydroxyl group are unknown.

A new class of alkaloids from a dendrobatid poison frog: a structure for alkaloid 251F.

Alkaloids of a new class are present in skin extracts of the dendrobatid poison frog, Minyobates bombetes, of Colombia. The structure of the major alkaloid of this class, 251F, has been determined as a trimethylcyclopenta[b]quinolizidinemethanol 1 by nmr, gc-Ft-ir, and ms studies including ms-ms. At least nine congeners of 251F were detected in these extracts.

Alkaloids from dendrobatid poison frogs: Further cis-decahydroquinolines and 8-methylindolizidines

Skin extracts from one population of the poison frog Dendrobates auratus contain a variety of alkaloids, including 2,5-disubstituted-cis-decahydroquinolines and 5-substituted-8-methylindolizidines. Three of the major alkaloids are cis-decahydroquinolines, whose structures based on NNR spectral analyses are 2,5-diallyl-cis-decahydroquinoline (cis- 219A), 2-allyl-5(pent-2-en-4-ynyl)-cis-decabydroquinoline (cis- 243A) and 5-methyl-2-propyl-cis-decahydroquinoline (cis- 195A; the last is identical with “pumiliotoxin C” previously isolated from the poison frog Dendrobates pumilio. Alkaloids cis- 219A and cis- 243A differ fron cis- 195A in configuration at the C-2 position. Another poison frog, Dendrobates histrionicus, was previously shown to produce nearly exclusively trans-decahydroquinoline isomers of 219A and 243A. NMR analyses indicate that the structure of a minor alkaloid from Dendrobates auratus is 5-(pent-2-en-4-ynyl)-8-methylindolizidine ( 203A). Two additional 8-methylindolizidines isolated from Dendrobates pumilio are 5-(hept-4,6-dienyl)-8-methylindolizidine ( 233D) and 5-(hept-6-hydroxy-4-enyl)-8-methylindolizidine ( 251B). FTIR spectral analyses distinguish cis- and trans-decahydroquinolines and provide evidence for configurations of indolizidines.

Alkaloids from dendrobatid poison frogs: Further pumiliotoxins and allopumiliotoxins and a reassignment of the keto function in pumiliotoxin 307F

Skin extracts from the Panamanian poison-frog Dendrobates pumilio have afforded further trace alkaloids of the pumiliotoxin-A class (6-alkylidene-8-hydroxy-8-methylindolizidines) and an allopumiliotoxin subclass (7-hydroxy-PTX-As). Structures for pumiliotoxins 209F and 225F and allopumiliotoxin 225E, all with four carbon 6-alkylidene side chains, for allopumiliotoxins 309D, 325A′ and 325A″, all with ten carbon 6-alkylidene side chains, and a reassignment of the position of the keto function in the ten carbon 6-alkylidene side chain of pumiliotoxin 307F are presented. Carbon-13 magnetic resonance assignments for these and the 15-0-methyl ether artefacts of pumiliotoxin 307A and allopumiliotoxin 323B, are tabulated.

Enantioselective total synthesis of (+)- and (-)-pyrrolidine 197B, a new class of alkaloid from the dendrobatid poison frog: assignment of the absolute configuration

Enantioselective total synthesis of both enantiomers of pyrrolidine 197B (1), a new class of dendrobatid alkaloid, is described. The synthesis begins with the C 2 symmetric S,S or R,R diepoxides 4, derived from (S,S)-1,2,5,6-hexanetetraol (2) as a single common chiral synthon, and involves pyrrolidine formation via the cyclic sulfonates to afford (+)- or (−)-1, respectively. The (+) and (−) enantiomers of 1 were converted to the corresponding N-benzoyl derivatives (+)-27 and (−)-27, which were directly compared with 27 derived from natural 1 by HPLC using a Chiralcel column. This comparison established the absolute stereochemistry of the natural enantiomer of pyrrolidine 197B as 2S,5S [(+)-1]

Alkaloids from dendrobatid poison frogs: trans-decahydroquinolines and indolizidines

Skin extracts from one population of the Colombian poison-frog Dendrobates histrionicus contain a variety of histrionicotoxins (2,7-disubstituted 1-azaspiro(5.5)-undecan-8-ols) and other alkaloids. Two of tha major alkaloids are trans-dacahydro-quinolines whose gross structures based on mass and nuclear magnetic resonance spectral analysis are 2,5-diallyl- trans -dacahydroquinoline 219A and 2-allyl-5-pent-2-en-4-ynyl- trans -dacahydroquinoline 243A. A minor isomer, 243A°, is also present. X-ray crystalloraphic analysis of 219A-HC1 raveals the absolute configuration as [2S,4aS,5S,8aS]2,5-diallyl- trans -decahydroauinoline. A dihydroxydihydro-219A was also isolated and shown by nuclear magnetic resonance spectral analyais to be 2-allyl-5-(2,3-dihydroxypropyl)- trans -decahydroquinoline (253D). Four of the alkaloids from Dendrobates histrionicus are indolizidines namely (5E,9E)-3-n-butyl-5-n-propylndolizidine 223AE, its two ω-hydroxy congeners 239AE (hydroxypropyl) and 239CD (hydroxybutyl), and (5E,9E)-3-n-butyl-5-mathylindolizidine 195E. Structures are based on nuclear magnetic resonance spectral analysis.